Chapter 5: Origin and Evolution of Life
In December 1938, a museum curator named Marjorie Courtenay-Latimer went down to the docks at East London, South Africa, to look through a fishing trawler’s catch. Among the ordinary fish she saw something strange: a heavy, steel-blue fish about a metre and a half long, with thick scales and — most puzzling of all — fins that sat on stubby, fleshy, limb-like stalks. They did not look like fins. They looked like the beginnings of legs.
She could not identify it, so she sketched it and sent the drawing to a chemistry lecturer and fish expert, J. L. B. Smith. When he saw the sketch, he was astonished. This was a coelacanth — a lobe-finned fish known only from fossils, and believed to have died out long before the age of the dinosaurs ended. A creature that existed only in rock had just been pulled out of the sea, alive.
The coelacanth mattered because those fleshy fins tell a story. Fins like these are thought to be the kind of structure from which the limbs of the first land vertebrates arose. A living animal was carrying, in its own body, a clue to how life had changed over hundreds of millions of years.
This chapter is about that change — how life may have begun on a lifeless Earth, how it slowly diversified into millions of forms, and how we can read that history in fossils, bones, and genes.
Diagram
Fig. 5.0 – A living coelacanth swimming near the sea floor, with its lobed, limb-like paired fins clearly visible and labelled, and a small inset of a fossil coelacanth in rock for comparison
By the end of this chapter, you will be able to answer each of these questions for yourself.
5.1 Origin of Life (Protobiogenesis)
What Makes Matter “Living”?
Before we ask how life began, we must be clear about what we mean by “living”. Living matter is not simply matter that moves. It shows a definite set of attributes that non-living matter does not.
Living matter responds to changes around it (responsiveness), it grows, it carries out metabolism — the sum of all chemical reactions taking place inside the body. It also carries out energy transformations, changing energy from one form to another to run these reactions, and it reproduces, making more of its own kind.
Notice that these are not random properties. Each of them is needed for a body to stay organised and to continue existing. A living system constantly spends energy to keep itself in order, and it copies itself before it wears out. Any explanation for the origin of life must therefore explain how a bundle of chemicals first came to do all of these things together.
An Unanswered Question
The origin of life has remained an enigma — a deep puzzle — for thinkers of every age. Even today, despite great progress in biochemistry, astrobiology, geography and molecular biology, scientists have not been able to establish the complete truth of how life first appeared.
Because the event happened roughly four billion years ago and left no direct witness, various theories and hypotheses have been proposed to find the probable answer. Some of these are religious, some speculative, and some testable by experiment. Only the testable ones are of scientific value. The main lines of thought are compared in FC 5.1.
Flowchart
FC 5.1 – Theories of the origin of life: Question “How did life begin?” → branch 1: Theory of Special Creation (created by a supernatural power; no scientific proof) → branch 2: Cosmozoic Theory / Panspermia (life arrived from space as spores) → branch 3: Theory of Spontaneous Generation / Abiogenesis (life arose suddenly from non-living matter; disproved by Pasteur) → branch 4: Theory of Biogenesis (life comes only from pre-existing life; explains continuity, not the first life) → leads to → Chemical Evolution (Section 5.2)
a. Theory of Special Creation
This is the oldest of all the theories. It is based on religious belief and has no scientific proof behind it. According to this theory, all living organisms were created by a super-natural power.
Because the theory offers nothing that can be tested, measured, or disproved by observation, it lies outside the scope of science. Science does not reject it as a belief; it simply cannot examine it using experiments.
b. Cosmozoic Theory (Theory of Panspermia)
This theory advocates that life did not originate on the planet Earth at all. Instead, it may have descended to the earth from other planets in the form of spores or micro-organisms. These hypothetical travelling life-forms are called cosmozoa or panspermia.
The idea is not entirely far-fetched. Recently, NASA has reported fossils of bacteria-like organisms on a piece of Martian rock recovered from Antarctica — a meteorite that reached the earth from Mars.
Yet the theory has a fatal weakness. Even if life did arrive from another planet, this theory could not explain the origin of life on those other planets. It simply shifts the problem elsewhere instead of solving it, and hence it is not accepted.
c. Theory of Spontaneous Generation (Abiogenesis)
According to this theory, life originated from non-living (inanimate) material spontaneously. In other words, living organisms were thought to arise on their own, suddenly and repeatedly, out of mud, decaying meat, or grain.
Such beliefs were common for centuries. People saw maggots appear on rotting meat and concluded that the meat itself had produced them. This theory was disproved by Louis Pasteur.
Diagram
Fig. 5.20 – Redi’s experiment: three jars of meat side by side — open jar (flies enter, maggots appear), sealed jar (no flies, no maggots), gauze-covered jar (flies on gauze, maggots on gauze only); label flies, maggots and “no maggots”
Redi’s work weakened the theory, but supporters of spontaneous generation shifted their argument to micro-organisms: perhaps large animals do not appear on their own, they said, but microbes in a broth surely do.
Diagram
Fig. 5.21 – Pasteur’s experiment: swan-necked flask with broth being boiled; broth remains clear and free of micro-organisms; second flask with the curved neck removed showing micro-organisms growing in the broth
d. Theory of Biogenesis
According to this theory, living organisms are always produced from pre-existing living forms, by the process called reproduction. A bacterium comes from a bacterium; a mango tree comes from a mango seed; a child comes from parents.
This theory is strongly supported by everyday observation and by Pasteur’s experiment. But look closely at what it can and cannot do. Biogenesis explains the continuity of life — how life keeps going once it exists. It cannot explain the origin of the first life on earth, because there were no pre-existing living forms to reproduce from.
That gap is exactly why a further explanation was needed. If the first cell could not have been born from a parent, and could not have appeared magically from mud, then it must have been assembled slowly, step by step, from simple chemicals — over an enormous span of time. This idea is taken up in the next section.
5.2 Chemical Evolution of Life (Self Assembly Theory of Origin of Life)
Section 5.1 left us with a gap. Life cannot appear from nothing, and yet the very first living form had no parent. The only remaining possibility is that it was built — not in one leap, but by chemistry, over an immense stretch of time.
According to this theory, life originated on earth by the combination of several chemicals through constant chemical reactions carried out over a long period of time. Because the molecules gradually joined together and organised themselves without any outside designer, the theory is also called the self assembly theory of origin of life, or the biochemical origin of life.
The theory was first formulated by Haeckel. It was later developed in detail by the Russian scientist Alexander I. Oparin (1924) and the British biologist J. B. S. Haldane (1929). Working independently, both arrived at a strikingly similar picture of the early earth. The process of chemical evolution can be divided into the following steps, summarised in FC 5.2.
Flowchart
FC 5.2 – Steps of chemical evolution: Origin of earth and primitive reducing atmosphere → formation of \(CH_4\), \(NH_3\), \(H_2O\), \(H_2S\) → formation of simple organic molecules (monosaccharides, amino acids, purines, pyrimidines, fatty acids, glycerol) in the “hot dilute soup” → polymerisation into complex organic molecules (polysaccharides, fats, proteins, nucleosides, nucleotides) → formation of nucleic acids (RNA, DNA) with self-replicating ability → formation of protobionts (coacervates, microspheres) → eobionts/protocell → first cell (anaerobic, heterotrophic)
a. Origin of Earth and Primitive Atmosphere
To understand how life began, we must first understand the stage on which it began. The origin of the universe was explained by the Big-Bang theory of Georges Lemaitre (1931). According to this theory, the universe originated about 20 billion years ago by a single huge titanic explosion.
As the universe expanded, the temperature decreased, and various galaxies of solid objects were formed. The Milky Way is one such galaxy, and our solar system is one small part of it. The earth is one of the planets of the solar system, and it originated about 4.6 billion years ago.
When it was formed, the earth was a rotating cloud of hot gases and cosmic dust called a nebula. As this mass condensed and cooled, it separated into layers — a process called stratification. Heavier elements like nickel and iron sank to the core, while lighter ones such as helium, hydrogen, nitrogen, oxygen and carbon remained on the surface. These lighter elements formed the atmosphere of the earth.
This point is crucial. The primitive atmosphere of the earth was quite different from the present one: it was of a reducing type, devoid of free oxygen. A reducing atmosphere is one rich in hydrogen and hydrogen-containing gases, in which molecules tend to gain electrons rather than lose them. Without free oxygen, delicate organic molecules could form and survive instead of being burnt up.
b. Formation of Ammonia, Water and Methane
The primitive atmosphere was very hot. As it slowly cooled, the lighter elements began to react with one another, because at lower temperatures atoms can hold on to the bonds they form instead of being knocked apart.
The early atmosphere was rich in hydrogen, carbon, nitrogen and sulphur. Of these, hydrogen was the most active, and so it reacted with the other elements to form the first chemicals on earth: methane, ammonia, water and hydrogen sulphide — \(CH_4\), \(NH_3\), \(H_2O\) and \(H_2S\).
c. Formation of Simple Organic Molecules
As the temperature of the earth decreased further, steam condensed into water. This resulted in heavy rainfall, which cooled the earth still more. The rain water got accumulated on the land to form rivers, streams, lakes, seas and oceans.
The atmosphere at that time did not contain an ozone layer. Today, ozone in the upper atmosphere absorbs most of the sun’s ultra-violet radiation. Without it, ultra-violet radiation reached the surface of the earth directly, and it carried enormous energy.
Under the influence of the available energy sources — ultra-violet rays, other radiations, lightning and volcanic activity — the early molecules of hydrocarbons, ammonia, methane and water underwent reactions such as condensation, polymerisation, oxidation and reduction. Energy is essential here, because building a larger molecule from smaller ones does not happen on its own; it must be driven.
These reactions resulted in the formation of simple molecules like monosaccharides, amino acids, purines, pyrimidines, fatty acids and glycerol. You will recognise these at once: they are the very building blocks from which carbohydrates, proteins, nucleic acids and fats are made in living bodies today.
All these simple organic molecules accumulated at the bottom of water bodies. Haldane described this accumulation as the “hot dilute soup” or “primitive broth”.
Why did the soup not simply decay away? Because there was no free oxygen to oxidise these molecules, and no enzymes to break them down — the reactions were taking place in a non-enzymatic, reducing atmosphere. So instead of being destroyed as fast as they formed, the organic molecules piled up over millions of years, growing steadily more concentrated.
d. Formation of Complex Organic Molecules
The primitive broth was neutral and free from oxygen — a chemically gentle environment in which fragile long chains could survive. In it, polymerisation took place: simple organic molecules aggregated and joined to form new, larger complex organic molecules such as polysaccharides, fats, proteins, nucleosides and nucleotides.
Polymerisation of amino acids formed protoproteins, which later formed proteins. The formation of protein molecules is considered a landmark in the origin of life.
Why such a landmark? Because proteins include enzymes — biological catalysts. Proteins (enzymes) accelerated the rate of other chemical reactions. Once catalysts existed, chemistry that would otherwise take centuries could happen quickly, and the whole system began to change at a new speed.
e. Formation of Nucleic Acids
Nucleotides may have been formed by reaction between phosphoric acid, sugar and nitrogenous bases (purines and pyrimidines). A nucleotide is thus made of three parts, and all three were already lying in the primitive broth.
A number of nucleotides then joined together to form nucleic acids — RNA and DNA.
The decisive step came next. Nucleic acids acquired self-replicating ability — the capacity to make copies of themselves. Self-replication is a fundamental property of living forms. Once a molecule can copy itself, whatever changes appear in it can be passed on, and the door to heredity, and therefore to evolution, is finally open.
f. Formation of Protobionts or Procells
Nucleic acids, along with other inorganic and organic molecules, formed the first form of life, called protobionts. Protobionts are prebiotic chemical aggregates having some properties of a living system.
Protobionts are formed due to coacervation, that is, the aggregation of organic molecules into droplets. Oparin (1924) called these droplets coacervates, while Sidney Fox called his own version protenoids or microspheres. Coacervates are shown in Fig. 5.1.
Diagram
Fig. 5.1 – Coacervates: a large spherical droplet enclosing numerous smaller colloidal globules, with the boundary labelled and the droplets marked “Coacervates”
Coacervates are colloidal aggregations of hydrophobic proteins and lipids — in effect, lipoid bubbles. They grew in size by taking up material from the surrounding aqueous medium. As they grew, they became thermodynamically unstable and split into smaller units, comparable to the daughter cells of budding organisms. Growth and division, two hallmarks of life, thus appeared before life itself did.
Microspheres are protenoids formed from colloidal hydrophilic complexes surrounded by water molecules. These bodies may have had an outer double-membrane, like a primitive cell. Diffusion and osmosis may have occurred across this membrane, allowing the inside to be chemically different from the outside. They were more stable than coacervates.
It must be stated plainly: coacervates and microspheres were non-living colloidal aggregations — of lipids and of proteinoids respectively. They were not organisms. Yet they had some basic properties of living cells, such as growth and division. These colloidal aggregations turned into the first primitive living system, called eobionts or protocells.
g. Formation of First Cell
When an RNA or DNA system developed within the protocells, they came to look like bacteria or viruses. Now the droplet no longer merely grew and split — it carried instructions. With genetic material inside, the protocell could regulate various metabolic activities in an organised, repeatable way.
The first cell was anaerobic and heterotrophic, and obtained energy by chemoheterotrophic processes. Each of these words follows logically from the conditions we have described. It had to be anaerobic because there was no free oxygen in the atmosphere. It had to be heterotrophic because it could not manufacture its own food — but it did not need to, since the primitive broth around it was already full of ready-made organic molecules, which it simply absorbed and broke down for energy.
Urey and Miller’s Experiment
All of this was, until 1953, only a chain of reasoning. Stanley L. Miller and his teacher Harold C. Urey provided the first experimental evidence in support of the chemical evolution theory of Oparin. Instead of arguing about the early earth, they built a small model of it.
They designed a glass apparatus called the spark-discharge apparatus, shown in Fig. 5.2.
Diagram
Fig. 5.2 – Urey and Miller’s spark-discharge apparatus: flask of boiling water supplying water vapour, connected to an upper chamber containing \(CH_4\), \(NH_3\) and \(H_2\) with two electrodes producing a carbon-arc spark, leading to a condenser with water in/water out, water droplets falling into a U-tube water trap, and the liquid containing organic compounds collected there; also label the outlet to the vacuum pump
The apparatus was first sterilized and evacuated — sterilized so that no living micro-organism could sneak in and be mistaken for newly formed life, and evacuated so that no present-day air, especially oxygen, remained inside.
Methane, ammonia and hydrogen gases were then pumped in the proportion of 1:2:2 into the glass chamber, recreating the reducing atmosphere of the primitive earth. A tube carrying water vapour was also connected to the chamber, standing in for the early oceans and the water vapour above them.
The lightning effect was mimicked by an electric discharge carbon arc spark inside the chamber. The processes of evaporation and precipitation — the ancient cycle of steam rising and rain falling — were simulated by the use of a heating mantle and a condenser respectively.
The mixture of \(CH_4\), \(NH_3\) and \(H_2\) was exposed continuously to electric discharge for several days, causing the gases to interact, after which these were condensed. The liquid collected in the U-tube turned brown — the first visible sign that something new had been made.
Chemical analysis of this liquid reported the presence of simple organic compounds: urea, amino acids, lactic acid and others. Amino acids — the very units of proteins — had appeared from nothing but gases, water and sparks.
This experiment strongly supports the view that the simple molecules present in the earth’s early atmosphere combined to form the organic building blocks of life.
RNA World Hypothesis
The Oparin–Haldane theory and the Miller–Urey experiment together give us an understanding that the pathway of the origin of life on earth goes from non-living to living. But a difficulty remains. The rule ‘like begets like’ requires the presence of a stable genetic material to store information, and cellular machinery to carry out the routine activities essential for survival.
Here lies a puzzle that looks like a vicious circle. In modern cells, DNA carries the information needed to make proteins — but proteins (enzymes) are needed to copy DNA. Which of the two, then, came first?
We are aware that certain proteins, which we call enzymes, catalyse the chemical reactions in the cell. Then, in 1980, Sidney Altman and Thomas Cech independently found out that RNAs can also act as biocatalysts. These catalytic RNAs are called ribozymes. For this discovery, Altman and Cech earned the Nobel Prize in Chemistry in 1989.
This discovery breaks the circle. If a single molecule can both carry information and catalyse reactions, neither DNA nor protein had to come first — RNA could do both jobs by itself.
The discovery therefore provided important support for the RNA World hypothesis. The hypothesis suggests that early (first) life must have been based exclusively on nucleic acids, most probably RNA. Remarkably, it was first proposed by Carl Woese, Francis Crick and Leslie Orgel in 1960 — long before ribozymes were discovered. The idea was later confirmed by a discovery no one had anticipated when it was proposed.
Several facts support this hypothesis. RNA is found abundantly in all living cells. It is structurally related to DNA. Chains of RNA can evolve or undergo mutations, can replicate, and can catalyse reactions — that is, RNA is capable of everything the earliest life would have needed. This sequence is shown in Fig. 5.3.
Diagram
Fig. 5.3 – RNA World Hypothesis: RNA forms from inorganic sources → RNA self-replicates via ribozymes → RNA catalyses protein synthesis → membrane formation changes internal chemistry, allowing new functionality → RNA codes for both DNA and protein → proteins catalyse cellular activities → DNA becomes the master template
Besides, biomolecules like Acetyl-Co-A have a nucleotide in their molecular structure — a fossil of an older, RNA-dominated chemistry preserved inside our own metabolism. The major evidence, however, is the existence of the ribosome, the protein assembly unit of the cell. In the ribosome, the translation process is catalysed by RNA. (Refer to the chapter Molecular Basis of Inheritance.) In other words, in every cell of your body at this moment, protein is being made by an RNA catalyst.
These early molecules might have undergone repeated replication and mutation, forming varieties of RNA molecules with varying sizes and catalytic properties. Eventually they might have developed their own protein coats and the machinery needed to survive, assembling into the primitive cell. In due course, a double-stranded, more stable structure — DNA — might have been formed and taken over the job of storing information, which it does better than RNA because two strands protect the message and allow damage to be repaired from the other strand.
Thus the ongoing journey continued, and it resulted in the rich biodiversity we see on earth today. How that diversity arose from those first cells is the subject of the rest of this chapter.
5.3 Organic Evolution
What the Word Means
The word evolution comes from Latin: e = from, and volvere = to roll. Literally, it means the act of unrolling or unfolding of nature — as though nature were a scroll being slowly opened, revealing more of itself with time.
Evolution brings about orderly changes from one form to another. The word “orderly” is important. These are not random, meaningless changes; they follow from what came before. The result is that descendants become different from their ancestors.
Organic evolution can be defined as the slow, gradual, continuous and irreversible changes through which the present-day complex forms of life developed (or evolved) from their simple pre-existing forms.
Each word in this definition carries weight. Slow and gradual tell us that evolution is not something you can watch in an afternoon. Continuous tells us it has never stopped — it is going on now. Irreversible tells us that a lineage does not simply turn back into its ancestor; even when a group returns to an old habitat, it does so with a new body built from what it had become.
According to Charles Darwin, evolution is ‘descent with modification’. This short phrase contains the whole idea: offspring descend from parents, and in each generation they are slightly modified. Add up small modifications over millions of generations, and you get a new kind of organism.
Lamarck’s Theory of Inheritance of Acquired Characters
Long before Darwin, another explanation had been offered. You have already studied Lamarck’s Theory of inheritance of acquired characters in Std. X.
According to this theory, the traits of an organism are acquired during its own lifetime — due to an internal force, changes in the environment, new needs, and the use and disuse of organs. An organ that is used constantly becomes better developed, while an organ that is not used degenerates.
Lamarck further held that these characters acquired during life are passed on to the offspring. After several generations, the accumulation of such acquired traits gives rise to a new species.
The theory sounds reasonable at first. A blacksmith’s arms do grow stronger with use. But the crucial question is a different one: does his son inherit those stronger arms? This is precisely where the theory was tested — and failed.
Weismann’s Test and the Germplasm Theory
Weismann concluded that variations produced in somatic cells (somatoplasm) — the ordinary body cells, such as those of the tail — are not inherited, while variations produced in germ cells (germplasm) — the reproductive cells that form gametes — are inherited by the next generation. On this basis he proposed the Theory of continuity of Germplasm.
The logic behind this becomes clear once you see how a body is built, as shown in Fig. 5.22. The germ cells are set aside early and are passed on, generation after generation, in an unbroken line. The body — the somatoplasm — is built afresh from them each time. Whatever happens to the body, therefore, does not reach the germ line. Cutting a tail damages the body, not the instructions from which the next body will be made.
Diagram
Fig. 5.22 – Weismann’s germplasm theory: a continuous germplasm line running from parent to offspring to grand-offspring, with the somatoplasm (body) branching off from it in each generation as a dead end; arrows show germplasm → germplasm continuity and germplasm → somatoplasm one-way development
With Lamarck’s mechanism ruled out, the central question of biology remained open: if acquired characters are not inherited, then what actually causes species to change over time? The answer proposed by Darwin is taken up in the next section.
5.4 Darwinism
Before Darwinism, several theories were proposed to explain the process of organic evolution. Lamarck (1809) published his ‘Theory of origin of acquired characters’, which, as we have just seen, was ruled out.
What was needed was not merely another guess, but a mechanism — some natural process, visible in the living world itself, that could turn one species into another without anyone directing it. That mechanism was supplied by Charles Darwin (Fig. 5.4), in his Darwinism, also called the Theory of origin of species by Natural Selection.
Diagram
Fig. 5.4 – Portrait of Charles Darwin (1809–1882)
The Voyage and the Influences
Before publishing his book, Darwin was influenced by several thinkers, and each one gave him a piece of the argument.
He was much influenced by C. Lyell, a geologist, who pointed out that the natural forces which existed in the past are the same as those existing at present. This mattered enormously. If the same slow forces have always been at work, then the earth is very old, and slow processes have had enough time to produce very large changes.
Darwin himself observed the changes in the earth’s crust along the coast of South America, caused by natural forces — direct evidence that the ground beneath our feet is not fixed.
On the Galapagos islands and other nearby islands of the Archipelago, he observed a variety of strange plants and animals, particularly the variations between the tortoises and the finches from one island to another. Similar observations were also made by Wallace, who independently arrived at much the same idea.
He was also influenced by R. Malthus, an economist, who worked on human population and pointed out that the reproductive potential of humans is greater than the rate at which food can be produced. This results in multiplication of the human population without a corresponding increase in the food supply. Such an increase must lead to competition, and therefore to a struggle for existence within the human species.
Finally, Darwin was influenced by plant and animal breeders, who, by using artificial selection, had altered the characteristics of cultivated plants and domestic animals. If a farmer could reshape a species in a few decades merely by choosing which individuals breed, what could nature do by the same method over millions of years?
The Five Postulates of Darwinism
Darwinism is based on five main postulates. Read them in order, because they form a single chain of reasoning, summarised in FC 5.3 — each postulate makes the next one unavoidable.
Flowchart
FC 5.3 – The logic of Darwinism: Overproduction (far more offspring born than can survive) → Struggle for existence (for food, space, mate, escape from enemies) → Organic variations (individuals differ from one another) → Natural selection (individuals with favourable variations survive and reproduce; others perish) → Origin of new species (favourable variations accumulate over generations → speciation)
1. Overproduction (Prodigality of Nature)
Overproduction is the natural tendency of organisms to produce a larger number of progeny, in geometric ratio, for the perpetuation of the species. Nature is extravagant — it produces far more offspring than the environment can possibly support.
Darwin observed this prodigality in many species of plants and animals. A salmon fish, for example, produces about 28 lakh eggs in a single season. In a span of 750 years, a single pair of elephants would produce 19,000,000 elephants — and the elephant is one of the slowest-breeding animals on earth.
And yet, the size of a given species in a given area remains relatively constant, apart from the fluctuations that occur seasonally. Here is the puzzle that drives the entire theory: if reproduction is so lavish, why do populations not explode? The only possible answer is that most of the offspring produced must die before they can reproduce.
2. Struggle for Existence
The tendency towards overproduction leads directly to a struggle for existence between the members of a population. Since the number of individuals produced is far greater than the resources available, they must compete.
This struggle is for a limited supply of food, for space and for a mate; it is also a struggle to overcome adverse environmental conditions and to escape from enemies. It is not necessarily a violent fight. A seedling that reaches sunlight before its neighbours is “struggling for existence” quite as much as two stags locking horns.
3. Organic Variations
If all individuals were identical, the struggle would be decided by chance alone and nothing would change over time. But they are not identical.
The organic variations speak for all kinds of differences that occur among individuals — in morphology, physiology, nutrition, habit, behavioural patterns, and so on. Darwin recognised these variations as the raw material for evolution. Variations were observed among members of the same species, and even between different species.
This is the single most important postulate to grasp, because everything that follows depends on it. Selection cannot create anything; it can only choose among differences that already exist.
4. Natural Selection
Organic variations can serve as evidence that some organisms are better adapted to survive under the existing environmental conditions than others. In the struggle for existence, organisms with favourable variations are selected by nature, while those with unfavourable variations perish.
According to Darwin, the principle by which useful variations are preserved by nature is called ‘Natural Selection’. H. Spencer named this process ‘survival of the fittest’.
Notice carefully what “fittest” means here. It does not mean the strongest, the largest, or the fastest. It means the best suited to that particular environment at that particular time. A thick fur coat is a superb adaptation in the Arctic and a fatal one in a desert. Fitness is measured only by one yardstick: how many surviving offspring an individual leaves behind.
5. Origin of New Species (Speciation)
Favourable variations are transmitted from generation to generation. As a result, the successive generations become better adapted to their environment than the ones before them.
Gradually, these adaptations — along with a few new modifications appearing in each generation — become fixed in the life cycle of the organism. Given enough time, the accumulated differences become so great that the population is no longer the same species it started as. It has become a new species. This is speciation, and it is examined in detail later in the chapter.
Evidences for Darwinism
Evolution of the long-necked giraffe. Giraffes with longer necks could pluck and eat more leaves from tall trees and woody climbers, and so were better fed than their short-necked companions. This adaptation became fixed in the population because it aided survival. The taller-born giraffe could survive in famine-struck, arid areas where only tall trees still bore leaves, and this adaptation was transmitted to their offspring. In this way, the present-day long-necked giraffe came into existence.
Black-coloured peppered moths evolved gradually as a new form. This striking case — known as industrial melanism — is examined closely in Section 5.6, where you will see natural selection working within a single century.
DDT resistance in mosquitoes. Intensive DDT spraying destroyed almost all types of mosquitoes. However, some mosquitoes already carried a variation that made them resistant to DDT, and these survived the onslaught of the spray. Such resistant mosquitoes survived and reproduced, giving rise to more resistant offspring, until the whole population was resistant and the insecticide no longer worked.
Drawbacks and Objections to Darwinism
Darwinism was a landmark, but it was not complete. Darwin knew nothing of genes — Mendel’s work was published in his lifetime but was unknown to him. The gaps that follow are all, in one way or another, gaps about heredity.
He considered minute fluctuating variations to be the principal factor in evolution. Many such small variations are not heritable, and so cannot be part of evolution. In other words, he was unable to distinguish between environmental and hereditary variations.
He also did not distinguish between somatic and germinal variations, and considered all variations to be heritable — the very error that Weismann’s experiment exposed.
He explained the ‘survival of the fittest’, but he did not explain the ‘arrival of the fittest’ — that is, where the favourable variations come from in the first place.
He did not explain the cause, origin and inheritance of variations, nor the origin of vestigial organs, nor could he explain the extinction of species.
According to natural selection, new species are formed by the gradual accumulation of useful variations. If this is so, then intermediate forms should exist between the old species and the new one. But in most cases such intermediate forms were not recognised. Moreover, Darwinism also could not explain the occurrence of neutral flowers, or the sterility of hybrids — for how can a character that leaves no offspring at all be favoured by selection?
Objection (c) is the deepest of them, and it points straight ahead. Darwin could explain what happens to variations once they exist. He could not explain how they arise. The very next theory — de Vries’ mutation theory — was an attempt to answer exactly that question.
5.5 Mutation Theory
Darwinism left one question unanswered: where do variations come from? An answer was proposed by Hugo de Vries in 1901, soon after the rediscovery of Mendel’s work in 1900 — a rediscovery that, for the first time, gave biologists a way of thinking about heredity in terms of discrete units.
The Evening Primrose
De Vries proposed this theory on the basis of his observations on seven generations of the plant evening primrose (Oenothera lamarckiana), which he grew and studied patiently in his garden, as shown in Fig. 5.23.
Diagram
Fig. 5.23 – Evening primrose (Oenothera lamarckiana): a normal plant with its flowers, alongside a variant offspring plant that differs sharply in size and form from the parent
He found that although most of the offspring resembled their parents in many characters, some of the offspring showed the appearance of sudden or spontaneous variation, clearly different from the phenotypic expression of the parent. These were not slight, graded differences of the kind Darwin had emphasised; they were abrupt jumps.
De Vries called these sudden variations mutations, or discontinuous variations.
Two further observations turned an oddity into a theory. First, the variant offspring produced variants, and not normal plants — that is, the change was inheritable and bred true. Second, some of these variants themselves produced still more variations. Since these sudden changes were heritable, they could serve as the raw material that Darwin’s theory badly needed. On this basis, de Vries proposed the Mutation theory.
Main Features of Mutation Theory
The main features of the mutation theory are as follows. Their overall logic is summarised in FC 5.4.
- Mutations are large, sudden, random and discontinuous variations appearing in a population.
- These changes are inheritable, and they may not be directional — that is, a mutation does not appear because the organism needs it, and it does not point in any particular direction.
- Mutations provide the raw material for organic evolution.
- A mutation may be useful or harmful. Useful mutations are selected by nature.
- Accumulation of these mutations over a period of time leads to the origin and establishment of new species.
- Harmful mutations may persist in the population, or may get eliminated by nature.
Flowchart
FC 5.4 – Mutation theory: sudden, random mutation appears in a population → the change is heritable → branch: if useful → selected by nature → accumulates over generations → origin of a new species; if harmful → either eliminated by nature or persists in the population
Read the second and fourth points together, because that is where students most often go wrong. A mutation is random with respect to need. The mosquito did not mutate because DDT was sprayed; the mutation was already there, and the spraying merely revealed which individuals happened to carry it. Nature does not order the mutation — nature only edits the result afterwards.
Objections to Mutation Theory
Like Darwinism, the mutation theory could not stand alone. Three objections were raised against it.
The large and discontinuous variations observed by Hugo de Vries were actually due to chromosomal aberrations — changes in whole chromosomes — whereas gene mutations usually bring about only minor changes. In other words, what de Vries had seen in his primroses was not the ordinary raw material of evolution.
The rate of mutation is very low as compared with the requirement of evolution. Mutations alone occur far too rarely to account for the pace at which species have actually changed.
Chromosomal aberrations have little significance in evolution, as they are quite unstable and are not reliably passed on.
Notice how the picture now stands. Darwin had a mechanism (natural selection) but no source of variation. De Vries had a source of variation (mutation) but no adequate mechanism, and too few mutations to work with. Each theory held a piece the other lacked.
The obvious step was to put them together, and to bring in everything that genetics, ecology and palaeontology had learned in the meantime. That combination is the Modern Synthetic Theory, taken up next.
5.6 Modern Synthetic Theory of Evolution
Why a “Synthesis” Was Needed
Neither Darwin nor de Vries was wrong; each was incomplete. The Modern Synthetic Theory is the result of a true synthesis of all the biological disciplines. Studies drawn from genetics, ecology, anatomy, geography, palaeontology and other fields were pursued together to explain the mechanism of evolution. In this synthesis, due importance was given to both mutations and natural selection — mutation supplying the variation, selection deciding its fate.
The main contributors to the modern theory of evolution include R. Fischer, J. B. S. Haldane, T. Dobzhansky, J. Huxley, E. Mayr, Simpson, Stebbins, Fisher, Sewall Wright, Mendel and T. H. Morgan.
Stebbins, in his book, discussed five key factors that contribute to the evolution of new species: gene mutations, mutations in chromosome structure, mutations in chromosome number, genetic recombinations, natural selection, and reproductive isolation.
The Population, Not the Individual
Before going further, one shift of viewpoint must be made, and it is the heart of the modern theory. An individual is born with a fixed set of genes and dies with them; an individual cannot evolve. What changes over time is the population.
All individuals of the same species constitute a population. Populations occur in small groups of ‘interbreeding populations’. Such a small interbreeding group within a population is referred to as a Mendelian population.
The total genetic information encoded in the sum total of genes in a Mendelian population is called the gene pool. Simply put, the gene pool means the total number of genes of all the individuals in a population. The gametes produced by the individuals furnish the pool of genes for the next generation, from which the genes of that generation will be drawn.
The migration of individuals into or out of a population effectively alters the gene pool. The gene pool also changes as one generation is replaced by another in the Mendelian population. Thus, any change in the gene pool affects the population as a whole.
Genes are arranged linearly on the chromosome and each occupies a definite position, called a locus. According to Mendel, every gene that influences a trait has two alleles. The proportion of one particular allele in the gene pool, relative to the total number of alleles at that locus, is called the gene frequency (allele frequency).
Now the definition of evolution itself can be made precise and measurable. Evolution, in the language of the modern theory, is simply a change in gene frequency in a population across generations. If the frequencies stay the same, the population is not evolving. If they shift, it is.
The Three Main Concepts
The modern synthetic theory comprises five main factors, which are broadly grouped under three main concepts:
- Genetic variations, caused by various aspects of mutation, recombination and migration;
- Natural selection; and
- Isolation.
Together, these three explain the evolution of species. The relationship between them is shown in FC 5.5.
Flowchart
FC 5.5 – Modern Synthetic Theory: Genetic variations (gene mutation, genetic recombination, gene flow, genetic drift, chromosomal aberrations) → variation in the gene pool → Natural selection (favourable variations reproduce more) → change in gene frequency → Isolation (geographical or reproductive) prevents gene flow between groups → separate gene pools → new species
a. Genetic Variations
The change in genes and in gene frequencies is known as genetic variation. Genetic variations are caused by the following factors.
i. Gene Mutation
A sudden, permanent, heritable change is called a mutation. A mutation can occur in the gene, in the chromosome morphology, and in the chromosome number.
A mutation that occurs within a single gene is called a point mutation or gene mutation. Since a gene carries the instructions for building a protein, a change in the gene changes the protein, and so leads to a change in the phenotype of the organism — causing variation.
ii. Genetic Recombination
Mutation is not the only source of new combinations. In sexually reproducing organisms, during gamete formation, an exchange of genetic material occurs between the non-sister chromatids of homologous chromosomes. This exchange is called crossing over.
Crossing over produces new genetic combinations, which result in variation. Importantly, it does not create new alleles; it simply deals the existing alleles into new hands.
Fertilization between opposite mating gametes then leads to further recombinations, resulting in phenotypic variations and causing a change in the frequencies of alleles.
iii. Gene Flow
Gene flow is the movement of genes into or out of a population. This gene movement may take place in the form of migration of organisms, or of gametes (for example, the dispersal of pollen), or even of segments of DNA (transformation).
Gene flow also alters gene frequency, and so causes evolutionary changes. A single migrating individual that breeds successfully adds its alleles to a gene pool that never had them before.
iv. Genetic Drift
Any random fluctuation (alteration) in allele frequency, occurring in a natural population by pure chance, is called genetic drift.
Notice the words “by pure chance”. Genetic drift has nothing to do with fitness. An allele may disappear from a population not because it was harmful, but simply because the individuals carrying it happened not to reproduce.
For example, when the size of a population is severely reduced due to natural disasters such as earthquakes, floods or fires, particular alleles may be eliminated from the population altogether — regardless of whether they were useful or harmful.
Smaller populations have a greater chance of genetic drift. This follows from simple probability: in a population of ten individuals, the accidental death of two can wipe out an allele entirely; in a population of ten thousand, the same two deaths change almost nothing. Drift results in a change in gene frequency, and it is therefore an important factor in evolutionary change.
v. Chromosomal Aberrations
The changes in chromosome number (gains and losses) and the changes in the morphological structure of chromosomes (deletion, inversion, and so on) together constitute the chromosomal aberrations. Since the order or sequence of genes on the chromosome is altered, the result is variation.
Chromosomal aberrations occur due to the following four changes, which are shown in Fig. 5.5.
- Deletion: Loss of genes from a chromosome.
- Duplication: Genes are repeated, or doubled in number, on the chromosome.
- Inversion: A particular segment of the chromosome is broken and gets reattached to the same chromosome in an inverted position, due to a \(180^\circ\) twist. There is no loss or gain of the gene complement of the chromosome — the same genes are present, but their order is reversed.
- Translocation: The transfer (transposition) of a part of a chromosome, or of a set of genes, to a non-homologous chromosome is called translocation. It is effected naturally by the transposons present in the cell.
Diagram
Fig. 5.5 – Chromosomal aberrations: for each type, a normal chromosome marked A B C D E F G H alongside the altered chromosome — Deletion (A B C E F G H, loss of a segment), Duplication (A B C B C D E F G H, repeat of a segment), Inversion (A D C B E F G H, a segment reversed), Translocation (M N O C D E F G H, a segment moved from one chromosome to another)
b. Natural Selection
According to Darwin, natural selection is the main driving force behind evolution. The modern theory keeps this idea, but restates it in the language of genes.
The theory holds that genetic variations arise within the population. The ‘fittest’ individuals will be at a selective advantage and will be more likely to produce more offspring than the rest. The ‘fit’ continue to enjoy greater survival and greater reproductivity. Over many generations, new species will eventually evolve.
Stated another way, natural selection is the process by which better-adapted organisms grow and produce a greater number of offspring in the population.
It brings about evolutionary changes by favouring the differential reproduction of genes, which brings about changes in gene frequency from one generation to the next generation. That last phrase is the whole modern definition in miniature: selection is not about who lives longest, but about whose alleles are more numerous in the next generation.
Selection against harmful mutations leads to a mutation balance, in which the allele frequency of harmful recessives remains constant generation after generation. Such alleles are never quite eliminated, because in a heterozygote the recessive allele is hidden from selection by the dominant one, and so it survives quietly and reappears in later generations.
Industrial Melanism
Natural selection encourages those genes or traits that ensure the highest degree of adaptive efficiency between a population and its environment. Industrial melanism is one of the best examples of natural selection, and one of the very few in which human beings have watched the process happen within a single century (Fig. 5.6).
Diagram
Fig. 5.6 – Natural selection: a lichen-covered tree trunk with a grey-white peppered moth (Biston betularia) well camouflaged and a black moth (Biston carbonaria) clearly visible, alongside a soot-blackened trunk where the situation is exactly reversed; label both moths
In Great Britain, before industrialization (1845), grey-white winged moths (Biston betularia) were more in number than the black-winged moth (Biston carbonaria).
These moths are nocturnal, and during the day they rest on tree trunks — motionless and exposed. Their survival therefore depends entirely on not being noticed. The white-winged moth could camouflage itself (hide in the background) well against the lichen-covered trees, and this helped it to escape from predatory birds. The black-winged moth resting on the same lichen-covered trunks stood out sharply, was an easy victim for the predatory birds, and so its numbers remained low.
Then the environment changed. During the industrial revolution, a large number of industries came up in Great Britain. The industries released black sooty smoke that covered and killed the lichens growing on the trees, and turned the tree trunks black due to pollution.
This change became an advantage to the black-winged moth, which now camouflaged well against the black tree trunks, and its numbers increased. The white-winged moth, now conspicuous against the dark bark, became the victim of predatory birds, and its numbers were reduced.
Thus natural selection resulted in the establishment of a particular phenotypic trait under changing environmental conditions.
Notice what did not happen. No moth turned black because of the soot. Both forms existed all along; pollution merely changed which of them the birds could see. The environment did not create the variation — it selected among variations that were already present.
c. Isolation
Variation and selection can change a population, but they cannot, by themselves, split it into two species. As long as individuals continue to interbreed, their genes keep mixing, and the population stays one gene pool. Something must first cut the gene flow. That something is isolation.
Isolation is the separation of the population of a particular species into smaller units, which prevents interbreeding between them. Any barrier that prevents gene flow — the exchange of genes between the isolated populations — is called an isolating mechanism.
A number of isolating mechanisms operate in nature. They may promote divergence between the separated groups, and speciation may occur. The isolating mechanisms are of two types: geographical isolation and reproductive isolation.
I. Geographical Isolation
Geographical isolation is also called physical isolation. It occurs when an original population is divided into two or more groups by geographical barriers, such as a river, an ocean, a mountain range or a glacier. These barriers prevent interbreeding between the isolated groups.
Once separated, the groups are exposed to different kinds of environmental factors, and they acquire new traits by mutations. Since no gene flow occurs between them, the mutations arising in one group never reach the other. The separated populations therefore develop distinct gene pools, and they do not interbreed. In this way, new species have been formed by geographical isolation. For example, Darwin’s finches.
II. Reproductive Isolation
Reproductive isolation occurs due to a change in the genetic material, in the gene pool, and in the structure of the genital organs. It prevents interbreeding between populations even when they live in the same area.
Types of Isolating Mechanisms
The isolating mechanisms are classified by when they act — before mating, or after it. The full classification is shown in FC 5.6.
Flowchart
FC 5.6 – Types of isolating mechanisms: Isolating mechanisms → branch A: Pre-mating (pre-zygotic) barriers — prevent fertilization and zygote formation → habitat (ecological) isolation, seasonal (temporal) isolation, ethological isolation, mechanical isolation; branch B: Post-mating (post-zygotic) barriers — act after mating → gamete mortality, zygote mortality, hybrid sterility
A. Pre-mating or Pre-zygotic Isolating Mechanisms
These mechanisms prevent fertilization and zygote formation. In other words, they act before any zygote is ever made, so no energy is wasted on offspring that cannot succeed.
Habitat isolation (Ecological isolation): Members of a population live in the same geographic region but occupy separate habitats, so that potential mates do not meet.
Seasonal or temporal isolation: Members of a population live in the same geographic region but become sexually mature in different years, or at different times of the year, so that their breeding periods never overlap.
Ethological isolation: Due to specific mating behaviour, the members of the population do not mate. Courtship songs, dances or displays act as passwords — an individual that gives the wrong signal is simply not accepted.
Mechanical isolation: Members of two populations have differences in the structure of their reproductive organs, so that mating is physically not possible.
B. Post-mating or Post-zygotic Barriers
These act after mating has taken place, and they ensure that even a successful mating leaves no fertile descendants.
Gamete mortality: Gametes have a limited life span. Due to one reason or another, if the union of the two gametes does not occur within the given time, the result is gamete mortality.
Zygote mortality: Here the egg is fertilized, but the zygote dies due to one or another reason, before it can develop.
Hybrid sterility: Hybrids develop to maturity but become sterile, due to the failure of proper gametogenesis (meiosis). For example, the mule is an intergeneric hybrid, and it is sterile.
The reason for hybrid sterility is worth understanding rather than memorising. Meiosis requires chromosomes to pair up with their homologous partners. In a hybrid, the two sets of chromosomes have come from different species, so they do not match properly, pairing fails, and functional gametes cannot be formed. The hybrid may be strong and healthy in body, yet it is an evolutionary dead end.
5.7 Mechanism of Organic Evolution
Having examined the theories one by one, we can now put the working machinery of evolution together in a single place.
One must give importance to the population while considering the mechanism of evolution. It is the population that evolves, and not its individual members. The individual’s role is only to pass on its genetic variation to its offspring; the individual itself cannot change its genes during its lifetime.
The following are the basic processes which bring about evolution: mutations, gene recombination, gene flow (migration), genetic drift, natural selection, isolation and speciation. Their combined action is summarised in FC 5.7.
Flowchart
FC 5.7 – Mechanism of organic evolution: Mutations + gene recombination → new genetic variation enters the gene pool → gene flow and genetic drift alter allele frequencies → natural selection favours the better-adapted variants → isolation stops gene flow between the diverging groups → speciation (a new species is formed)
Mutations
Mutations are permanent, heritable changes in the genetic material of an organism. They have already been described earlier in this chapter. Gene mutations will alter the genetic make-up of the individual and, through it, the gene pool of the population.
Mutation is the only process in this list that creates a genuinely new allele. Every other process merely shuffles, spreads, or filters the alleles that mutation has already supplied.
Gene Recombination
These are the variations produced due to the coming together of alleles during sexual reproduction. Gene recombinations occur due to three events: the random union of gametes at fertilization, the anaphasic separation of chromosomes during meiosis, and crossing over.
Notice that all three are shuffling events, and all three happen in every sexually reproducing organism, in every generation. This is why sexual reproduction, despite its costs, is so powerful: it guarantees a constant supply of fresh combinations for selection to test.
Gene Flow
Gene flow is the transfer of genes during interbreeding between populations that are genetically different. As explained earlier in this chapter, gene flow occurs due to emigration (individuals leaving) and immigration (individuals entering). It brings about changes in allele frequency.
Gene flow has an important secondary effect that is easy to miss: by mixing gene pools, it reduces the differences between populations. It therefore works against speciation — which is precisely why isolation is needed before two populations can become two species.
Genetic Drift
Any alteration in the allele frequency of a natural population by chance is called genetic drift. The concept of genetic drift was first given by Sewall Wright, and it is therefore also called the Sewall Wright effect.
For example, an allele may be eliminated from a population because of events such as the accidental death of an organism before mating. Genetic drifts are random, or directionless — they do not push a population towards better adaptation.
The effect of genetic drift is more significant in a small population than in a large population. Due to genetic drift, some alleles of a population are lost or reduced by chance, while some others may be increased.
Founder Effect
Sometimes, a few individuals become isolated from a large population and produce a small new population in a new geographical area. Because these few founders carry only a small, accidental sample of the parent population’s alleles, the allele frequency of the new population becomes suddenly different from that of the original one.
The original members that drifted away and established the new population become the ‘founders’, and the effect is called the founder effect.
Bottleneck Effect
The bottleneck effect is a type of genetic drift, seen when much of a population is killed due to a natural disaster — a tsunami, floods, a tornado, a disease epidemic, and so on — and only a few individuals are left to begin a new population.
The name is a picture of the process. Think of tipping a jar of mixed marbles through a narrow neck: only a few marbles get through, and which colours they are is largely a matter of chance. When the population recovers in numbers, it recovers from those few survivors, and its gene pool is permanently poorer in variation than before.
Natural Selection
Natural selection is the process by which better-adapted individuals, carrying useful variations, are selected by nature and leave a greater or larger number of progenies. This is called differential reproduction.
Selection, however, does not always work in the same direction. Depending on which part of the range of variation is favoured, three types of natural selection are recognised.
Types of Natural Selection
To follow this, picture the population’s variation for a trait — say, body weight — plotted as a curve, with the number of individuals on the y-axis and the character value along the curve. Most individuals cluster near the average, and fewer lie at the extremes. Selection can now do one of three things: cut off both extremes, cut off one extreme, or cut out the middle.
a. Stabilizing Selection (Balancing Selection)
- Here, more individuals of a population acquire the mean character value.
- It tends to favour the intermediate forms and eliminate both the phenotypic extremes. For example, a greater number of infants with intermediate birth weight survive better as compared with those who are over-weight or under-weight.
- It reduces variation in the population.
- It does not lead to evolutionary change, but tends to maintain phenotypic stability within the population; it is therefore described as stabilizing selection.
- Genetically, stabilizing selection represents a situation where a population is already well adapted to its environment.
This last point explains the whole type. If a population is already suited to a stable environment, then any departure from the average is a departure from what works — so selection punishes change and preserves the existing state, as shown in Fig. 5.7.
Diagram
Fig. 5.7 – Stabilising selection: a bell-shaped curve of number of individuals against time/character value, with two downward arrows marked “selection against both extremes”; the dotted original population curve is broad, and the solid curve of the population after selection is tall and narrow
b. Directional Selection
- In this type, more individuals acquire a value other than the mean character value.
- Natural selection acts to eliminate one of the extremes of the phenotypic range and to favour the other — for example, the systematic elimination of homozygous recessives.
- Directional selection operates for many generations, and it results in an evolutionary trend within a population, with the peak of the curve shifting in one direction (Fig. 5.8).
- Examples: industrial melanism, DDT-resistant mosquitoes, and similar cases.
Directional selection is the type that produces visible evolutionary change, and it is what you should expect whenever the environment itself changes in a consistent direction.
Diagram
Fig. 5.8 – Directional selection: a curve of number of individuals against time/character value, with a single downward arrow marked “selection against an extreme”; the dotted original population curve and the solid curve after selection, the peak having shifted to one side
c. Disruptive Natural Selection
- Here, a greater number of individuals acquire the peripheral character values at both ends of the distribution curve.
- Nature selects the extreme phenotypes and eliminates the intermediate ones. Hence two peaks are formed in the distribution of the trait (Fig. 5.9).
- This kind of selection is rare.
- It has an effect on the entire gene pool of a population, considering all mating types or systems.
- Example: it was observed in the different beak sizes of African seed-cracker finches. The birds have beaks of different sizes, and they feed on seeds. The available seeds were of two kinds — small and large. Large-beaked birds could feed on the large seeds, while small-beaked birds could feed on the small seeds, and the numbers of both increased. The intermediate-beaked birds were unable to feed efficiently on either type of seed, so their population decreased gradually and was then eliminated by natural selection.
Diagram
Fig. 5.9 – Disruptive selection: a curve of number of individuals against time/character value, with a downward arrow in the centre marked “selection against the intermediates”; the dotted original single-peaked curve becomes a solid two-peaked curve after selection
The seed-cracker example repays a second look. Being “average” is usually the safest thing to be — but not here. When the food supply itself comes in two separate sizes, an average beak is the one that fits nothing. Disruptive selection is therefore the one type that can split a single population into two distinct groups, and it is an important route to the formation of new species.
Isolation
Isolation is the separation of a single interbreeding population into subunits. Isolation restricts gene flow between the discrete (non-continuous) populations, because of different barriers such as geographical barriers. (This part has already been explained in detail earlier in this chapter.)
Speciation
The subunits of a single interbreeding population, separated by geographical barriers such as a river, mountains, a desert or a sea, become isolated in such a way that their interbreeding is prevented. This will finally lead to the origin of a new species — that is, to speciation. (Speciation is discussed in detail ahead in this chapter.)
Putting the seven processes together, the mechanism of evolution can be stated in one sentence: mutation and recombination create variation; gene flow and genetic drift move allele frequencies about; natural selection sorts the variation according to fitness; and isolation preserves the differences long enough for speciation to occur.
5.8 Hardy–Weinberg’s Principle
A Ruler Against Which Evolution Can Be Measured
We have said that evolution is a change in gene frequency. But how would we ever know that a frequency has changed unless we first knew what it should look like when nothing is happening?
This is exactly what the Hardy–Weinberg principle provides. It describes an imaginary, non-evolving population — a baseline. Any real population that departs from this baseline must be evolving, and the size of the departure tells us how much.
The principle is also known as Hardy–Weinberg’s equilibrium law. The law states that ‘at equilibrium point, both the gene (allele) frequency and the genotypic frequency remain constant from generation to generation.’
The Conditions
This equilibrium occurs only under a strict set of conditions. It occurs only in a population that is:
- diploid, that is, carrying two sets of chromosomes;
- sexually reproducing;
- large in size;
- freely interbreeding, with random mating; and
- free from selection or any other factor that could change the allele frequency.
Every one of these conditions is examination-critical, and each is there for a reason. The population must be large, because in a small population genetic drift alone would shift the frequencies by chance. Mating must be random, because if individuals choose partners of a particular genotype, some alleles will be over-represented in the next generation. And there must be no selection, because selection is, by definition, a change in frequency.
The Equations
Consider a single locus with two alleles, A and a. Let the frequencies of these two alleles be \(p\) and \(q\) respectively.
Since these are the only two alleles at that locus, every allele in the gene pool must be either A or a. The allele frequency for any locus is therefore always one:
\[p + q = 1\]
Now, in a diploid organism, each individual carries two alleles at this locus, one from each parent. The genotypic frequencies of both alleles taken together are therefore represented by the square of the allele frequencies:
\[(p + q)^2 = 1\]
The binomial expansion of this is:
\[p^2 + 2pq + q^2 = 1\]
Here each term corresponds to one genotype, and this is the step to understand rather than memorise:
- \(AA = p^2\) — the chance of drawing an A allele from the father and an A allele from the mother, that is, \(p \times p\);
- \(aa = q^2\) — likewise, \(q \times q\);
- \(Aa = 2pq\) — the heterozygote can be formed in two ways (A from father and a from mother, or a from father and A from mother), and so the term carries the factor 2.
Hence \(p^2 + 2pq + q^2 = 1\). This is simply the binomial expansion of \((p + q)^2\).
In these expressions, \(p\) = frequency of the dominant allele A, \(q\) = frequency of the recessive allele a, \(p^2\) = frequency of the homozygous dominant genotype (AA), \(2pq\) = frequency of the heterozygous genotype (Aa), and \(q^2\) = frequency of the homozygous recessive genotype (aa). All of these are proportions, and so they have no units.
Explanation by the Punnett Square
This can be explained by a Punnett square, as follows. The gametes of one parent, carrying A with frequency \(p\) and a with frequency \(q\), are set against the gametes of the other parent, and every possible union is filled in.
| Hybrid A a | ||
|---|---|---|
| A (p) | a (q) | |
| Hybrid Aa — A (p) | AA (\(p^2\)) | Aa (\(pq\)) |
| Hybrid Aa — a (q) | Aa (\(pq\)) | aa (\(q^2\)) |
Look at the four boxes. AA appears once, giving \(p^2\); aa appears once, giving \(q^2\); and Aa appears in two boxes, giving \(pq + pq = 2pq\). This is where the factor of 2 in the middle term comes from — it is not an arbitrary number, it is simply the count of the boxes.
Like the allele frequencies, the genotypic frequencies together are also equal to 1:
\[AA + 2Aa + aa = 1\]
What Disturbs the Equilibrium
There are a few factors which affect or change the Hardy–Weinberg equilibrium: gene migration (gene flow), genetic drift, mutation, genetic recombination, natural selection and non-random mating.
You will recognise every item on this list. These are precisely the processes described in Section 5.7 as the mechanisms of evolution. That is not a coincidence — it is the whole point. The forces that break Hardy–Weinberg equilibrium are exactly the forces that cause evolution.
If these factors do not occur in the population, then the population is genetically stable, or a non-evolving population. The relationship is set out in FC 5.8.
Flowchart
FC 5.8 – Hardy–Weinberg equilibrium: Population satisfies all conditions (large, diploid, sexually reproducing, random mating, no selection) → allele and genotypic frequencies remain constant → non-evolving population; if any factor operates — mutation, gene flow (migration), genetic drift, recombination, natural selection, or non-random mating → allele frequencies change → equilibrium is disturbed → the population evolves
5.9 Adaptive Radiation
The process of evolution which results in the transformation of an original species into many different varieties is called adaptive radiation.
The word “radiation” is a picture of what happens. From a single ancestral stock, lines of descent spread outwards in different directions — like rays from a centre — each one becoming fitted to a different way of life. The pattern arises because the descendants of one species find themselves in different habitats, facing different food supplies and different enemies, and natural selection therefore pulls each group along a different path.
Darwin’s Finches
Darwin’s finches are one of the best examples of adaptive radiation. During his visit to the Galapagos Islands, Charles Darwin noticed a variety of small birds. These birds are called Darwin’s finches (Fig. 5.10).
Diagram
Fig. 5.10 – Darwin’s finches: a fan of finch species radiating outwards from a common ancestral finch at the centre, each bird drawn with a distinctly different beak — heavy seed-crushing beaks, slender probing beaks, and sharp insect-catching beaks — all against the Galapagos island background
Darwin concluded that the American mainland species of bird was the original one, from which the finches had migrated to the different islands of the Galapagos. On reaching these islands, they adapted to the different environmental conditions found on each.
From the original seed-eating features, many other forms evolved, with altered beaks, developing insectivorous features. The chain of events is set out in FC 5.9.
Flowchart
FC 5.9 – Adaptive radiation of Darwin’s finches: original seed-eating finch on the American mainland → migration to the different Galapagos islands → geographical isolation of the island populations → different food and conditions on each island → natural selection acts on beak variations → beaks altered for different diets (large seed-crushers, small seed-eaters, insect-eaters) → 14 distinct species from one ancestor
Australian Marsupials
Another example of adaptive radiation is that of the Australian marsupials. In Australia there are many marsupial mammals, all of which evolved from a common ancestor.
Here the same principle worked on a whole continent. Isolated from the rest of the world, a single marsupial stock radiated into forms that burrow, forms that graze, forms that climb, forms that leap across open grassland, and forms that hunt — each occupying the kind of role filled elsewhere in the world by placental mammals.
Keep this idea of divergence in mind. In the next section you will meet homologous organs — structures built to the same underlying plan but put to entirely different uses — and you will see that they are simply the anatomical record of the very process described here.
5.10 Evidences of Organic Evolution
The theory of organic evolution states that the present-day complex organisms have originated from earlier, simpler forms of life. That is a large claim about events no human being ever saw. It must therefore be supported by evidence.
The process of evolution is supported by evidences provided by various branches of biology: palaeontology, comparative anatomy (morphology), embryology and molecular biology. Each of these lines of evidence is independent of the others — and yet all of them point to the same conclusion, which is what gives the theory its strength.
A. Palaeontology
The study of ancient life with the help of fossils is called palaeontology. Fossils are the dead remains of plants and animals that lived in the past, preserved in various geological layers.
- The study of fossils provides the most convincing and direct evidence of evolution. It is direct because a fossil is not an argument — it is the organism itself, or its imprint, lying in the rock.
- Fossils are formed in sedimentary rocks, in amber (yellowish fossil resin), in ice, in peat bogs, and so on.
- During fossilization, the primitive forms of organisms occupy the older, lower layers, and the advanced forms occupy the upper, more recent layers of the earth.
That third point is the most important one, and it deserves a moment’s thought. Sediment settles layer upon layer, so the deeper a layer lies, the older it is. When we dig down and find simple organisms below and complex organisms above — never the reverse — the rock itself is recording the order in which life appeared. This is the closest thing biology has to a photograph of the past.
Types of Fossils
Actual remains: These are the most common type of fossils. Plants, animals and human bodies that got embedded in the permafrost of arctic or alpine snow remain preserved in the actual state — for example, the Wooly Mammoth of Siberia, found frozen with its flesh, hair and even its last meal intact. Amber, or hardened resin, contains the preserved bodies of many insects and arthropods.
Moulds: These are the hardened encasements formed in the outer parts of organic remains, which later decayed leaving cavities behind. The body parts of the plant or animal decay, but the impression remains and becomes permanent. Footprints, for example, are formed in this manner.
Casts: These are hardened pieces of mineral matter deposited in the cavities of moulds. If the mould is the hollow, the cast is the filling — a solid replica of the original body.
Compressions: Here the internal structure is absent, but a thin carbon film indicates the outline of the external features. The organism has been flattened and reduced to a carbon impression, yet its shape survives.
Significance of Palaeontology
- It is useful in the reconstruction of phylogeny — the evolutionary family tree of a group.
- It helps in studying the various forms and structures of extinct animals.
- It provides a record of the missing link between two groups of organisms.
- It helps in the study of the habits of extinct organisms.
- It provides the types of evidence described below.
Connecting Link (Missing Link)
These are fossil forms that are transitional, or intermediate, between two groups of organisms. They show some characters akin to both groups, and thus they indicate the evolutionary line joining them.
Two important examples are Seymouria (between amphibians and reptiles) and Archaeopteryx (between reptiles and birds).
This is a powerful kind of evidence. If reptiles and birds had been separately created, there is no reason why an animal should exist that is half one and half the other. But if birds evolved from reptiles, then such an animal is exactly what we should expect to find.
Archaeopteryx lithographica
Archaeopteryx lithographica is a fossilized, crow-sized, toothed bird found in Jurassic rocks in Germany. It is known as the missing link between reptiles and birds, because it shows the characters of both (Fig. 5.11).
Diagram
Fig. 5.11 – Archaeopteryx: a crow-sized feathered animal with a beak bearing teeth, scales on the body, long feathered tail, wings, and clawed digits on the forelimb; label beak, teeth, scales, feathers, wings and clawed digits
Reptilian characters: 1. Presence of a long tail, claws and scales on the body. 2. Single-headed ribs. 3. Abdominal ribs are present, which look like the ribs of a crocodile. 4. Jaws with homodont teeth — that is, teeth that are all of the same kind. 5. Sternum without a keel. 6. Bones are solid (non-pneumatic), unlike the hollow, air-filled bones of a modern bird. 7. Hind limbs had four clawed digits.
Avian characters: 1. Feathery exoskeleton. 2. Forelimbs are modified into wings. 3. Jaws are modified into a beak. 4. Skull bone is completely fused. 5. Large rounded cranium. 6. Cranium with large orbits and a single condyle. 7. Limb bones are bird-like. 8. Hind limbs with four toes, of which the first toe is opposable.
Look carefully at what the reptilian list contains: a keel-less sternum and solid bones. A modern bird needs a keeled sternum to anchor the powerful flight muscles, and hollow bones to stay light. Archaeopteryx had neither. It possessed feathers and wings, and yet its skeleton was still that of a reptile — it was a bird still under construction.
Thus, from the above study, it is very clear that birds evolved from reptiles. Huxley justified this by calling birds ‘glorified reptiles’.
| Animal | Connecting link between |
|---|---|
| 1. Balanoglossus | …………………………….. |
| 2. ………………. | Annelida and Arthropoda |
| 3. Lung fishes | …………………………….. |
| 4. ………………. | Reptiles and mammals |
| 5. Seymouria | …………………………….. |
| 6. Ichthyostega | Fishes and Amphibia |
B. Morphology and Anatomy
Morphology deals with the study of external structures, while anatomy deals with the study of internal structures. From a comparative study of morphology and anatomy, we can understand the evolutionary relationships between organisms, in the form of homologous, analogous and vestigial organs.
The reasoning here is simple but profound. If two organisms inherited a structure from a common ancestor, that structure should have the same underlying plan, even if it has since been put to very different uses. And that is exactly what we find.
a. Homologous Organs
Homologous organs are those organs which are structurally similar but perform different functions.
Examples:
The forelimbs of vertebrates such as the lizard, bird, bat, horse, whale and man. All of them have the same set of bones in their forelimbs — humerus, radius–ulna, carpals, metacarpals and phalanges — arranged in the same order (Fig. 5.12). The forelimbs of these vertebrates are structurally similar, but they perform entirely different functions: the bat’s forelimb flies, the whale’s swims, the horse’s runs, and the human’s writes and grasps.
The vertebrate heart and brain.
In plants, the thorns of Bougainvillea and the tendrils of Cucurbita represent homology — both are modified stems, arising in the same position, one turned into a weapon and the other into a climbing organ. The thorns of Bael and the tendrils of passion flower are likewise homologous.
Diagram
Fig. 5.12 – Homologous organs: (A) forelimbs of lizard, horse, whale, bat, bird and man drawn side by side with corresponding bones shaded alike and labelled humerus, radius, ulna, carpals, metacarpals, phalanges; (B) thorn of Bougainvillea and tendril of Cucurbita, both shown arising in the axil of a leaf on the stem
The structural similarities between homologous organs indicate that these organisms have a common ancestry. And the differences between homologous organs are examples of divergent evolution, or adaptive radiation — the very process described in the previous section, now written into the bones.
b. Analogous Organs
Analogous organs are those which are structurally dissimilar but functionally similar. These organs have an external, superficial similarity because they do a similar job, but they are different anatomically.
For example, the wings of a butterfly (an insect) and the wings of a bird look superficially alike, but they are not anatomically similar structures, even though they perform a similar function (Fig. 5.13). The bird’s wing is supported by bones — humerus, radius and ulna — and covered with feathers, while the insect’s wing has no bones at all; it is a fold of the body wall.
Diagram
Fig. 5.13 – Analogous organs: the fore wing and hind wing of an insect (no internal bones) shown beside the wing of a bird with humerus, radius, ulna and feathers labelled
Other examples of analogous organs:
- The eye of the octopus (Mollusca) and of mammals. They differ in the position of the retina, the structure of the lens, and the origin of the different eye parts.
- The flippers of penguins (birds) and of dolphins (mammals).
- The sweet potato (a root modification) and the potato (a stem modification) — both store food in the form of starch, but they are modifications of entirely different organs.
- The terminal leaflets of pea and the apical bud in Vitis, both modified into tendrils.
Analogous organs lead to convergent evolution — that is, different organisms come to show the same superficial structural similarities because they face similar functions or similar habitats. Selection, working on unrelated ancestors, arrives at similar solutions to the same problem, since the laws of physics leave only so many ways of flying or of swimming.
These organs do not help to trace common ancestry. Thus, analogous organs do not have a significant role as evidence in evolution.
c. Vestigial Organs (Rudimentary Organs)
Vestigial organs are imperfectly developed, non-functional, degenerate structures which were functional in some related animals, or in the ancestors of the organism. The vestigial organs are no longer required by the organism, but they indicate its relationship with those organisms in which the same organs are fully developed.
Vestigial organs are awkward for any theory except evolution. Why should a body contain a useless structure at all? The answer is that it is a leftover — an organ that was useful to an ancestor and has not yet been fully lost.
Examples: Human beings show several vestigial organs (Fig. 5.14):
- The presence of the vestigial nictitating membrane — the plica semilunaris, the small fold at the inner corner of the eye. In many other vertebrates this is a full third eyelid that sweeps across the eye.
- The presence of wisdom teeth (third molars), which our ancestors, chewing coarse raw plant food, needed and used.
- The coccyx (tail bone), which is greatly reduced in man, since a tail is of no use because of our erect posture.
- The vermiform appendix and the caecum. These are functional in herbivorous mammals, where they house microbes that digest cellulose. In man, because of the eating of cooked food, the structure has lost its function.
Diagram
Fig. 5.14 – Vestigial organs in man: (1) plica semilunaris at the corner of the eye; (2) the third molar or wisdom tooth in the jaw, with the canine marked; (3) the sacrum and the coccyx (fused tail vertebrae); (4) the caecum and vermiform appendix at the junction of the small intestine and the colon
The presence of these vestigial organs provides evidence that man has evolved, or descended, from simpler primates.
Embryological Evidences
Comparative embryology adds a further line of evidence. The early embryos of very different vertebrates — a fish, a bird, and a mammal — resemble one another far more closely than the adults do. At an early stage, all of them possess features such as a tail and gill clefts, structures that the fish keeps and uses, while the bird and the mammal go on to lose or remodel them.
The point is not that a human embryo passes through a “fish stage”. It is that development starts from an inherited plan shared with distant relatives, and is then modified. Similarity of embryos therefore points to the same conclusion as similarity of bones: common ancestry.
D. Molecular Evidences
The deepest evidence of all lies not in bones but in molecules — because the same molecular machinery is found in every living thing, from a bacterium to a blue whale.
- The cell is the basic structural and functional unit of life in all organisms.
- Similarities in the proteins and genetic material performing a similar function among diverse organisms give evidence of a common ancestry.
- Basic metabolic activities also occur in a similar manner in all organisms.
- ATP is the energy currency, and the energy source, in all living organisms.
Consider what point 2 really means. The genetic code — which triplet of bases stands for which amino acid — is virtually the same in a bacterium, a mango tree and a human being. There is no chemical necessity for this. It is common only because all of them inherited it from the same ancestral population of cells.
5.11 Speciation
The process of formation of a new species from a pre-existing species is called speciation. It is the end point towards which everything in this chapter has been building: variation, selection and isolation acting together, until one kind of organism has become two.
Before we can speak of a new species, we must be clear about what a species is. A species is a group of similar organisms that can interbreed and produce fertile offspring in nature.
Two words in that definition do the real work. Interbreed tells us that the members share a single gene pool. Fertile tells us that the gene pool must continue into the next generation — which is why the mule, healthy but sterile, is not a new species. Speciation, then, is nothing other than the permanent splitting of one gene pool into two.
New species are formed by the following modes, shown in FC 5.10.
Flowchart
FC 5.10 – Modes of speciation: Speciation → branch a: Intraspecific speciation (within one species) → (i) Allopatric speciation — geographical barrier separates the population → separate gene pools → new species; (ii) Sympatric speciation — no geographical barrier, reproductive isolation within the same area → new species; branch b: Interspecific speciation → Hybridisation — two different species cross → hybrid → doubling of chromosomes → new fertile species
a. Intraspecific Speciation
Here the new species arises from within a single existing species.
i. Allopatric Speciation
Allopatric speciation is the formation of a new species due to the separation of a segment of a population from its original population — either by sheer distance, or by a geographical barrier cutting across the species range. Examples of such barriers are creeping glaciers and the development of mountains. Migration of individuals to a distant area also causes allopatric speciation.
The mode of evolution here is called adaptive radiation, which you met in Section 5.9. The separated segment, cut off from gene flow, accumulates its own mutations and is shaped by its own environment, until it can no longer interbreed with the parent population.
Examples: the 14 different species of finches in the Galapagos Islands, and the several marsupial species of the Australian continent.
ii. Sympatric Speciation
Sympatric speciation is the formation of new species within a single population, without any geographical isolation. The two groups live side by side, in the same place — and yet they split.
Such species are formed due to reproductive isolation. Some biological barrier — a difference in breeding season, in mating behaviour, or in the genetic material itself — arises within the population and stops the two groups from interbreeding, even though nothing physical separates them. Mutations are helpful in sympatric speciation, because a single chromosomal change can make a group instantly unable to breed with its neighbours.
Example: the Cichlid fishes in Lake Victoria, where a great number of species have arisen within one lake.
b. Interspecific Speciation
Hybridisation
Here the new species arises between two existing species.
Two different species, on crossing, may give rise to a hybrid, which on doubling of chromosomes forms a new species.
The doubling step is essential, and the reason takes us back to hybrid sterility. A hybrid between two species usually cannot form gametes, because its chromosomes have no proper homologous partners to pair with during meiosis. But if the entire chromosome set is doubled, every chromosome suddenly has an identical partner. Meiosis can now proceed normally, the hybrid becomes fertile — and, being unable to breed successfully with either parent species, it is a new species at once.
Examples: Triticum aestivum (hexaploid) and Gossypium herbaceum (tetraploid).
5.12 Geological Time Scale
Reading the Calendar of the Earth
The planet earth, with its present biodiversity, was not the same when it was born. The study of fossils tells us that the life forms of millions of years ago (MYA) were not like today’s at all.
The geological time scale is used to understand the sequence of events that took place on the earth in different ages, over a long period of time. It is, in effect, the calendar of life — a way of putting every fossil, every extinction and every new group in its proper place in time.
The scale is divided into six major ‘Eras’. Eras ended with major environmental changes on earth, which resulted in the extinction of some species and the emergence of new ones. That is precisely why the boundaries fall where they do: an era ends when the living world is largely replaced. The eras are further divided into periods and epochs, based on minor but landmark events within each era. Table 5.15 shows the geological time scale at a glance, and Fig. 5.16 presents the same history as a pie diagram.
Flowchart
FC 5.11 – Divisions of the geological time scale: Geological time scale → Era (ends with a major environmental change and mass extinction) → Period (a smaller division within an era) → Epoch (the smallest division, based on landmark events)
How the Age of a Fossil Is Found
One question must be answered before the table can mean anything: how do we know that a rock is 65 million years old rather than 65 thousand?
The layers themselves give the order of events — deeper means older — but not the actual age in years. The age is obtained by radiometric dating. Certain elements in the rock are radioactive, and they decay into other elements at a fixed, unchanging rate. By measuring how much of the parent element is left, and how much of the daughter element has accumulated, the time elapsed since the rock formed can be calculated.
For relatively recent remains — up to a few tens of thousands of years — carbon dating is used. While an organism is alive, it keeps taking in carbon, including a small, steady proportion of the radioactive isotope carbon-14. Once it dies, intake stops, and the carbon-14 already in the body begins to decay at a known rate. Measuring how much is left therefore tells us how long ago the organism died. For older fossils, carbon-14 has decayed almost entirely, and slower-decaying elements in the surrounding rock must be used instead.
The Story the Table Tells
The first life appeared on the earth some 2000 million years ago. It took billions of years for this to happen, and the transition from protenoids to the first cells is still a mystery.
Once formed, the living forms diversified into various groups. Life began in the sea water, and plants were the first living beings to adapt to terrestrial life. This order makes sense: land is a hostile place — dry, exposed to radiation, offering no support against gravity — and any organism moving on to it must first solve those problems. Plants, which need only light and water, solved them first, and in doing so they created the food supply that later allowed animals to follow.
Fishes evolved and diversified. The lobefin group of fishes also diversified. Some of them developed stout and strong fins, with which they could go on to land and come back to water.
The coelacanth was long considered a living fossil. Lobefins were thought to be extinct, but a variety of lobefin fish, the coelacanth, was caught in 1938 in South Africa — the discovery with which this chapter opened.
Reptiles evolved from amphibians. They were the first true land vertebrates, because they do not have to go back to water for reproduction. (Compare an amphibian’s egg, a soft mass of jelly that would dry out instantly on land, with a reptile’s shelled egg, which carries its own water supply inside it. That single innovation set the vertebrates free from the water’s edge.)
But about 200 million years ago (mya), some reptiles moved back to an aquatic mode of life and gained a fish-like form, as seen in Ichthyosaurs. The giant reptiles, the Dinosaurs, once dominated the earth, but are now extinct — around 65 million years ago. At around the same time, giant ferns that were present on earth also became extinct, and were converted into fossil fuels.
The decline of the giant reptiles marked the beginning of the dominance of mammals. These viviparous organisms were more intelligent, and could avoid danger. Early mammals were small, shrew-like organisms, but this group diversified remarkably: whales, dolphins, seals and sea cows live in water; bats are the flying mammals; kangaroo rats are fossorial (burrowing); and lemurs are arboreal (tree-dwelling).
Notice that this is adaptive radiation once again, this time on the grandest scale — one small shrew-like ancestor giving rise to swimmers, fliers, burrowers and climbers, because the extinction of the dinosaurs had emptied every way of life at once.
Continental Drift and Its Consequences
Major physical disturbances of the earth led to the phenomenon of continental drift — that is, the continents moved from their original positions.
The biological consequences were dramatic. When South America joined North America, the ancestral forms of horse, hippos, rabbits and other animals native to South America were dominated by the North American animals that now poured across the new land bridge. Two faunas that had evolved separately were suddenly forced to compete, and one lost.
At the same time, marsupials in Australia diversified into different habitats. These survived precisely because of the lack of competition — Australia remained isolated, and no placental mammals arrived to displace them. The same isolation that permits adaptive radiation is what allowed the marsupials to keep the continent to themselves.
From the fossil records, we can trace the complete evolutionary history of the horse, the elephant, the dog, and other animals. Human beings are the most evolved animals on the earth.
Diagram
Fig. 5.16 – Geological time scale pie diagram: a circular chart of the earth’s 4.6-billion-year history with the eons Hadean, Archean, Proterozoic and Phanerozoic (Palaeozoic, Mesozoic, Cenozoic) marked as coloured arcs, and key events labelled — 4550 Ma formation of the Earth, 4527 Ma formation of the Moon, c. 4000 Ma end of the Late Heavy Bombardment and first life, c. 3200 Ma earliest photosynthesis, c. 2300 Ma atmosphere becomes oxygen-rich and first Snowball Earth, 750–635 Ma two Snowball Earths, c. 530 Ma Cambrian explosion, c. 380 Ma first vertebrate land animals, 230–66 Ma non-avian dinosaurs, 2 Ma first hominins
Table 5.15: Geological Time Scale
| Era | Period | Time (MYA) | Epoch | Plant life | Animal life |
|---|---|---|---|---|---|
| Cenozoic | Quaternary | 0.01–0.6 | Recent (Holocene) | Angiosperms, dicots, monocots; spread of agriculture | Age of mammals: development of modern man, birds, fishes and insects; development of human culture |
| Cenozoic | Quaternary | 0.6–2.0 | Pleistocene | Increase in herbs | Extinction of the great mammals; appearance of primitive man |
| Cenozoic | Tertiary | 2–13 | Pliocene | Hard woody plants, conifers, bryophytes, monocots; grasslands dominated | Emergence or origin of man; evolution of ruminants — horse, camel, elephant |
| Cenozoic | Tertiary | 13–26 | Miocene | Abundance of deciduous trees; origin of grasses | Formation of the first man-like apes; adaptive radiation or spread of mammals |
| Cenozoic | Tertiary | 26–38 | Oligocene | Rise of monocots and flowering plants | Extinction of archaic mammals; appearance of apes and monkeys; turtles and crocodiles attained development |
| Cenozoic | Tertiary | 38–54 | Eocene | Development of angiosperms | Diversification of placental mammals and modern birds |
| Cenozoic | Tertiary | 54–65 | Palaeocene | Advancement of flowering plants | Arrival of the early or first primates; rise of placental mammals |
| Mesozoic | Cretaceous | 65–135 | — | Decline of ferns, sphenopsids (horsetails) and Bennettitales; Ginkgos, Gnetales; first appearance of flowering plants | Extinction of dinosaurs and toothed birds; appearance of placental mammals and the first modern birds |
| Mesozoic | Jurassic | 135–165 | — | Origin of angiosperms; dominance of herbaceous lycopods, ferns, conifers, cycads | Age of reptiles; dinosaurs dominant; appearance of toothed birds (Archaeopteryx); rise of marsupials |
| Mesozoic | Triassic | 165–225 | — | Dominance of Bennettitales, gymnosperms; extinction of seed ferns | Appearance and rise of dinosaurs; extinction of primitive amphibians; diversification of reptiles; rise of oviparous mammals; therapsids; diversification of flies |
| Palaeozoic | Permian | 225–280 | — | Origin of conifers; decline of arborescent lycopods; abundance of ferns, cycads and advanced conifers | Rise of modern insects; disappearance of trilobites; appearance of mammal-like reptiles (pelycosaurs); decline of amphibians |
| Palaeozoic | Carboniferous | 280–345 | — | Development of diverse arborescent lycopods, mosses, seed ferns and primitive conifers; age of ferns and coal forests; different fungal groups | Abundance of amphibians (age of amphibians); appearance of reptiles and winged insects |
| Palaeozoic | Devonian | 345–400 | — | Appearance of the first progymnosperms; formation of forests; wood-decaying fungi, chytrids; origin of bryophytes | Diversification of fishes; evolution of amphibians; appearance of ammonites |
| Palaeozoic | Silurian | 400–440 | — | Appearance of lycopods and ferns; dominance of algae and ascomycete fungi | Appearance of the first terrestrial animals, wingless insects and jawed fish |
| Palaeozoic | Ordovician | 440–500 | — | Appearance of the first seedless vascular land plants; abundant algae | Abundance of diverse invertebrates; appearance of the first vertebrates — jawless fishes; appearance of corals and giant cephalopods like Nautilus |
| Palaeozoic | Cambrian | 500–590 | — | Rhynia-like plants; all types of marine algae | Abundance, or age, of trilobites; diversification of invertebrate phyla |
| Proterozoic | — | 600–1600 | — | Tracheophyte ancestors, chlorophyte ancestors, bacterial single-celled protista, blue-green algae | Primitive flatworms, annelids, sponges, coelenterates, primitive metazoans; scanty fossils of prokaryotes |
| Archaeozoic | — | 1600–3800 | — | No fossil records | Origin of life; simple unicellular forms like viruses, bacteria and algae |
| Azoic | — | 3800–4600 | — | No life | Absence of living beings; chemical evolution; formation of the earth |
(MYA = Million Years Ago)
5.13 Human Evolution
Everything in this chapter — variation, selection, isolation, speciation, the slow filling of the geological time scale — applies to our own species as much as to any other. This last section traces that story.
From a Shrew-like Ancestor
It has been traced that human evolution appeared to have begun from a tree-dwelling, shrew-like animal. This process began in the Palaeocene epoch.
During this period, the forests were dwindling. Arboreal mammals that had spent their lives in the branches were forced to adapt to life on the ground. This descent from the trees must have been the driving force of everything that followed — because an animal that lives on the ground, and no longer needs its forelimbs to grip branches, has its hands free for something else.
Where We Sit Among the Mammals
Our position in the classification of mammals is shown in Chart 5.17, and it can be traced step by step in FC 5.12. Read it carefully, and you will see that we are most closely related to the gibbons, chimpanzees and gorillas.
Flowchart
FC 5.12 – Classification of mammals (Chart 5.17): Class Mammalia → branches into Marsupials (pouched mammals), Monotremes (egg-laying mammals) and Eutherians (placental mammals) → Eutherians → Order Primate → branches into Sub-order Prosimii (Lemurs, Tarsier) and Sub-order Anthropoidea → Anthropoidea branches into New World monkeys (long non-prehensile tail; e.g. squirrel monkey, spider monkey), Old World monkeys (short non-prehensile tail; e.g. baboons, macaques, langur) and Hominoidea (tailless great apes) → Hominoidea branches into Hyalobatidae (arboreal apes, gibbons), Pongidae (semi-arboreal apes — gorilla, chimpanzee, orangutan) and Hominidae (erect posture — human beings)
Notice the two dividing lines within this chart. Monkeys have tails; the Hominoidea do not. Among the Hominoidea, the Pongidae are still apes of the trees and the forest floor, while the Hominidae are set apart by one thing above all — erect posture.
Major Evolutionary Trends from Ape to Man
Special characteristics have been acquired by man in the course of evolution. The major changes that took place in the evolution of man include:
- increase in the size and complexity of the brain, and enhanced intelligence;
- increase in cranial capacity;
- bipedal locomotion;
- opposable thumb;
- erect posture;
- shortening of the forelimbs and lengthening of the hind limbs;
- development of a chin;
- broadening of the pelvic girdle;
- development of the lumbar curvature; and
- social and cultural development — articulated speech, art, the making of tools, and so on.
These are not ten separate accidents. They form a chain, in which each change makes the next one possible or necessary.
The cranial capacity of humans increased steadily over a period of time, and the large size of the frontal lobe helped in the development of a high forehead.
The increase in intelligence necessitated physical development, so that the body and the brain could be used effectively and productively. The freedom of the forelimbs from the locomotory function, together with the opposable thumb, led to a far better utilisation of the hands — for holding objects effectively, and for the development of motor skills.
Bipedal locomotion and upright posture, coupled with stereoscopic vision, helped man to move around safely on land. Stereoscopic vision — two forward-facing eyes whose fields overlap — gives depth perception, which an animal leaping between branches needs absolutely, and which a tool-maker needs just as much.
Fossil Ancestors of Man
The evolutionary history of man was traced with the help of fossil remains found over a period of time. Some of our ancestors, and their evolutionary history, are shown in Table 5.18. The overall sequence is illustrated in Fig. 5.19.
Diagram
Fig. 5.19 – Human Evolution: a series of six figures walking from left to right, progressing from a stooped, ape-like, knuckle-walking form through increasingly upright intermediate forms to a fully erect modern man; show the gradual straightening of the back, shortening of the arms and enlargement of the skull
Table 5.18: Human Evolution
| Heads | Dryopithecus | Ramapithecus | Australopithecus | Homo habilis | Homo erectus | Neanderthal man | Homo sapiens |
|---|---|---|---|---|---|---|---|
| Appearance | Ape-like | Man-like | Connecting link between ape and man | Handy-man like | Ape man | Advanced prehistoric man | Modern man |
| Site of fossil record | Lake Victoria of Africa; Haritalyangar of Himachal Pradesh | Shivalik Hills in India, and also in Kenya | Taung in South Africa; Ethiopia; Tanzania | Olduvai Gorge, Tanzania, in Africa | Java and Peking | Neanderthal valley, Germany | Africa |
| Period | Miocene epoch, 20–25 mya | Miocene and early Pliocene epoch, 14–12 mya | Late Pliocene or early Pleistocene epoch, about 4–1.8 mya | Late Pliocene or early Pleistocene, 2.5 to 1.4 mya | Middle of the Pleistocene epoch, 1.5 mya | Late Pleistocene epoch, 100000 to 40000 years ago | — |
| Skeletal features | — | Lower jaw lightly built; dentition more like modern man; smaller molars | Not taller than 4 feet; jaws larger; prognathous face; chin absent; lumbar curvature present | — | 5 feet in height; prognathous face; massive jaws; huge teeth; chin absent; bony eyebrow ridges present | Heavily built and short; prominent brow ridges; low forehead; deep jaws; chin absent; outwardly curved thigh bones | — |
| Posture | Semi-erect | Erect | Upright | Erect | Erect | Erect | Erect |
| Cranial capacity | — | — | 450 to 600 cc | 650 to 800 cc | 900 cc | 1400 cc | 1450 cc |
| Special features, if any | Close similarity to the chimpanzee | Man with an ape brain | Probably did not eat meat | Made tools from stones; nicknamed ‘handy man’ | Probably ate meat; omnivorous; might have used fire | Used hide; buried their dead; constructed flint tools | Developed distinct races; developed cave art about 18000 years ago |
The table clearly shows the gradual increase in cranial capacity, and the changes in the shape of the skull and in the dentition of our ancestral humans, right up to the present day.
5.14 Summary
Origin of life
- Living matter shows responsiveness, growth, metabolism, energy transformations and reproduction. How such matter first appeared is still an unsolved question.
- Four early theories were proposed. The theory of special creation (life created by a super-natural power) has no scientific proof. The cosmozoic theory (panspermia) holds that life reached the earth from other planets as spores; it cannot explain how life began on those planets, and is not accepted.
- The theory of spontaneous generation (abiogenesis) claimed that life arises from non-living matter on its own. It was disproved by Louis Pasteur.
- The theory of biogenesis states that living organisms arise only from pre-existing living forms, by reproduction. It explains the continuity of life, but not the origin of the first life.
Chemical evolution
- According to the chemical evolution (self assembly) theory, life arose on earth by the combination of chemicals over a long time. It was formulated by Haeckel and developed by Oparin (1924) and Haldane (1929).
- The steps were: origin of the earth and its primitive reducing atmosphere, devoid of free oxygen → formation of \(CH_4\), \(NH_3\), \(H_2O\) and \(H_2S\) → formation of simple organic molecules (monosaccharides, amino acids, purines, pyrimidines, fatty acids, glycerol) which accumulated as Haldane’s “hot dilute soup” → polymerisation into complex molecules (polysaccharides, fats, proteins, nucleotides) → formation of nucleic acids with self-replicating ability → formation of protobionts (Oparin’s coacervates, Sidney Fox’s microspheres), which became eobionts or protocells → formation of the first cell, which was anaerobic and heterotrophic.
- Urey and Miller provided the first experimental support. In a sterilized, evacuated spark-discharge apparatus they exposed \(CH_4\), \(NH_3\) and \(H_2\) in the proportion 1:2:2, along with water vapour, to an electric discharge for several days, and obtained urea, amino acids and lactic acid.
- The RNA World hypothesis (Carl Woese, Francis Crick and Leslie Orgel, 1960) suggests the first life was based on RNA. It was strongly supported when Sidney Altman and Thomas Cech showed in 1980 that RNA can act as a biocatalyst (ribozyme); they received the Nobel Prize in Chemistry in 1989.
Theories of evolution
- Organic evolution is the slow, gradual, continuous and irreversible change by which complex present-day life forms developed from simple pre-existing forms. Darwin called it ‘descent with modification’.
- Lamarck’s theory of inheritance of acquired characters held that characters acquired during life, through use and disuse, are inherited. August Weismann disproved it by cutting the tails of rats for 21 generations with no effect, and proposed the theory of continuity of germplasm — only germ-cell (germplasm) changes are inherited, not body-cell (somatoplasm) changes.
- Darwinism (On the Origin of Species, 1859) rests on five postulates: overproduction, struggle for existence, organic variations, natural selection (Spencer’s ‘survival of the fittest’), and origin of new species. Its evidences include the long-necked giraffe, black peppered moths, and DDT resistance in mosquitoes.
- Darwinism’s chief drawbacks: it could not distinguish hereditary from environmental (or somatic from germinal) variations, could not explain the ‘arrival of the fittest’, the origin of variations, vestigial organs, extinction, neutral flowers or hybrid sterility.
- Mutation theory (Hugo de Vries, 1901), based on the evening primrose (Oenothera lamarckiana), states that mutations are large, sudden, random, discontinuous and heritable variations, which provide the raw material for evolution. Objections: de Vries’ variations were really chromosomal aberrations; the mutation rate is too low; chromosomal aberrations are unstable. A single-step large mutation is called saltation.
Modern synthetic theory
- It combines all biological disciplines and gives importance to both mutation and natural selection. A small interbreeding group is a Mendelian population; the sum of its genes is the gene pool; the proportion of an allele at a locus is the gene frequency. Populations evolve, not individuals.
- Its three concepts are genetic variations, natural selection and isolation.
- Genetic variations arise from gene mutation, genetic recombination (crossing over between non-sister chromatids of homologous chromosomes), gene flow, genetic drift, and chromosomal aberrations — deletion, duplication, inversion (\(180^\circ\) twist, no loss or gain of genes) and translocation (transfer to a non-homologous chromosome).
- Industrial melanism in Great Britain is a classic case of natural selection: before industrialization (1845) the grey-white Biston betularia outnumbered the black Biston carbonaria; soot killed the lichens and blackened the trunks, and the black form then had the advantage.
- Isolation prevents interbreeding. It is geographical (physical) — e.g. Darwin’s finches — or reproductive. Isolating mechanisms are pre-mating (pre-zygotic) — habitat, seasonal, ethological, mechanical — or post-mating (post-zygotic) — gamete mortality, zygote mortality, hybrid sterility (e.g. the sterile mule).
Mechanism of evolution
- The basic processes are mutation, gene recombination, gene flow, genetic drift, natural selection, isolation and speciation.
- Genetic drift is random change in allele frequency by chance (the Sewall Wright effect); it matters most in small populations. The founder effect occurs when a few individuals establish a new population; the bottleneck effect occurs when most of a population is destroyed by a disaster.
- Natural selection is of three types: stabilizing (favours the mean, reduces variation, no evolutionary change — e.g. intermediate infant birth weight), directional (favours one extreme, shifts the peak — e.g. industrial melanism, DDT-resistant mosquitoes), and disruptive (favours both extremes, eliminates intermediates, produces two peaks — e.g. beak size in African seed-cracker finches; this type is rare).
Hardy–Weinberg principle
- At equilibrium, allele and genotypic frequencies remain constant from generation to generation, provided the population is diploid, sexually reproducing, large, freely interbreeding, randomly mating, and free from selection or any other factor that changes allele frequency.
- \(p + q = 1\) and \((p+q)^2 = 1\), giving \(p^2 + 2pq + q^2 = 1\), where \(AA = p^2\), \(Aa = 2pq\) and \(aa = q^2\). Also \(AA + 2Aa + aa = 1\).
- Gene migration, genetic drift, mutation, recombination, natural selection and non-random mating disturb the equilibrium. If none of these operates, the population is genetically stable and non-evolving.
Adaptive radiation and speciation
- Adaptive radiation is the transformation of one original species into many different varieties — e.g. Darwin’s finches of the Galapagos and the Australian marsupials.
- Speciation is the formation of a new species from a pre-existing one. A species is a group of similar organisms that interbreed and produce fertile offspring in nature.
- Intraspecific speciation is of two types: allopatric (separation by distance or a geographical barrier; e.g. 14 finch species of the Galapagos) and sympatric (no geographical barrier; caused by reproductive isolation; e.g. Cichlid fishes of Lake Victoria).
- Interspecific speciation occurs by hybridisation: two species cross, and doubling of chromosomes makes the hybrid a new fertile species — e.g. Triticum aestivum (hexaploid) and Gossypium herbaceum (tetraploid).
Evidences of organic evolution
- Palaeontology: fossils give the most convincing and direct evidence. Fossils form in sedimentary rock, amber, ice and peat bogs; primitive forms lie in older, lower layers and advanced forms in upper, recent layers. Types: actual remains (Wooly Mammoth of Siberia), moulds (footprints), casts, and compressions.
- A connecting (missing) link shows characters of two groups — Seymouria (amphibians–reptiles) and Archaeopteryx (reptiles–birds). Archaeopteryx lithographica, a crow-sized toothed bird from the Jurassic rocks of Germany, had reptilian characters (long tail, scales, clawed digits, homodont teeth, keel-less sternum, solid bones) and avian characters (feathers, wings, beak, fused skull). Huxley called birds ‘glorified reptiles’.
- Morphology and anatomy: homologous organs are structurally similar but functionally different (forelimbs of lizard, bird, bat, horse, whale and man; thorn of Bougainvillea and tendril of Cucurbita) and indicate common ancestry and divergent evolution. Analogous organs are structurally dissimilar but functionally similar (wings of insect and bird; flippers of penguin and dolphin) and indicate convergent evolution; they do not trace ancestry. Vestigial organs are degenerate, non-functional structures — in man, the plica semilunaris, wisdom teeth, coccyx, and the vermiform appendix with the caecum.
- Molecular evidences: the cell as the basic unit of all life; similar proteins and genetic material; similar basic metabolic activities; ATP as the energy source in all organisms.
Geological time scale and human evolution
- The scale is divided into six eras, further divided into periods and epochs. The first life appeared some 2000 million years ago. Life began in sea water; plants adapted to land first; reptiles were the first true land vertebrates; dinosaurs became extinct around 65 million years ago, after which mammals became dominant. Continental drift reshaped the distribution of animals, and Australian marsupials survived through lack of competition.
- Human evolution began from a tree-dwelling, shrew-like animal in the Palaeocene epoch. Major trends: increase in brain size and cranial capacity, bipedal locomotion, erect posture, opposable thumb, shortened forelimbs, chin, broadened pelvic girdle, lumbar curvature, and social and cultural development.
- The fossil sequence runs Dryopithecus → Ramapithecus → Australopithecus → Homo habilis → Homo erectus → Neanderthal man → Homo sapiens, with cranial capacity rising from 450–600 cc in Australopithecus to 1450 cc in Homo sapiens.
Flowchart
FC 5.13 – The chapter in one chain: Non-living chemicals (\(CH_4\), \(NH_3\), \(H_2O\), \(H_2S\)) → simple organic molecules → complex organic molecules → nucleic acids → protobionts → first cell → mutation and recombination create variation → gene flow and genetic drift shift allele frequencies → natural selection favours the fit → isolation stops gene flow → speciation → adaptive radiation → the diversity recorded in the geological time scale → human evolution → (the process continues)
5.15 Points to Ponder
Pasteur showed that life does not arise from non-living matter. Chemical evolution claims that, once upon a time, it did. Are these two statements really in conflict — or does everything depend on the conditions? What exactly was different about the early earth?
If the first cell was anaerobic and heterotrophic, it fed on the organic molecules already lying in the primitive broth. What must have happened to life once that free supply began to run out?
Free oxygen would have destroyed the fragile organic molecules from which life was assembled. Yet today most organisms cannot live without oxygen. How did a gas that would have prevented life from beginning become the gas on which life depends?
Why is the human brain capable of what it is? Why did we evolve along these lines and not some other? And what is the true extent of the similarity between humans, chimpanzees and monkeys?
The cranium of an elephant is larger than that of a human being. On what grounds, then, do we call ourselves the more intelligent species — and is “intelligence” a single thing that can be ranked at all?
Human beings are said to be the most evolved and most intelligent living beings. Yet we are not self-sufficient. Think of all the ways in which our survival depends on other living organisms. Does “most evolved” really mean “most independent”?
Darwin could explain the survival of the fittest, but not the arrival of the fittest. Which later idea filled that gap — and is the gap now completely closed?
Natural selection can only choose among variations that already exist; it cannot create the variation it needs. What does this tell you about the limits of what evolution can produce?
Genetic drift can eliminate a useful allele purely by chance. Does this mean that evolution does not always produce better-adapted organisms?
The Hardy–Weinberg principle describes a population in which nothing changes. Does any real population in nature actually satisfy all its conditions? If not, of what use is the principle?
A mule is strong, healthy and long-lived, yet it is an evolutionary dead end. What does this tell you about how biologists define “fitness”?
The dinosaurs disappeared around 65 million years ago, and so did the giant ferns. Can we give an affirmative reason for these extinctions? What conditions could have destroyed both a group of huge reptiles and a group of large plants at roughly the same time?
The coelacanth was declared extinct on the evidence of the fossil record, and then turned up alive. What does this tell us about the difference between “no fossils found” and “did not exist”?
Vestigial organs are useless to their owner. Why has natural selection not removed them entirely? What would have to be true for an organ to be lost completely?
Marsupials survived in Australia because there was no competition from placental mammals. What is likely to happen to such a fauna when humans introduce new species to the continent?
Fossil parks and rock shelters preserve the record of life and of early human culture — the Wadadham fossil park in Gadchiroli district of Maharashtra, and the cave paintings of prehistoric humans at the Bhimbetka rock shelter in Raisen district, Madhya Pradesh. Why should a country invest in protecting such sites? What would be lost if they were destroyed?
Our journey of evolution still continues. In what ways might natural selection be acting on human populations today — and in what ways have medicine, technology and culture changed the rules?
5.16 Objective Questions
- Who proposed that the first form of life could have come from pre-existing non-living organic molecules?
- Alfred Wallace
- Oparin and Haldane
- Charles Darwin
- Louis Pasteur
- The sequence of origin of life may be —
- Organic materials – inorganic materials – eobiont – colloidal aggregates – cell
- Inorganic materials – organic materials – colloidal aggregates – eobiont – cell
- Organic materials – inorganic materials – colloidal aggregates – cell
- Inorganic materials – organic materials – eobiont – colloidal aggregates – cell
- In the Hardy–Weinberg equation, the frequency of homozygous recessive individuals is represented by —
- \(p^2\)
- \(pq\)
- \(q^2\)
- \(2pq\)
- Select the analogous organs.
- Forelimbs of whale and bat
- Flippers of dolphin and penguin
- Thorn and tendril of Bougainvillea and Cucurbita
- Vertebrate hearts or brains
- Archaeopteryx is known as a missing link because it is a fossil and shares characters of both —
- Fishes and amphibians
- Annelida and Arthropoda
- Birds and reptiles
- Chordates and non-chordates
- Identify the wrong statement regarding evolution.
- Darwin’s variations are small and directional.
- Mutations are random and non-directional.
- Adaptive radiation leads to divergent evolution.
- Mutations are non-random and directional.
- Gene frequency in a population remains constant due to —
- Mutation
- Migration
- Random mating
- Non-random mating
- Which of the following characteristics is NOT shown by the ape?
- Prognathous face
- Tail is present
- Chin is absent
- Forelimbs are longer than the hind limbs
- ……………… can be considered as the connecting link between ape and man.
- Australopithecus
- Homo habilis
- Homo erectus
- Neanderthal man
- The cranial capacity of Neanderthal man was —
- 600 cc
- 940 cc
- 1400 cc
- 1600 cc
- Match the following.
| Column I | Column II |
|---|---|
| a. August Weismann | i. Mutation theory |
| b. Hugo de Vries | ii. Germplasm theory |
| c. Charles Darwin | iii. Theory of inheritance of acquired characters |
| d. Lamarck | iv. Theory of natural selection |
- Complete the chart.
| Era | Dominating group of animals |
|---|---|
| 1. Cenozoic | ……………………………. |
| 2. ……………….. | Reptiles |
| 3. Palaeozoic | ……………………………. |
| 4. ……………….. | Invertebrates |
5.17 Check Your Concepts
Pasteur’s swan-necked flask experiment showed that a sterile broth stays free of micro-organisms indefinitely, even though air can reach it. Explain precisely which theory this disproved, and explain why it does NOT disprove the chemical evolution theory of the origin of life.
Assertion: The primitive atmosphere of the earth was of a reducing type. Reason: The absence of free oxygen allowed newly formed organic molecules to accumulate without being degraded. State whether the assertion and the reason are true, and whether the reason correctly explains the assertion.
Arrange the following in the correct sequence of chemical evolution: nucleic acids, simple organic molecules, protobionts, first cell, complex organic molecules, formation of \(CH_4\) and \(NH_3\), eobionts.
In Urey and Miller’s apparatus, why was the chamber first sterilized and evacuated? What error in interpretation would have been possible if this step had been omitted?
Why did the discovery of ribozymes provide such strong support for the RNA World hypothesis? In your answer, explain the “chicken-and-egg” problem that DNA and protein present.
Compare coacervates and microspheres with respect to: the scientist who described them, their composition, their boundary, and their stability.
Weismann cut the tails of rats for 21 generations and found no change in tail length. What theory was he testing, what conclusion did he draw, and which theory did he propose as a result?
A gardener prunes a rose plant every year to give it a particular shape. Will the seeds of this plant produce pruned-looking plants? Justify your answer using the germplasm theory.
Assertion: Overproduction of offspring is essential for natural selection to operate. Reason: Populations of most species remain relatively constant in size from year to year. Explain the connection between the assertion and the reason.
Darwin could explain the “survival of the fittest” but not the “arrival of the fittest”. Explain what this criticism means, and state which later theory attempted to answer it.
“The spraying of DDT did not create resistance in mosquitoes; it only revealed it.” Justify this statement.
Distinguish between Darwin’s variations and de Vries’ mutations on the basis of size, suddenness, direction, and their proposed role in speciation.
What happens to the gene complement of a chromosome in (i) deletion, (ii) duplication, (iii) inversion, and (iv) translocation? For which one is there neither a loss nor a gain of genes, and why?
Crossing over and translocation both involve the exchange of chromosome segments. Why is one a normal event of meiosis and the other an aberration?
Explain why genetic recombination is important for evolution even though it does not create a single new allele.
Why is the effect of genetic drift much more significant in a small population than in a large one? Illustrate your answer with a simple example.
Distinguish between the founder effect and the bottleneck effect. In which one does the original population survive?
Both natural selection and genetic drift change allele frequencies. State the one essential difference between them, and explain why a useful allele can be lost by drift but not by selection.
Before industrialization, why was the black-winged moth rare, and after industrialization, why did it become common? Which type of natural selection does this illustrate, and why?
Three graphs of number of individuals against character value are given. In the first, the peak becomes narrower without shifting; in the second, the peak shifts sideways; in the third, one peak becomes two. Identify each type of selection, and state, for each, which individuals are being eliminated.
Why is disruptive selection the only one of the three types of selection that can split a single population into two distinct forms? Support your answer with the example of the African seed-cracker finches.
A population is in Hardy–Weinberg equilibrium. Explain what would happen to the allele frequencies if (i) the population became very small, (ii) individuals began choosing mates of a particular genotype, and (iii) a group of migrants entered the population.
Why does the middle term of the Hardy–Weinberg equation carry the factor 2? Answer using the Punnett square.
“The forces that break the Hardy–Weinberg equilibrium are exactly the forces that cause evolution.” Explain this statement.
Both Darwin’s finches and the Australian marsupials are cited as examples of adaptive radiation. What do these two cases have in common that made adaptive radiation possible in each?
Two populations of frogs live in the same pond, but one breeds in the early monsoon and the other in the late monsoon, and they never interbreed. Name the isolating mechanism, state whether it is pre-zygotic or post-zygotic, and explain your reasoning.
A mule is healthy and strong, yet sterile. Explain the cellular reason for hybrid sterility, and explain why a sterile hybrid is an evolutionary dead end.
Distinguish between allopatric and sympatric speciation. How can two groups living in the same lake stop interbreeding without any geographical barrier?
In interspecific speciation, why is the doubling of chromosomes essential before a hybrid can become a new species?
Assertion: Fossils in the lower, older layers of sedimentary rock are simpler than those in the upper layers. Reason: Sediment is deposited layer upon layer over time. Explain how these two statements together provide evidence for evolution.
Distinguish between a mould and a cast as types of fossils.
Archaeopteryx had feathers and wings, and yet its sternum lacked a keel and its bones were solid. Explain why these two skeletal facts show that it was not a fully developed flying bird.
Label the following in a diagram of the vertebrate forelimb: humerus, radius, ulna, carpals, metacarpals, phalanges. Then explain how the same bones give rise to a bat’s wing, a whale’s flipper and a human hand.
The wings of a bat and the wings of a bird are analogous, and yet the bones inside them are homologous. Explain how both statements can be true at the same time.
Why do analogous organs fail to establish common ancestry, whereas homologous organs succeed in doing so?
If the vermiform appendix is non-functional in man, why has natural selection not eliminated it completely? What condition would have to be satisfied for an organ to be lost entirely?
How does the fact that a bacterium can read a human gene and manufacture human insulin serve as molecular evidence for evolution?
Why can carbon dating not be used to determine the age of a dinosaur fossil? What must be used instead?
Arrange the following eras in order from oldest to most recent, and name the dominant animal group of each: Mesozoic, Archaeozoic, Cenozoic, Palaeozoic, Proterozoic, Azoic.
Reptiles are described as the first true land vertebrates. What single reproductive feature justifies this description, and why could amphibians not achieve it?
Ichthyosaurs were reptiles that returned to the water and became fish-like in shape. Does this contradict the statement that evolution is irreversible? Justify your answer.
Why did the marsupials of Australia survive and diversify, while the South American mammals were displaced when the continents joined?
Explain how erect posture, the opposable thumb, tool-making and increasing cranial capacity form a connected chain in human evolution, each step making the next one advantageous.
The cranium of an elephant is larger than that of a human. On what grounds, then, is the human considered more intelligent?
Arrange in the correct evolutionary sequence, and give the cranial capacity of each where it is known: Homo erectus, Dryopithecus, Homo sapiens, Australopithecus, Ramapithecus, Neanderthal man, Homo habilis.
Match the following.
| Column I | Column II |
|---|---|
| a. Sewall Wright | i. Ribozymes |
| b. Sidney Fox | ii. Genetic drift |
| c. Altman and Cech | iii. Microspheres |
| d. Sidney Fox’s contemporary, Oparin | iv. Coacervates |
| e. Huxley | v. Birds as glorified reptiles |
- Match the fossil with the two groups it connects.
| Column I | Column II |
|---|---|
| a. Archaeopteryx | i. Fishes and amphibia |
| b. Seymouria | ii. Reptiles and birds |
| c. Ichthyostega | iii. Amphibians and reptiles |
- Define the following terms: (30–40 words)
- Gene pool
- Gene frequency
- Organic evolution
- Population
- Speciation
What is adaptive radiation? (30–40 words)
If a variation occurs in a population by chance alone and not through natural selection, and it brings about a change in the frequency of an allele — what is it called? (30–40 words)
State the Hardy–Weinberg equilibrium. (30–40 words)
What are homologous organs? (30–40 words)
What is a vestigial organ? (30–40 words)
What is the scientific name of modern man? (30–40 words)
What is a coacervate? (30–40 words)
Which period is known as the “age of Reptilia”? (30–40 words)
Name the ancestor of humans which is described as “man with an ape brain”. (30–40 words)
Write a note on genetic drift. (80–100 words)
Enlist the different factors that are responsible for changing gene frequency. (80–100 words)
Draw a graph to show that natural selection leads to disruptive change. (80–100 words)
Give the significance of fossils. (80–100 words)
Write the objections to Hugo de Vries’ mutation theory. (80–100 words)
What is disruptive selection? Give an example. (80–100 words)
Would you consider the wings of a butterfly and of a bat as homologous or analogous, and why? (150–200 words)
What is adaptive radiation? Explain with a suitable example. (150–200 words)
Taking industrial melanism as an example, explain the concept of natural selection. (150–200 words)
Describe Urey and Miller’s experiment. (150–200 words)
What is isolation? Describe the different types of reproductive isolation. (150–200 words)
What are genetic variations? Explain the different factors responsible for genetic variations. (150–200 words)
5.18 Practice With Numericals
In a large, randomly mating population in Hardy–Weinberg equilibrium, the frequency of the recessive allele a is 0.2. Calculate the frequency of the dominant allele A, and the frequencies of the AA, Aa and aa genotypes.
In a population of 10,000 individuals in Hardy–Weinberg equilibrium, 400 individuals show a recessive trait. Find the frequency of the recessive allele, the frequency of the dominant allele, and the number of heterozygous individuals in the population.
A recessive genetic disorder affects 1 in 2500 newborns in a large, randomly mating community. Assuming Hardy–Weinberg equilibrium, calculate the frequency of the disease allele and the percentage of the population that are healthy carriers.
In a population at Hardy–Weinberg equilibrium, the frequency of heterozygous individuals (Aa) is 0.48 and the frequency of the dominant allele is greater than that of the recessive allele. Determine p and q, and hence the frequencies of AA and aa.
In Urey and Miller’s experiment, methane, ammonia and hydrogen were pumped into the chamber in the proportion 1:2:2. If a total of 250 units of gas were pumped in, how many units of each gas were used?
A single pair of elephants would, over a span of 750 years, produce 19,000,000 elephants if all offspring survived and reproduced. Given that elephant populations in fact remain relatively constant, express in one sentence what this comparison demonstrates about the struggle for existence — and state what fraction of the potential 19,000,000 must, on average, fail to reproduce.
The cranial capacity of Australopithecus ranged from 450 to 600 cc, while that of Homo sapiens is 1450 cc. Calculate the increase in cranial capacity from the upper limit of Australopithecus to Homo sapiens, and express this increase as a percentage of the Australopithecus value.