Evolution | CBSE Class 12 Biology Notes

This note covers NCERT Class 12 Biology Chapter 6, Evolution: the origin of life, Darwin's theory of natural selection, the evidences for evolution, adaptive radiation, the mechanism of evolution, the Hardy-Weinberg principle, a brief account of the evolution of life forms, and the origin and evolution of man. The dates, names and numbers are set out in tables.

How did the universe, the earth and life begin?

Evolutionary biology is the study of the history of life forms on earth. To understand the changes in flora and fauna over millions of years, we need the context of the origin of life: the evolution of the earth, of stars and of the universe itself.

The Big Bang theory attempts to explain the origin of the universe. It speaks of a single huge explosion, unimaginable in physical terms. The universe expanded and its temperature came down. Hydrogen and helium formed some time later, and the gases condensed under gravitation to form the galaxies of the present-day universe.

The early earth

1. There was no atmosphere on the early earth. Water vapour, methane, carbon dioxide and ammonia released from the molten mass covered the surface.
2. UV rays from the sun broke up water into hydrogen and oxygen, and the lighter H₂ escaped.
3. Oxygen combined with ammonia and methane to form water, CO₂ and others. The ozone layer was formed.
4. As the earth cooled, the water vapour fell as rain, filling all the depressions to form the oceans.

Where did life come from?

Louis Pasteur demonstrated by careful experimentation that life comes only from pre-existing life. In pre-sterilised flasks life did not come from killed yeast, while in another flask open to air, new living organisms arose from "killed yeast". This did not, however, answer how the first life form came on earth.

What did Oparin, Haldane and Miller show?

Oparin of Russia and Haldane of England proposed that the first form of life could have come from pre-existing non-living organic molecules (for example RNA and protein), and that the formation of life was preceded by chemical evolution: the formation of diverse organic molecules from inorganic constituents.

The conditions on the early earth were high temperature, volcanic storms and a reducing atmosphere containing CH₄, NH₃ and so on.

Miller's experiment

1. In 1953, S.L. Miller, an American scientist, created similar conditions on a laboratory scale.
2. He created an electric discharge in a closed flask containing CH₄, H₂, NH₃ and water vapour at 800°C.
3. He observed the formation of amino acids.
4. In similar experiments, others observed the formation of sugars, nitrogen bases, pigment and fats.

Analysis of meteorite content also revealed similar compounds, indicating that similar processes are occurring elsewhere in space. With this limited evidence, chemical evolution was more or less accepted.

A closed loop of glass apparatus. At the top is a large flask of gases, labelled CH₄, NH₃, H₂O and H₂, with two electrodes producing a spark discharge.

The gases pass down through a condenser, with water in and water out, where water droplets form. These collect in a trap as water containing organic compounds. Below, a flask of boiling water sends vapour back up to the gas flask. A tap near the top leads to a vacuum pump.

See Fig. 6.1 in your NCERT textbook

We have no idea how the first self-replicating metabolic capsule of life arose. The first non-cellular forms of life would have been giant molecules (RNA, protein, polysaccharides and so on), which perhaps reproduced their molecules.

All life forms were in a water environment only. This version of abiogenesis, that the first form of life arose slowly through evolutionary forces from non-living molecules, is accepted by the majority.

What did Darwin propose?

Conventional religious literature tells us about the theory of special creation, which has three connotations:

1. All living organisms (species or types) that we see today were created as such.
2. The diversity was always the same since creation and will be the same in future also.
3. The earth is about 4000 years old.

All these ideas were strongly challenged during the nineteenth century. Based on observations made during a sea voyage round the world in a sail ship called H.M.S. Beagle, Charles Darwin concluded:

• Existing living forms share similarities to varying degrees among themselves, and also with life forms that existed millions of years ago, many of which do not exist any more.
• There had been extinctions of different life forms, just as new forms of life arose at different periods of the history of the earth. There has been a gradual evolution of life forms.
• Any population has built-in variation in characteristics. Those characteristics that enable some to survive better in natural conditions would outbreed others that are less endowed.
• Those better fit in an environment leave more progeny than others. They therefore survive more and are selected by nature. He called this natural selection and implied it as a mechanism of evolution.

Definition: Fitness, according to Darwin, refers ultimately and only to reproductive fitness: those who are better fit in an environment leave more progeny than others.

Alfred Wallace, a naturalist who worked in the Malay Archipelago, had come to similar conclusions at around the same time. The geological history of the earth closely correlates with its biological history, and a common permissible conclusion is that the earth is billions of years old, not thousands.

What are the evidences for evolution?

Homology and analogy

Part (a) shows a stem of Bougainvillea with a thorn and a stem of Cucurbita with a coiled tendril. Part (b) shows a man, a cheetah, a whale and a bat, each with an arrow pointing down to its forelimb skeleton: the human arm, the cheetah's leg, the whale's flipper and the bat's wing.

All four are built from the same set of bones in a different shape.

See Fig. 6.3 in your NCERT textbook

Industrial melanism

The explanation is that predators spot a moth against a contrasting background, so the moths that were able to camouflage themselves survived. In rural areas, where industrialisation did not occur, the count of melanic moths was low. Lichens can be used as industrial pollution indicators, because they will not grow in polluted areas. No variant is completely wiped out.

Two pictures, each showing a dark-winged moth (drawn orange-brown) and a white-winged moth on a tree trunk. In (a), an unpolluted area, the trunk is light golden-brown. In (b), a polluted area, the trunk is dark and the white-winged moth stands out sharply against it.

See Fig. 6.4 in your NCERT textbook

Similarly, excess use of herbicides and pesticides has resulted in the selection of resistant varieties in a much shorter time, and so has the use of antibiotics and drugs.

Resistant organisms and cells appear in months or years, not centuries. These are examples of evolution by anthropogenic action. They also show that evolution is not a directed process in the sense of determinism. It is a stochastic process, based on chance events in nature and chance mutation in organisms.

What is adaptive radiation?

Definition: Adaptive radiation is the process of evolution of different species in a given geographical area starting from a point and literally radiating to other areas of geography (habitats).

When more than one adaptive radiation appears to have occurred in an isolated geographical area (representing different habitats), one can call this convergent evolution.

Figure 6.5 shows the heads of four of Darwin's finches, numbered 1 to 4, with beaks of different shapes, from a thick, deep beak (1) to a slender, pointed one (4).

Figure 6.6 shows ten Australian marsupials, including the kangaroo, koala, wombat and Tasmanian wolf, each joined by an arrow to a central circle labelled Marsupial radiation. Figure 6.7 shows pairs of animals side by side, each Australian marsupial next to the placental mammal it resembles.

See Figs. 6.5, 6.6 and 6.7 in your NCERT textbook

What is the essence of Darwin's theory?

Evolution by natural selection, in a true sense, would have started when cellular forms of life with differences in metabolic capability originated on earth. The essence of Darwinian theory is natural selection.

An example with bacteria

1. A colony of bacteria (say A) growing on a given medium has built-in variation in terms of ability to use a feed component.
2. A change in the medium composition would bring out only that part of the population (say B) that can survive under the new conditions.
3. In due course of time this variant population outgrows the others and appears as a new species. For microbes this would happen within days.
4. For the same thing to happen in a fish or fowl would take millions of years, as their life spans are in years. The fitness of B is better than that of A under the new conditions.

The rate of appearance of new forms is linked to the life cycle or life span. The so-called fitness is based on characteristics that are inherited, so there must be a genetic basis for getting selected and evolving. Branching descent and natural selection are the two key concepts of the Darwinian theory of evolution.

Lamarck

Even before Darwin, the French naturalist Lamarck said that evolution of life forms had occurred, but driven by the use and disuse of organs. His example was the giraffe: in an attempt to forage leaves on tall trees, giraffes elongated their necks and passed on this acquired character to succeeding generations. Nobody believes this conjecture any more.

The observations behind natural selection

It is possible that the work of Thomas Malthus on populations influenced Darwin. Natural selection is based on facts:

• natural resources are limited;
• populations are stable in size, except for seasonal fluctuation;
• members of a population vary in characteristics, and no two individuals are alike;
• most variations are inherited;
• population size would grow exponentially if everybody reproduced maximally, but in reality it is limited, so there has been competition for resources.

Darwin's insight was that variations which are heritable, and which make resource use better for a few, enable only those to reproduce and leave more progeny. Over many generations there would be a change in population characteristics, and new forms appear to arise.

What is the mechanism of evolution?

Mendel had talked of inheritable "factors" influencing phenotype, but Darwin either ignored these observations or kept silent. In the first decade of the twentieth century, Hugo de Vries, based on his work on evening primrose, brought forth the idea of mutations: large differences arising suddenly in a population.

Later, studies in population genetics brought some clarity.

The view that emerged is as follows. Variation due to mutation, variation due to recombination during gametogenesis, and gene flow or genetic drift all change the frequency of genes and alleles in future generations. Coupled with enhanced reproductive success, natural selection then makes the population look like a different one. The Hardy-Weinberg principle gives a way to measure such change.

What is the Hardy-Weinberg principle?

Definition: The Hardy-Weinberg principle says that allele frequencies in a population are stable and constant from generation to generation. The gene pool (total genes and their alleles in a population) remains a constant. This is called genetic equilibrium.

The sum total of all the allelic frequencies is 1. In a diploid, let p and q be the frequencies of allele A and allele a.

• The frequency of AA individuals is p², the product of the probabilities that allele A appears on both chromosomes.
• The frequency of aa individuals is q².
• The frequency of Aa individuals is 2pq.
• Hence p² + 2pq + q² = 1, which is the binomial expansion of (p + q)².

Worked example: genotype frequencies at equilibrium

1. In a population at Hardy-Weinberg equilibrium, the frequency of allele A is p = 0.7. Find the expected genotype frequencies.
2. Since p + q = 1, q = 1 − 0.7 = 0.3.
3. AA = p² = 0.7 × 0.7 = 0.49; aa = q² = 0.3 × 0.3 = 0.09; Aa = 2pq = 2 × 0.7 × 0.3 = 0.42.
4. Check: 0.49 + 0.42 + 0.09 = 1.00.

When the frequency measured differs from the expected values, the difference, and its direction, indicates the extent of evolutionary change. A disturbance in genetic equilibrium, that is a change of the frequency of alleles in a population, is interpreted as resulting in evolution.

Five factors that affect Hardy-Weinberg equilibrium

Three ways natural selection acts

Each graph plots the number of individuals against a phenotype as a bell-shaped curve. The starting curve on the left marks the phenotypes favoured by natural selection, with medium-sized individuals favoured.

In (a), stabilising, the peak gets higher and narrower. In (b), directional, the peak shifts in one direction. In (c), disruptive, two peaks form, one towards each end of the curve.

See Fig. 6.8 in your NCERT textbook

How did life forms evolve through geological time?

• The first organisms to invade land were plants. They were widespread on land when animals invaded land.
• In 1938, a fish caught in South Africa was a Coelacanth, which had been thought to be extinct. These lobefins evolved into the first amphibians, ancestors of modern frogs and salamanders.
• Amphibians evolved into reptiles, which lay thick-shelled eggs that do not dry up in the sun. Their modern descendants include turtles, tortoises and crocodiles.
• The biggest dinosaur, Tyrannosaurus rex, was about 20 feet in height. We do not know the true reason for the disappearance of the dinosaurs: some say climatic changes killed them, and some say most of them evolved into birds.
• The first mammals were like shrews. Mammals were viviparous and more intelligent in sensing and avoiding danger.
• Due to continental drift, the mammals of South America were overridden by North American fauna when the two continents joined, while the pouched mammals of Australia survived because of lack of competition.

A family tree of dinosaurs and their living modern-day counterparts, crocodiles and birds. Labelled animals on a branching yellow shape with upward arrows include Triceratops, Tyrannosaurus, Stegosaurus, Brachiosaurus, Pteranodon, a crocodilian and Archaeopteryx.

See Fig. 6.2 in your NCERT textbook

How did man originate and evolve?

Pre-historic cave art developed about 18,000 years ago. Cave paintings by pre-historic humans can be seen at the Bhimbetka rock shelter in Raisen district of Madhya Pradesh. Agriculture came around 10,000 years back, and human settlements started.

Three skulls are drawn one below the other, each with a drawing of the matching head beside it: adult human (top), baby chimpanzee (middle) and adult chimpanzee (bottom). The skull of the baby chimpanzee, with its rounded top and small face, is more like the adult human skull than like the skull of the adult chimpanzee, which has a sloping forehead and a large jutting jaw.

See Fig. 6.11 in your NCERT textbook

Short answers to the NCERT exercise

Glossary

• Panspermia — The idea that units of life called spores were transferred to different planets, including earth.
• Spontaneous generation — The discarded belief that life came out of decaying and rotting matter like straw and mud.
• Chemical evolution — The formation of diverse organic molecules from inorganic constituents, which preceded the formation of life.
• Natural selection — Darwin's mechanism of evolution, in which those better fit in an environment leave more progeny.
• Fitness — In Darwin's sense, reproductive fitness: the ability to leave more progeny than others in an environment.
• Homologous structures — Structures with similar anatomy but different functions, which indicate common ancestry and result from divergent evolution.
• Analogous structures — Structures that are not anatomically similar but perform similar functions, resulting from convergent evolution.
• Industrial melanism — The increase of dark-winged moths in polluted areas of England after industrialisation, as an example of natural selection.
• Adaptive radiation — The evolution of different species in a geographical area, starting from a point and radiating to other habitats.
• Saltation — De Vries's term for speciation caused by a single-step large mutation.
• Gene pool — The total genes and their alleles in a population.
• Genetic equilibrium — The state in which allele frequencies in a population remain constant from generation to generation.
• Genetic drift — A change in allele frequency in a population that occurs by chance.
• Founder effect — The effect when a drifted population becomes so different that it forms a new species, with the original drifted individuals as founders.

Common errors and misconceptions

• Misconception: Pasteur explained how the first life form arose. Correct: Pasteur showed that life comes only from pre-existing life and dismissed spontaneous generation. This did not answer how the first life form came on earth.
• Misconception: Miller's flask contained oxygen. Correct: It contained CH₄, H₂, NH₃ and water vapour, a reducing mixture, at 800°C.
• Misconception: Fitness means physical strength. Correct: Fitness, according to Darwin, refers ultimately and only to reproductive fitness.
• Misconception: The wings of a bird and a butterfly are homologous. Correct: They are analogous: similar in function but not in anatomy, a result of convergent evolution.
• Misconception: Homologous structures result from convergent evolution. Correct: Homology is based on divergent evolution. Analogy results from convergent evolution.
• Misconception: Industrial pollution wiped out the white-winged moths completely. Correct: The dark moths became more numerous, but no variant is completely wiped out.
• Misconception: De Vries agreed with Darwin that small variations cause evolution. Correct: De Vries believed that mutations, which are large, random and directionless, cause evolution by saltation.
• Misconception: Homo erectus was the first human-like being. Correct: Homo habilis was called the first human-like being, the hominid. Homo erectus came later, about 1.5 mya.

Exam-style questions with model answers

Q1. What did Louis Pasteur demonstrate about the origin of life? [1 mark]

1. Pasteur demonstrated that life comes only from pre-existing life: in pre-sterilised flasks life did not arise from killed yeast, which dismissed the theory of spontaneous generation.

Q2. Distinguish between homologous and analogous structures, with one example each. [2 marks]

1. Homologous structures have similar anatomy but different functions and result from divergent evolution, for example the forelimbs of whales, bats, cheetah and human.
2. Analogous structures are not anatomically similar but perform similar functions and result from convergent evolution, for example the wings of a butterfly and of a bird.

Q3. State the Hardy-Weinberg principle and write its equation. [2 marks]

1. The principle states that allele frequencies in a population are stable and constant from generation to generation, so the gene pool remains constant (genetic equilibrium).
2. If p and q are the frequencies of alleles A and a, then p² + 2pq + q² = 1, the binomial expansion of (p + q)².

Q4. Describe Miller's experiment and its significance. [3 marks]

1. In 1953 S.L. Miller created conditions like those of the early earth in the laboratory: an electric discharge in a closed flask containing CH₄, H₂, NH₃ and water vapour at 800°C.
2. He observed the formation of amino acids. Others, in similar experiments, observed sugars, nitrogen bases, pigment and fats.
3. This supported the idea of chemical evolution proposed by Oparin and Haldane, that organic molecules formed from inorganic constituents before life arose.

Q5. Explain industrial melanism as evidence for natural selection. [3 marks]

1. In England, a collection of moths made in the 1850s, before industrialisation, had more white-winged moths than dark-winged moths. A collection from the same area in 1920, after industrialisation, had more dark-winged moths.
2. After industrialisation, tree trunks became dark with smoke and soot, so white-winged moths were spotted by predators and dark moths survived. Before it, trunks were covered with white lichen, and dark moths were picked out.
3. Moths that could camouflage themselves survived. In rural areas without industrialisation the count of melanic moths stayed low, which supports this explanation.

Q6. What is adaptive radiation? Give two examples. [3 marks]

1. Adaptive radiation is the evolution of different species in a given geographical area, starting from a point and radiating to other habitats.
2. Darwin's finches of the Galapagos Islands evolved from an original seed-eating form into varieties with altered beaks, including insectivorous and vegetarian finches.
3. Australian marsupials evolved into a number of different forms from an ancestral stock within the Australian island continent.

Q7. Name the five factors that affect Hardy-Weinberg equilibrium and explain any two. [5 marks]

1. The five factors are gene migration or gene flow, genetic drift, mutation, genetic recombination and natural selection.
2. Gene migration: when a section of a population migrates to another place, gene frequencies change in both the original and the new population; new alleles are added to one and lost from the other.
3. Gene flow occurs if this gene migration happens multiple times.
4. Genetic drift: the change in gene frequency occurs by chance. The new sample may become so different that it forms a different species.
5. In that case the original drifted population becomes the founders, and the effect is called the founder effect.

Q8. Trace the stages in the evolution of man, with brain capacities where known. [5 marks]

1. About 15 mya, Dryopithecus (more ape-like) and Ramapithecus (more man-like) existed; they were hairy and walked like gorillas and chimpanzees.
2. About 3 to 4 mya, man-like primates not taller than 4 feet walked upright in eastern Africa. About 2 mya, Australopithecines lived in East African grasslands, hunting with stone weapons but essentially eating fruit.
3. Homo habilis, the first human-like being, had a brain capacity of 650 to 800 cc and probably did not eat meat. Homo erectus, about 1.5 mya, had a brain of around 900 cc and probably ate meat.
4. Neanderthal man, with a brain size of 1400 cc, lived in the near east and central Asia between 1,00,000 and 40,000 years back, used hides and buried the dead.
5. Homo sapiens arose in Africa, and modern Homo sapiens arose during the ice age between 75,000 and 10,000 years ago. Cave art developed about 18,000 years ago and agriculture about 10,000 years back.

Key takeaways

• The universe is almost 13.8 billion years old, the earth about 4.5 billion years old, and life appeared almost four billion years back.
• Oparin and Haldane proposed chemical evolution, and Miller's 1953 experiment produced amino acids from CH₄, H₂, NH₃ and water vapour.
• Darwin's theory rests on branching descent and natural selection; fitness means reproductive fitness, and Wallace reached similar conclusions.
• Homologous structures come from divergent evolution and show common ancestry; analogous structures come from convergent evolution.
• Industrial melanism in moths and resistance to pesticides and antibiotics are examples of natural selection observed in action, with resistance appearing in months or years and not centuries.
• Adaptive radiation is seen in Darwin's finches and Australian marsupials; de Vries proposed mutation and saltation against Darwin's gradual change.
• Hardy-Weinberg equilibrium, p² + 2pq + q² = 1, is disturbed by gene flow, genetic drift, mutation, recombination and natural selection.
• Brain capacity rose from 650 to 800 cc in Homo habilis to around 900 cc in Homo erectus and 1400 cc in Neanderthal man.

Test yourself

When did life appear on earth?

Life appeared 500 million years after the formation of the earth, that is almost four billion years back.

Which gases did Miller use in his experiment?

Miller used CH₄, H₂, NH₃ and water vapour in a closed flask at 800°C, with an electric discharge.

What are the three connotations of the theory of special creation?

That all living organisms were created as such, that diversity has always been the same, and that the earth is about 4000 years old.

Who disapproved Heckel's embryological evidence, and why?

Karl Ernst von Baer disapproved it, noting that embryos never pass through the adult stages of other animals.

Give one plant example each of homology and analogy.

The thorn of Bougainvillea and the tendril of Cucurbita are homologous; sweet potato and potato are analogous.

What was Lamarck's explanation for the long neck of the giraffe?

Lamarck said giraffes elongated their necks to reach leaves on tall trees and passed this acquired character to succeeding generations; nobody believes this now.

What was significant about the Coelacanth caught in 1938?

The Coelacanth caught in South Africa in 1938 was a fish thought to be extinct; such lobefins evolved into the first amphibians.

Where can pre-historic cave paintings be seen in India?

Pre-historic cave paintings can be seen at the Bhimbetka rock shelter in Raisen district of Madhya Pradesh.