Evolution

NCERT opens with the age of things. The universe is very old, almost 20 billion years, and is made of huge clusters of galaxies. The Big Bang theory says it began with a singular huge explosion; the universe expanded, temperature fell, hydrogen and helium formed, and gases condensed under gravity into galaxies. The earth formed about 4.5 billion years back. There was no atmosphere at first. Water vapour, methane, carbon dioxide and ammonia released from the molten mass covered the surface. UV rays from the sun broke water into hydrogen and oxygen; the lighter hydrogen escaped, oxygen combined with ammonia and methane to form water, carbon dioxide and other compounds, and an ozone layer formed. As it cooled, water vapour fell as rain and filled the depressions to make oceans. Life appeared 500 million years after the formation of earth, that is, almost four billion years back.

Three older ideas are listed so that you can reject them with reasons. Early Greek thinkers proposed that units of life called spores were transferred to earth from other planets; this is panspermia, still favoured by some astronomers. The theory of spontaneous generation held that life arose from decaying matter such as straw and mud. Louis Pasteur disproved it: in pre-sterilised flasks life did not arise from killed yeast, while in a flask open to air new organisms appeared from killed yeast. Life comes only from pre-existing life, but that leaves the first life unexplained.

The accepted answer is chemical evolution, proposed by Oparin of Russia and Haldane of England. The first form of life could have come from pre-existing non-living organic molecules such as RNA and protein, and the formation of life was preceded by chemical evolution of these molecules from inorganic constituents. Conditions on the early earth favoured this: high temperature, volcanic storms and a reducing atmosphere containing methane and ammonia. In 1953, S.L. Miller, an American, created these conditions in a closed flask containing CH₄, H₂, NH₃ and water vapour at slightly below 100 °C, passed an electric discharge through it, and observed the formation of amino acids. Similar experiments by others produced sugars, nitrogen bases, pigments and fats, and analysis of meteorite content revealed similar compounds. This is experimental support for chemical evolution.

The last two dates matter: the first non-cellular forms of life (giant molecules such as RNA, protein and polysaccharides) could have originated about 3 billion years ago, and the first cellular forms did not possibly originate till about 2000 million years ago; they were single cells and all life arose in water. A 3-mark question in the 2024 paper asked for the significance of Miller's experiment, so learn the flask contents, the temperature and the product.

Conventional religious literature holds the theory of special creation, with three ideas: all living organisms were created as they are, the diversity was always the same and will stay the same, and the earth is about 4000 years old. Charles Darwin challenged this after his sea voyage on H.M.S. Beagle round the world. He concluded that existing life forms share similarities to varying degrees among themselves and also with life forms that existed millions of years ago, many of which are extinct. Life forms have varied gradually over millions of years; NCERT uses the phrase gradual evolution for this.

Darwin's explanation was natural selection. Any population has built-in variation. Individuals with characters that give them a better chance to survive and reproduce in the prevailing environment leave more progeny; this is called fitness, and Darwin measured it as reproductive fitness, not strength or speed. Over time, the fitter types out-reproduce the others and the population changes. Another naturalist, Alfred Wallace, working in the Malay Archipelago, came to similar conclusions at about the same time, which is why the theory is sometimes called the Darwin–Wallace theory.

The two key concepts of the Darwinian theory that the examiner expects you to name are branching descent (all life forms are related through common ancestors from which lineages branch) and natural selection. Note the subtlety NCERT states: the adaptation of a species does not mean the environment changes the organism; the environment merely selects, from the variation already present, those individuals that fit it best.

The Galapagos finches (Darwin's finches) are the classic illustration. On each island Darwin found small black birds whose beaks differed, and he reasoned that they all evolved on the islands themselves from one ancestral seed-eating stock. That example returns in section 6.4 as adaptive radiation, and a 2025 MCQ turned on the point that all the finch varieties arose from one original ancestor. For a 2-mark question on this section write: variation exists, fitter individuals reproduce more, characters are heritable, so the population changes over generations.

Learn this section under four heads, because 3-mark and 5-mark questions ask for evidences classified this way.

1. Paleontological evidence (fossils). Fossils are remains of hard parts of life forms found in rocks. Rocks form sediments, and a cross-section of the earth's crust shows sediments laid one over the other through the history of earth. Different-aged sediments contain fossils of different life forms that probably died during the formation of that sediment. Some resemble modern organisms, some are extinct (dinosaurs). Study of fossils in different sedimentary layers thus shows the geological period in which they existed and that life forms varied over time. This is direct evidence of evolution.

2. Comparative anatomy and morphology. Homologous organs have the same basic structure but different functions: the forelimbs of whale, bat, cheetah and human all have humerus, radius, ulna, carpals, metacarpals and phalanges, yet are used for swimming, flying, running and grasping. This is the same structure developing in different directions for different needs, so homology indicates common ancestry, and the pattern is divergent evolution. Other examples: vertebrate hearts and brains; in plants, the thorn of Bougainvillea and the tendril of Cucurbita (both modified axillary buds). Analogous organs are not anatomically similar but perform the same function: the wings of butterfly and bird, the eye of octopus and mammals, the flippers of penguins and dolphins, and sweet potato (root modification) and potato (stem modification). Different structures evolving for the same function because of a similar habitat is convergent evolution.

3. Embryological evidence. Ernst Haeckel proposed that embryos of all vertebrates pass through common stages absent in adults, for example a row of vestigial gill slits just behind the head, functional only in fish and non-functional in reptiles, birds and mammals. Karl Ernst von Baer disproved this on careful study: embryos never pass through the adult stages of other animals. Haeckel's proposal was a 1-mark item in 2024.

4. Biochemical (molecular) evidence. Similarities in proteins and genes performing a given function across diverse organisms (cytochrome c, haemoglobin, DNA sequences) point to common ancestry; the fewer the differences, the closer the relationship. Case-based questions give a table of amino-acid differences and ask you to rank relatedness.

Natural selection seen directly: industrial melanism. In England, moths (Biston betularia) collected in the 1850s, before industrialisation, were mostly white-winged; collections from the same area in 1920 were mostly dark-winged (melanised). Before industrialisation white lichens covered tree trunks and white moths were camouflaged from predators; soot from industries killed lichens and darkened trunks, so dark moths survived and white moths were eaten. Both forms existed all along; the environment selected. In rural unpolluted areas the white form still dominates. Lichens are therefore pollution indicators. Similarly, excess use of herbicides and pesticides selects resistant varieties in a much shorter time, and microbes become resistant to antibiotics within months or years, not centuries: evolution by anthropogenic action. NCERT concludes that evolution is not a directed, deterministic process; it is stochastic, based on chance events in nature and chance mutations in organisms.

Darwin's finches give the definition. On the Galapagos Islands Darwin observed many varieties of finch and conjectured that all of them evolved on the islands themselves from one ancestral stock. From the original seed-eating features, other forms with altered beaks arose that became insectivorous and vegetarian finches. NCERT defines adaptive radiation as 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). Write the definition and the finch example for 2 marks; a 2-mark question asking exactly this has appeared more than once.

The second NCERT example is Australian marsupials. A number of marsupials, each different from the other, evolved from an ancestral stock, but all within the Australian island continent: the marsupial mole, numbat (anteater), marsupial mouse, kangaroo, koala, wombat, bandicoot, Tasmanian tiger cat and Tasmanian wolf occupy different habitats and feeding roles, yet all carry their young in a pouch.

Now the connection the examiner tests. When more than one adaptive radiation appears to have occurred in an isolated geographical area representing different habitats, we can call it convergent evolution. Placental mammals in Australia also show adaptive radiation into varieties, each of which appears similar to a corresponding marsupial: the placental wolf and the Tasmanian wolf (marsupial), placental mole and marsupial mole, anteater and numbat, flying squirrel and flying phalanger, mouse and marsupial mouse, lemur and spotted cuscus, bobcat and Tasmanian tiger cat. Two unrelated stocks radiating in the same range of habitats produced look-alike pairs, which is convergence.

So the same section carries both terms: adaptive radiation is divergent (one ancestor, many descendants in different habitats), while the parallel radiations of placentals and marsupials filling matching niches are convergent. A 3-mark question in 2025 asked you to compare and contrast convergent and divergent evolution; give definitions, the mechanism (different structures → same function versus same structure → different functions), one organ example of each and one radiation example of each.

Evolution by natural selection in the true sense began when cellular forms with differences in metabolic capability arose. NCERT uses bacteria to show how fast it can happen. A colony of bacteria, say A, growing on a medium has built-in variation in its ability to use a feed component. If the medium composition changes, only that part of the population, say B, that can survive the new conditions grows; in time B outgrows the others and appears as a new species. Because bacteria divide in about 20 minutes, this happens within days; in a fish or fowl, whose life spans are in years, the same change takes millions of years. The rate of appearance of new forms is linked to the life cycle or life span. Here fitness of B is better than that of A under the new conditions; nature selects for fitness. Fitness is based on inherited characteristics, so there is a genetic basis for being selected. Another way to say it: some organisms are better adapted to survive in a hostile environment; adaptive ability is inherited; fitness is the end result of the ability to adapt and get selected.

Before Darwin, the French naturalist Lamarck said evolution occurred but was driven by use and disuse of organs. His example was the giraffe, which stretched its neck to forage leaves on tall trees and passed the acquired elongated neck to the next generation. NCERT says plainly that nobody believes this any more, because acquired characters are not inherited.

Darwin's theory rests on observations partly influenced by Malthus's work on populations: natural resources are limited; populations are stable in size except for seasonal fluctuations; members of a population vary in characteristics (no two individuals are alike) though they look superficially similar; and most variations are inherited. Since a population would grow exponentially if everyone reproduced maximally, yet real populations stay limited, there must be competition for resources; only some survive and grow at the cost of others. Darwin's insight was this: variations that are heritable and make resource utilisation better for a few (the fitter) enable only those to reproduce and leave more progeny. Over many generations the survivors change the population's characteristics and new forms appear. Remember the two key concepts once more: branching descent and natural selection.

This section asks what supplies the variation on which natural selection acts. 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, from his work on the evening primrose, brought forth the idea of mutations: large differences arising suddenly in a population. He believed that mutation, not the minor heritable variations Darwin spoke of, causes evolution.

Learn the contrast as a table because it is asked as a 2-mark or 3-mark question and as an Assertion–Reason. Mutations are random and directionless; Darwinian variations are small and directional. Evolution for Darwin was gradual; de Vries believed mutation caused speciation in a single step and called it saltation (single-step large mutation). The 2024 MCQ asked exactly this term, and another 2024 item asked who proposed the mutation theory in favour of organic evolution: de Vries.

Studies in population genetics later combined these ideas, which is the modern synthetic theory that the CBSE syllabus names: variation arises by mutation and by recombination during gametogenesis, gene flow and genetic drift change allele frequencies, and natural selection acting on heritable variation changes the population over generations. In exam terms, the mechanism of evolution has two parts: a source of variation (mutation, recombination) and a sorting process (natural selection, plus chance effects such as drift). The measure of whether any of this has happened is a change in allele frequencies, which is what the next section, the Hardy–Weinberg principle, provides.

NCERT also mentions that microbial experiments show that pre-existing advantageous mutations, when selected, produce new phenotypes, and over a few generations this results in speciation. Keep this sentence for antibiotic-resistance questions: the mutation existed before the antibiotic was applied; the antibiotic only selected it.

In a population one can find the frequency of occurrence of the alleles of a gene. The Hardy–Weinberg principle states that allele frequencies in a population are stable and remain constant from generation to generation; the gene pool (the total genes and their alleles in a population) remains constant. This is genetic equilibrium. The sum total of all allelic frequencies is 1.

Derivation asked for 3 marks. Let a gene have two alleles, A and a, with frequencies p and q, so p + q = 1. In a diploid, the probability that allele A appears on both chromosomes of an individual is the product of the probabilities, p × p = p²; this is the frequency of AA individuals. Similarly the frequency of aa is q², and of Aa is 2pq (Aa or aA). Since these three genotypes make up the whole population, p² + 2pq + q² = 1, which is the binomial expansion of (p + q)². When the measured frequency differs from the expected value, the difference and its direction indicate the extent of evolutionary change. Disturbance in Hardy–Weinberg equilibrium, that is, a change in allele frequency in a population, is interpreted as evolution.

Five factors disturb the equilibrium: gene migration (gene flow), genetic drift, mutation, genetic recombination and natural selection. When a section of a population migrates to another place and population, gene frequencies change in both the original and the new population: alleles are added to the new one and lost from the old. If this migration happens repeatedly there is gene flow. If the same change occurs by chance, it is genetic drift. Sometimes the allele frequency in the new, drifted sample is so different that it becomes a different species; the original drifted population becomes founders and the effect is the founder effect. Natural selection is a process in which heritable variations that enable better survival are enabled to reproduce and leave a greater number of progeny.

Three ways natural selection operates (the distribution-curve figure in NCERT): stabilisation, in which more individuals acquire the mean character value and the curve becomes narrower and taller; directional change, in which more individuals acquire a value other than the mean, and the peak shifts to one side; and disruption, in which more individuals acquire the peripheral values at both ends of the distribution and the curve becomes two-peaked. Practise the three graphs; they are asked with the phrase explain with graphs.

Worked calculation. If 16% of a population shows a recessive trait, q² = 0.16, so q = 0.4 and p = 0.6; heterozygotes 2pq = 2 × 0.6 × 0.4 = 0.48 (48%) and homozygous dominant p² = 0.36 (36%). Check: 0.36 + 0.48 + 0.16 = 1.

This is a timeline, and MCQs ask you to arrange events in order or to pick the group at a given date, so learn the sequence with its numbers. About 2000 million years ago (mya) the first cellular forms of life appeared. Some of these cells could release oxygen (photosynthesis), and slowly single-celled organisms became multicellular. By 500 mya invertebrates were formed and active. Jawless fish probably evolved around 350 mya. Sea weeds and a few plants existed probably around 320 mya. NCERT makes the point that the first organisms to invade land were plants, which then made animal life on land possible.

Fish with stout and strong fins could move on land and go back to water; some of these were lobefins, which evolved into the first amphibians that lived on both land and water. Their descendants today are the coelacanth, once thought extinct, one of which was caught in South Africa in 1938. Amphibians evolved into reptiles, which lay thick-shelled eggs that do not dry up in the sun like amphibian eggs; their modern descendants are turtles, tortoises and crocodiles. In the next 200 million years reptiles of different shapes and sizes dominated the earth. Some land reptiles went back into water about 200 mya to evolve into fish-like reptiles, the ichthyosaurs (a 2025 MCQ). The land reptiles were the dinosaurs; the biggest, Tyrannosaurus rex, was about 20 feet high with huge fearsome dagger-like teeth. About 65 mya the dinosaurs suddenly disappeared; whether by climatic change or because most evolved into birds is not known. Giant ferns (pteridophytes) of the same age fell and were slowly buried to form coal deposits.

Mammals. The first mammals were small, shrew-like, and their fossils are rare. Mammals are viviparous and protect their unborn young inside the mother's body; they are more intelligent in sensing and avoiding danger. When reptiles came down, mammals took over. Some mammals later went back to water: whales, dolphins, seals and sea cows. In South America there were mammals resembling horse, hippopotamus, bear and rabbit; when continental drift joined South America to North America, these were overridden by North American fauna. Because of the same continental drift the pouched mammals of Australia survived, since they faced no competition from any other mammal.

Plant order (from the NCERT evolutionary chart): marine algae and sea weeds → mosses and early land plants → ferns and horsetails (pteridophytes) → gymnosperms (conifers, cycads) → angiosperms (flowering plants), the last to appear. Animal order: invertebrates → jawless fish → jawed fish (lobefins) → amphibians → reptiles → birds and mammals. A 2025 MCQ asked for the correct evolutionary order of plants. Sequence questions on animals are set from the dates above, so quote them: invertebrates (500 mya) before jawless fish (350 mya) before amphibians, reptiles and mammals.

Write human evolution as a dated sequence with brain capacity, posture, diet and tool use at each stage; that is what the 4-mark flow chart and the 5-mark timeline reward.

About 15 mya primates called Dryopithecus and Ramapithecus existed. They were hairy and walked like gorillas and chimpanzees; Ramapithecus was more man-like, Dryopithecus more ape-like. Few fossils of man-like bones have been found in Ethiopia and Tanzania. These revealed that 3 to 4 mya man-like primates walked in eastern Africa; they were probably not taller than 4 feet but walked upright. Two mya Australopithecines probably lived in East African grasslands; evidence shows they hunted with stone weapons but essentially ate fruit. Some of the bones among these were different, and this creature was called the first human-like being, the hominid, Homo habilis: brain capacity 650 to 800 cc, and they probably did not eat meat. Fossils discovered in Java in 1891 revealed the next stage, Homo erectus, about 1.5 mya, with a large brain of around 900 cc, which probably ate meat. The Neanderthal man, brain size 1400 cc, lived in near east and central Asia between 1,00,000 and 40,000 years back; they used hides to protect their bodies and buried their dead. Homo sapiens arose in Africa and moved across continents and developed into distinct races. During the ice age between 75,000 and 10,000 years ago modern Homo sapiens arose. Pre-historic cave art developed about 18,000 years ago; one such site is the Bhimbetka rock shelter in Raisen district of Madhya Pradesh. Agriculture came around 10,000 years back and human settlements started. The rest, as NCERT says, is part of human history.

Two exam angles. First, the 2025 paper asked which primates walked like gorillas 15 mya (Dryopithecus and Ramapithecus), which primates lived in East African grasslands 2 mya (Australopithecines) and which was the first human-like hominid (Homo habilis). Second, the NCERT exercise asks whether human evolution can be called adaptive radiation. The expected answer is no: adaptive radiation means one ancestral species diverging into many species occupying different habitats in a geographical area, whereas human evolution is a linear, successive replacement of one hominid form by the next (Australopithecines → Homo habilis → Homo erectus → Neanderthal → Homo sapiens) with a single living species at the end, a case of anagenesis rather than radiation.

When asked to trace the components of human evolution, organise by brain size (650–800 → 900 → 1400 cc), skeletal structure (upright walking, bipedal posture, reduced jaw), dietary preference (fruit → meat) and culture (stone weapons → hides and burial → cave art → agriculture).