Evolution — NEET Notes

Origin of life — peering into the past

When we look at the stars on a clear night we are looking back in time, because stellar light takes millions of years to reach us. The origin of life on earth must be set against this cosmic backdrop. The universe is roughly 13.8 billion years old, formed in a singular event called the Big Bang. The expanding universe cooled, hydrogen and helium condensed under gravity, galaxies formed, and within the Milky Way a small rocky planet condensed about 4.5 billion years ago — earth.

Early earth had no atmosphere as we know it. Water vapour, methane, carbon dioxide, and ammonia released from the molten interior covered the surface. Ultraviolet rays from the young sun split water vapour into hydrogen and oxygen; the lighter hydrogen escaped to space, while oxygen reacted with ammonia and methane to give water, carbon dioxide, and other simple molecules. As earth cooled, the water vapour condensed and fell as rain, filling depressions and forming the first oceans. Life appeared about four billion years ago — roughly 500 million years after the formation of the planet.

Two competing ideas dominated early thinking. The Greek thinkers proposed panspermia — that units of life ("spores") drifted from space to seed planets including earth. The medieval theory of spontaneous generation held that life arose from decaying matter — straw, mud, rotting flesh. Louis Pasteur demolished spontaneous generation by carefully designed flask experiments: in pre-sterilised flasks no life appeared from killed yeast, while in flasks open to air new organisms arose. Pasteur's conclusion stands: life comes only from pre-existing life. But how, then, did the very first life arise?

Chemical evolution — the Oparin-Haldane hypothesis

The Russian biochemist A.I. Oparin and the British scientist J.B.S. Haldane, working independently in the 1920s, proposed that the first life arose from pre-existing non-living organic molecules — RNA, proteins, polysaccharides. The formation of life, they argued, was preceded by a long phase of chemical evolution in which diverse organic molecules accumulated from inorganic precursors under the reducing conditions of the primitive earth.

The conditions are critical. The primitive atmosphere was a reducing atmosphere — methane (CH₄), ammonia (NH₃), water vapour (H₂O), and hydrogen (H₂) — with no free oxygen. Energy was abundant: ultraviolet radiation from the sun, lightning discharges, volcanic heat, and radioactive decay. Under these conditions, inorganic molecules slowly polymerised into amino acids, sugars, nitrogen bases, and fatty acids. These accumulated in the warm, shallow seas — what Haldane called the primordial soup.

Life arose slowly through evolutionary forces from non-living molecules — abiogenesis followed by biogenesis.

The Oparin-Haldane consensus, as stated in NCERT

The first non-cellular forms of life are thought to have originated about 3 billion years ago. They were giant molecules — RNA, protein, polysaccharides — possibly capable of self-replication. The first cellular forms appeared roughly 2 billion years ago, as single-celled organisms in an entirely aquatic environment. They were non-green and anaerobic; the first autotrophs were chemoautotrophs that did not release oxygen. Once formed, these cells diversified into the entire biosphere we see today — over the next two billion years.

The Urey-Miller experiment

The Oparin-Haldane hypothesis was a chain of plausible inferences until Stanley L. Miller, working in Harold Urey's laboratory at the University of Chicago in 1953, gave it a laboratory test. Miller built a closed glass apparatus and inside it created an analogue of primitive earth.

The result was decisive. From four simple gases and a spark, the building blocks of proteins formed spontaneously. Subsequent variations of the experiment, and the analysis of meteorite content, revealed that the same organic molecules form in interstellar space — indicating that chemical evolution is a universal, not a uniquely terrestrial, process. With Miller's evidence, the first part of the abiogenesis story — chemical evolution — became scientifically respectable.

Evolution of life forms — Lamarck vs Darwin

The earliest scientific theory of evolution belonged to the French naturalist Jean-Baptiste Lamarck. In 1809 he proposed that the diversity of life arose through the inheritance of acquired characters: organs that an organism used became larger over a lifetime, organs that fell into disuse atrophied, and these acquired modifications were passed to offspring. Lamarck's textbook example was the giraffe: ancestors of modern giraffes, striving to reach leaves on tall trees, stretched their necks slightly during life and passed the elongation to their young. Over generations, necks lengthened. The theory is intuitive — but wrong. Modern genetics has shown that bodily modifications acquired in the soma do not enter the germ line. No serious biologist accepts Lamarckism today.

The mechanism that did survive scrutiny belongs to Charles Darwin. From observations made during his 1831–1836 voyage on H.M.S. Beagle around the world, Darwin concluded that existing life forms share similarities not only with each other but also with life forms that existed millions of years ago, that there has been extinction of old forms and origination of new forms, and that earth itself must be very old — billions, not thousands of years. Darwin (and independently Alfred Russel Wallace, working in the Malay Archipelago) proposed natural selection as the mechanism. The argument has four observations and one inference:

Darwin's brilliance lay in synthesising four observations into one inference. Resources are limited; organisms produce more offspring than can survive; individuals vary in heritable characteristics (no two individuals are alike); therefore those variants better adapted to the local environment will leave more progeny than the less adapted. Darwin called this fitness — defined ultimately as reproductive success, not physical strength. Over many generations, the population shifts; new forms appear; species arise. The two concepts at the centre of Darwinism are branching descent (all life shares common ancestors that lived at different times) and natural selection (the differential reproductive success of variants).

Darwin was almost certainly influenced by Thomas Malthus's essay on populations: if every individual reproduced maximally, population would grow exponentially, but resources are finite, so competition is inevitable. From Malthusian arithmetic plus heritable variation, natural selection follows logically.

Evidences for evolution

Evolution is not assumed; it is supported by converging lines of evidence drawn from five major areas of biology. Each independently points to the same conclusion: today's life forms descended with modification from common ancestors.

Two additional lines of evidence supplement these. Biochemical similarity — similar proteins and gene sequences performing similar functions across diverse organisms — gives molecular confirmation of common ancestry. And artificial selection: man has bred dogs, pigeons, and crop plants into hundreds of distinct varieties within a few centuries; if humans can do this in centuries, Darwin reasoned, nature can do far more over millions of years.

Homology vs analogy — the most NEET-tested distinction

Of all confusions in this chapter, the homology–analogy distinction is the most frequently tested. NEET has asked it in 2016, 2018, 2020 and 2021 — sometimes by giving a structure and asking to classify, sometimes by giving the example and asking the direction of evolution. The trap always works the same way: homology and analogy look similar in casual language but point in opposite directions.

The peppered moth — natural selection caught in the act

England's industrial revolution provided the textbook demonstration of natural selection acting in real time. Before industrialisation (1850s), pale lichen covered the tree trunks in Manchester; collected moths of Biston betularia were overwhelmingly white-winged, camouflaged against the lichen. Dark melanic variants were picked off by birds. After industrialisation (1920s), soot blackened the tree trunks and killed the lichen; the same area's moth collection was overwhelmingly dark-winged. The white moths now stood out and were eaten; the melanics survived. No new mutation appeared — both variants were present all along — but the proportions reversed dramatically. This is industrial melanism.

The same principle, on faster time scales, explains antibiotic resistance in bacteria, pesticide resistance in insects, and herbicide resistance in weeds. Resistant variants exist in the population by chance mutation; the chemical applied by humans kills susceptible variants; resistant ones reproduce. These are examples of evolution by anthropogenic action — human intervention as the selective agent. NEET 2020 listed herbicide-resistant weeds, drug-resistant eukaryotes and domesticated dog breeds as the correct examples of anthropogenic evolution; Darwin's finches do not belong on that list because their evolution was natural.

Adaptive radiation

On the Galapagos Islands, Darwin observed something extraordinary: a single ancestral seed-eating finch had radiated, on those islands alone, into multiple distinct species with different beak shapes — some adapted to crushing seeds, others to probing for insects, some vegetarian. He realised that one geographically isolated stock could, in the absence of competition, fan out into an entire suite of species each occupying a distinct ecological niche. He named the birds Darwin's finches. The process is adaptive radiation: the evolution of different species in a given geographical area, starting from a point and literally radiating outward into different habitats.

The other classical example is Australian marsupials. When South America separated from Australia and the continents drifted apart, Australia's pouched mammals were left without placental competitors. From a single ancestral marsupial they radiated into wolves, moles, anteaters, flying squirrels, mice and bears — each ecologically equivalent to a placental mammal elsewhere in the world, but evolved independently. NEET 2023 tested this directly: numbat, spotted cuscus and flying phalanger are Australian marsupials; bobcat, mole, and flying squirrel are placental mammals from other continents.

When adaptive radiation occurs in two isolated geographical areas, producing strikingly similar end forms, the phenomenon overlays adaptive radiation onto convergent evolution. Australian marsupial wolves and placental Tasmanian wolves are the canonical pair: ecologically equivalent, anatomically similar, but evolved on different evolutionary stages. The marsupial mole resembles a placental mole, the marsupial flying squirrel resembles a placental flying squirrel — yet none share recent common ancestors.

The modern synthetic theory

Darwin's theory had one gaping hole. Darwin had no idea where heritable variation came from. He knew it existed and was inherited, but its mechanism eluded him. Mendel's work on inheritance was published in Darwin's lifetime but was overlooked by both Darwin and the wider biological community.

Two decades after Darwin's death, the Dutch botanist Hugo de Vries, working with evening primrose (Oenothera lamarckiana), described mutations — large, sudden, heritable differences that appeared without warning in his garden plants. De Vries believed it was these mutations, not Darwin's gradual variations, that caused evolution. He called the mechanism saltation — "single step large mutation." For de Vries, evolution proceeded by jumps, not slow drift.

De Vries's mutations were random and directionless; Darwin's variations were small and apparently directional. The two pictures looked irreconcilable until the work of population genetics in the early twentieth century reconciled them. By the 1940s a consensus emerged that is now called the modern synthetic theory of evolution: Darwin's natural selection acting on the heritable variation produced by Mendelian inheritance, mutation, and genetic recombination.

Evolution is the change in allele frequency in a population over time, brought about by mutation and recombination supplying variation, and natural selection, genetic drift and gene flow distributing it.

The modern synthetic theory in one sentence

The synthesis dissolves the apparent conflict. Mutations are indeed random and directionless — they supply the raw material. Natural selection is non-random and directional — it filters the raw material. The net result, played out over millions of generations, looks like the gradual, adaptive change Darwin described. NEET 2019 tested de Vries's view directly (mutations are "random and directionless") and 2018 asked his term for the mechanism (saltation).

Mechanism of evolution

Five mechanisms drive evolutionary change. They alter the frequencies of alleles in a population from one generation to the next.

Variation is the substrate. Without heritable differences between individuals, selection has nothing to act on. Variation enters the gene pool by two processes — mutation (errors in DNA replication producing new alleles) and genetic recombination (crossing-over during meiosis shuffling existing alleles into new combinations). RNA viruses, which lack DNA-repair machinery, mutate at much higher rates than DNA organisms; combined with their short generation times, this is why RNA viruses evolve so fast — a fact NEET 2023 tested as a statement-pair question.

Natural selection is the filter. Individuals carrying variants that increase survival and reproduction in the current environment leave more progeny; those alleles increase in frequency. Selection can act in three ways, depending on which part of the phenotypic distribution is favoured:

Genetic drift is change in allele frequency by pure chance — sampling error in small populations. When the population is small, the proportion of alleles in the next generation may differ from the parent generation simply because only a few parents happened to reproduce. Drift has no direction; it is random. Two special cases of drift are critical for NEET:

The bottleneck effect occurs when a population is drastically reduced — by famine, disease, or natural catastrophe — and a tiny surviving group rebuilds the population. The rebuilt population reflects only the allele frequencies of the survivors, not of the original. Cheetahs, northern elephant seals, and many endangered species pass through bottlenecks.

The founder effect occurs when a small group migrates and founds a new isolated population. The founders carry only a sample of the parent gene pool; the new population's allele frequencies reflect this sample. If the difference is large enough, the founders may diverge into a separate species. NEET 2021 asked precisely this — "the factor that leads to the founder effect is genetic drift."

Gene migration (also called gene flow) is the movement of alleles between populations through migrating individuals. Migration adds new alleles to the recipient population and removes them from the donor. Where migration is consistent, populations remain genetically connected; where it stops, populations diverge.

Hardy-Weinberg principle

In 1908, the British mathematician G.H. Hardy and the German physician Wilhelm Weinberg independently proved a deceptively simple result that became the null hypothesis of population genetics. In a large, randomly mating population, with no mutation, no migration, no selection and no drift, the frequencies of alleles and genotypes remain constant from generation to generation. The gene pool is in genetic equilibrium.

Consider a single locus with two alleles, A and a. Let p be the frequency of A and q be the frequency of a. Since these are the only alleles:

The equation gives a quick way to compute genotype frequencies if one allele frequency is known. NEET 2019 supplied a worked example: if the frequency of dominant allele A is 0.4, then q = 1 – 0.4 = 0.6, so AA = (0.4)² = 0.16, Aa = 2(0.4)(0.6) = 0.48, and aa = (0.6)² = 0.36. The numbers sum to one — a fast sanity check.

The equilibrium holds under five strict assumptions: no mutation, no migration, random mating, very large population (no drift), and no selection. Any departure from these assumptions disturbs the equilibrium. The very fact that real populations evolve means none of these assumptions is perfectly true — and that the gap between observed and predicted frequencies measures the extent of evolutionary change.

The five disturbing factors are tested directly. (1) Gene flow / migration changes frequencies in both donor and recipient populations. (2) Genetic drift alters frequencies by chance in small populations (founder effect, bottleneck effect). (3) Mutation introduces new alleles at low but non-zero rates. (4) Genetic recombination during gametogenesis produces new combinations. (5) Natural selection favours certain genotypes over others. Together, they constitute the entire mechanism of microevolution.

Speciation

The cumulative effect of these mechanisms, when allele frequencies in one population diverge sufficiently from another, is speciation — the origin of a new species. The classical biological species concept defines a species as a group of organisms that can interbreed and produce fertile offspring. When two populations of a species become reproductively isolated and accumulate enough genetic difference, they no longer interbreed even when brought back into contact — they are now separate species.

Speciation typically begins with geographic isolation. A river changes course, a mountain rises, a few individuals colonise an island — the population splits. With gene flow cut off, the two daughter populations evolve independently. Mutations arise in each separately; drift and selection act on different gene pools under different conditions. Given enough time, reproductive isolation hardens. When the populations eventually meet again, they may no longer interbreed. This is allopatric speciation — speciation through geographic separation. Darwin's finches and Australian marsupials are textbook outcomes of allopatric speciation followed by adaptive radiation.

Mutations that are advantageous in microbial experiments illustrate the same logic on a faster scale. A pre-existing advantageous mutation, when selected, produces a new phenotype in a few generations; over enough generations, this can amount to speciation. In a colony of bacteria growing on a given medium, a small subset has a heritable variation enabling it to use a new nutrient. Change the medium, and only that subset survives — within days, the new variant has outgrown the old and a new "species" (operationally) has emerged. The same speciation in a fish would take millions of years, because fish life cycles are slow.

A brief geological timeline of life

The major events of evolution unfold on a vast time scale. NEET tests the broad outlines rather than the precise dates, but a working timeline is essential for context.

• 13.8 bya: Big Bang; universe forms.
• 4.5 bya: Earth forms.
• 4 bya: First life — non-cellular, anaerobic.
• 3 bya: Giant self-replicating molecules.
• 2 bya: First cellular life — single-celled.
• 500 mya: Invertebrates active.
• 350 mya: Jawless fish; first amphibians (lobefins).
• 320 mya: Sea weeds, early plants.
• 200 mya: Reptile dominance; Ichthyosaurs.
• 65 mya: Dinosaurs go extinct; mammals rise.
• 15 mya: Dryopithecus & Ramapithecus.
• 2 mya: Australopithecus walks east African grassland.
• 1.5 mya: Homo erectus.
• 0.1 mya: Neanderthals.
• 0.075 mya: Homo sapiens arose.

Plants invaded land before animals. The first land animals were fish with stout, strong fins — lobefins — which could move on land and return to water. A famous lobefin, Coelacanth, was thought extinct until one was caught in South Africa in 1938. Lobefins gave rise to the first amphibians, which gave rise to reptiles with thick-shelled eggs that resisted drying out. Reptiles dominated for 200 million years — the dinosaurs, the largest of which (Tyrannosaurus rex) stood 20 feet tall. About 65 million years ago, the dinosaurs vanished — possibly through climate change, possibly through asteroid impact, possibly evolving into birds. The truth, NCERT notes, lies somewhere in between.

Mammals were small shrew-like animals during the reptile age; when the reptiles fell, mammals took over. Continental drift shaped the rest. South American mammals were overrun by North American fauna when the continents joined. Australian marsupials survived because no other mammals reached them. Some mammals returned to water — whales, dolphins, seals, sea cows.

Origin and evolution of man

The most discussed evolutionary lineage is our own. The story is read from fossils discovered mainly in East Africa and Java, supplemented by skull and brain-size comparisons. The sequence — Dryopithecus → Australopithecus → Homo habilis → Homo erectus → Homo neanderthalensis → Homo sapiens — should be at the fingertips of every NEET candidate, along with the brain capacities tested by NEET 2019.

Human evolution closely paralleled the evolution of the human brain and language. With each step, brain capacity expanded, tool use became more sophisticated, and behaviour grew increasingly cultural. Homo erectus ate meat and used fire. Neanderthals buried their dead. Modern Homo sapiens arose in Africa, spread across the globe, developed cave art by 18,000 years ago (the rock shelters at Bhimbetka in Madhya Pradesh preserve some of the oldest), invented agriculture around 10,000 years ago, and built the civilisations that followed. The skull of a baby chimpanzee, NCERT observes, more closely resembles the adult modern human skull than it does the skull of an adult chimpanzee — a striking signature of neoteny in our lineage.