EvolutionNEET Zoology · Class 12 · NCERT Chapter 6

Introduction

Evolution is the process by which populations of organisms change over generations. It is one of the central ideas in biology and explains the enormous diversity of life on Earth.

Expect 1 to 2 NEET questions from this chapter every year. The most reliable scoring areas are: the Miller-Urey experiment, homologous vs analogous organs, the Hardy-Weinberg principle and its five disturbing factors, industrial melanism, and the human evolution sequence from Australopithecus to Homo sapiens.

Origin of Life

The origin of life asks how the first living organisms appeared on Earth. The current scientific view is that life arose from non-living chemical molecules through a gradual process of chemical evolution.

The Big Bang and Early Earth

The universe is thought to have begun about 13 to 14 billion years ago in an event called the Big Bang: an enormous explosion that created all matter and energy. The Earth formed about 4.5 billion years ago from a cloud of gas and dust. The early Earth was very hot. As it cooled, water vapour condensed to form the first oceans. The first life is thought to have appeared about 3.8 to 4 billion years ago.

Chemical Evolution: Oparin and Haldane

In the 1920s, Alexander Oparin (1924) and J.B.S. Haldane (1929) independently proposed that the early atmosphere of Earth was reducing: it contained water vapour (H2O), methane (CH4), ammonia (NH3), and hydrogen (H2), but no free oxygen. Under this reducing atmosphere, with energy from UV light or lightning, simple inorganic molecules could combine to form more complex organic molecules. Over millions of years, these molecules concentrated in the oceans to form what Haldane called a primordial soup. Eventually, self-replicating molecules appeared and life began. This is the Oparin-Haldane theory of chemical evolution.

Miller and Urey Experiment

In 1953, Stanley Miller and Harold Urey tested the Oparin-Haldane idea experimentally. They set up a sealed flask containing:

• Water (H2O), methane (CH4), ammonia (NH3), and hydrogen (H2): to simulate the early atmosphere
• A heating coil: to simulate evaporation from warm oceans
• Electric sparks: to simulate lightning
• A condenser: to cool the gases and let them cycle back

After one week, Miller and Urey found that amino acids and other organic molecules (including sugars and nitrogen bases) had formed. This proved that organic molecules can be synthesised from inorganic chemicals under conditions similar to those on early Earth. It gave strong support to the Oparin-Haldane hypothesis.

Abiogenesis vs Biogenesis

Abiogenesis (also called spontaneous generation) is the old idea that living things can arise spontaneously from non-living matter. For example, the belief that maggots grow spontaneously from rotten meat.

Biogenesis is the principle that all life comes from pre-existing life.

Louis Pasteur (1859) disproved abiogenesis with his famous swan-neck flask experiment. He showed that sterilised broth in a sealed swan-neck flask did not grow microbes, but it did grow them as soon as the neck was broken. The swan-neck shape stopped dust and microbes from the air from reaching the broth. This proved that microbial growth came from microbes in the air, not from spontaneous generation.

Note: Pasteur disproved spontaneous generation of microbes in everyday conditions, but chemical evolution on early Earth (as studied by Miller-Urey) refers to the origin of organic molecules billions of years ago, under conditions that no longer exist. These two concepts are separate.

Evidence for Evolution

Several lines of evidence show that all living organisms are related and have evolved from common ancestors over billions of years.

Palaeontological Evidence

Fossils are the preserved remains or impressions of organisms that lived in the past. The study of fossils is called palaeontology. Fossils are found in sedimentary rock layers: older fossils are deeper, newer ones are closer to the surface.

• Fossils show organisms that no longer exist, proving life has changed over time.
• The sequence of fossils in rock layers shows a progression from simple to complex organisms.
• Connecting links: fossils that show intermediate features between two groups. For example, Archaeopteryx had features of both reptiles (teeth, clawed wings, long tail) and birds (feathers), showing that birds evolved from reptiles.

Comparative Anatomy and Morphology

Comparing body structures of different organisms reveals shared ancestry.

• Homologous organs: structurally similar (same bones, same embryonic origin) but functionally different. Evidence of divergent evolution from a common ancestor. Example: forelimbs of humans (grasping), whales (swimming), bats (flying), and horses (running). All have the same humerus-radius-ulna-carpals-phalanges arrangement.
• Analogous organs: structurally different (different origin) but functionally similar. Evidence of convergent evolution: different ancestors independently evolved similar adaptations. Example: wings of a butterfly (membranous thorax outgrowth) and wings of a bird (modified forelimb).
• Vestigial organs: structures reduced and non-functional in current organisms but were functional in ancestors. Examples in humans: vermiform appendix, wisdom teeth, nictitating membrane, ear muscles, coccyx (tail bones). These indicate shared ancestry with organisms in which these were fully functional.

Embryological Evidence

Ernst Haeckel proposed the biogenetic law: "ontogeny recapitulates phylogeny," meaning an individual's embryonic development replays its evolutionary history. For example, all vertebrate embryos pass through a stage with gill slits and a tail, suggesting a common aquatic ancestor.

However, Haeckel's exact claim has been disputed and is now treated with caution. Some drawings were later found to be exaggerated. But the broad observation that early embryos of very different vertebrates look similar does still support common ancestry. This evidence is therefore considered suggestive, not conclusive on its own.

Molecular Evidence

Molecular biology provides some of the strongest evidence for evolution:

• DNA similarity: organisms that share more of their DNA sequence are more closely related. For example, humans and chimpanzees share about 98% of their DNA.
• Universal genetic code: virtually all organisms use the same codons for the same amino acids, suggesting a common origin.
• Cytochrome c protein: comparing the amino acid sequence of cytochrome c across species closely mirrors the evolutionary tree built from other evidence.

Evolution by Anthropogenic Action

Human activity can drive evolution in other species.

• Industrial melanism in the peppered moth (Biston betularia): before industrialisation in England, the light-coloured form was common because it was camouflaged on pale, lichen-covered bark. After industrialisation, soot darkened the trees. The dark (melanic) form became better camouflaged; birds ate more of the light form. Over a few decades, the dark form became the majority. When anti-pollution laws were passed and trees became pale again, the light form increased again. This is a direct demonstration of directional natural selection in action.
• Drug resistance: bacteria with random mutations that resist an antibiotic survive treatment and reproduce. Within a few generations, resistant bacteria dominate. This is natural selection driven by human use of drugs.
• Pesticide resistance: similar to drug resistance. Insects with mutations that protect them from a pesticide survive and reproduce, producing pesticide-resistant populations.

Theories of Evolution

Lamarckism

Jean-Baptiste Lamarck (1809) proposed the first major theory of evolution. He had two key ideas:

1. Use and disuse of organs: organs used frequently become stronger and larger; organs not used become weaker and may disappear.
2. Inheritance of acquired characters: changes that an organism develops during its lifetime are inherited by its offspring.

Classic example: giraffes stretched their necks to reach leaves high in trees. The longer neck was then inherited by their young.

Why Lamarckism was disproved: Acquired characteristics (a bodybuilder's muscles, a pianist's dexterity) are not encoded in DNA. Only changes in the DNA of reproductive cells (mutations) can be inherited. Experiments (e.g. cutting tails of mice for many generations did not produce short-tailed offspring) also disproved it.

Darwin's Theory of Natural Selection

Charles Darwin (and independently, Alfred Russel Wallace) proposed the theory of evolution by natural selection in 1858. Darwin published his detailed evidence in On the Origin of Species (1859). The theory has four key observations:

1. Overproduction: all organisms produce far more offspring than can survive.
2. Variation: individuals within a population show heritable variation (they differ from each other in many ways).
3. Struggle for existence: because resources are limited, not all offspring can survive. There is competition for food, space, and mates.
4. Survival of the fittest (natural selection): individuals with variations that suit the environment better survive and reproduce more. They pass their favourable traits to offspring.

Over many generations, the proportion of individuals with favourable traits increases. The population changes. If populations become isolated, they may diverge into new species.

Darwin did not know the mechanism of inheritance (Mendel's work was not yet widely known), but his core theory has been strongly supported by modern genetics.

Hardy-Weinberg Principle

The Hardy-Weinberg principle (1908) states that allele and genotype frequencies in a population remain constant from generation to generation if there is no evolution. This is called genetic equilibrium.

The equation is:

p² + 2pq + q² = 1

where:

• p = frequency of dominant allele (A)
• q = frequency of recessive allele (a), with p + q = 1
• p² = frequency of homozygous dominant genotype (AA)
• 2pq = frequency of heterozygous genotype (Aa)
• q² = frequency of homozygous recessive genotype (aa)

If you know the frequency of the recessive phenotype (q²), you can calculate q, then p, then all the genotype frequencies. This is very useful in population genetics problems.

Factors That Disturb Hardy-Weinberg Equilibrium

For equilibrium to hold, the population must be large, mating must be random, and there must be no mutation, no natural selection, and no gene flow. Any factor that breaks these conditions causes evolution (changes in allele frequency). The five main disturbing factors are:

1. Gene migration (gene flow): movement of individuals (and their alleles) into or out of a population. If individuals from a different population join, they bring different allele frequencies, changing the population's gene pool.
2. Genetic drift: in a small population, allele frequencies can change randomly just by chance. Some individuals may not reproduce, so their alleles are lost. Over time, an allele can reach 100% (fixation) or 0% (loss) by pure chance.
3. Founder effect: a special case of genetic drift. A small group of individuals colonises a new area. This founder group carries only a small sample of the original gene pool, so the new population has different (often less varied) allele frequencies.
4. Mutation: creates new alleles. Even though mutation rates are low, over many generations they add new variation to the gene pool.
5. Natural selection: if certain genotypes survive or reproduce better, their alleles increase in frequency. This is the most directional of the five factors.

Types of Natural Selection

Natural selection can act on a population in three main ways, depending on which phenotypes are favoured:

• Stabilising selection: individuals with the average (intermediate) phenotype survive best. The range of variation narrows around the mean. Example: human birth weight. Babies who are too small or too large have higher mortality; babies of average weight survive best.
• Directional selection: individuals at one extreme of the phenotypic range survive best. The population mean shifts in that direction over generations. Example: industrial melanism in the peppered moth (dark form becomes more common). Drug resistance in bacteria.
• Disruptive (diversifying) selection: individuals at both extremes survive best, but those in the middle do not. The population may split into two groups. Example: birds in a habitat with only very hard seeds or very soft seeds; those with very strong or very delicate beaks survive; intermediate beaks are less useful.

Adaptive Radiation

Adaptive radiation is the process by which a single ancestral species rapidly diversifies into many new species, each adapted to a different ecological niche. It often happens when a species colonises a new environment with many empty niches and little competition.

• Darwin's finches (Galapagos Islands): a single ancestral finch species colonised the Galapagos Islands. Different islands had different food types. Over time, finch populations evolved very different beak shapes: large strong beaks for crushing hard seeds, thin pointed beaks for catching insects, wide flat beaks for scooping cactus. About 14 species evolved from the single ancestor. This is the classic example of adaptive radiation.
• Australian marsupials: when Australia separated from other continents (about 45 million years ago), only marsupials (pouched mammals) were present. With no competition from placental mammals, marsupials diversified into many niches: kangaroos (grazers), koalas (tree-leaf eaters), Tasmanian wolves (carnivores), marsupial moles, etc. These marsupials evolved independently but look very similar to placental mammals that fill the same niches on other continents. This is also an example of convergent evolution.

Origin and Evolution of Man

Humans belong to the order Primates, family Hominidae. The human evolutionary line split from other apes about 15 to 20 million years ago. The key stages, in chronological order, are:

• Dryopithecus and Ramapithecus (about 15 million years ago): both were ape-like primates. Dryopithecus was more like modern apes (walked on all fours, arboreal). Ramapithecus was more man-like (walked more upright, lived partly on the ground, found in Africa and Asia). These are studied from fossil teeth and jaw bones.
• Australopithecus (about 2 million years ago): lived in east Africa. Walked upright on two legs (bipedal). Brain size about 400 to 600 cc. Did not make tools but used natural objects. A. africanus is sometimes called the "ape-man."
• Homo habilis (about 2 million years ago): first of the genus Homo. "Handy man." Made and used simple stone tools (Oldowan tools). Brain size 650 to 800 cc. Did not eat meat.
• Homo erectus (about 1.5 million years ago): "Upright man." Used fire. Made more sophisticated tools. Brain size about 900 cc. First hominid to migrate out of Africa into Asia (Java man, Peking man). Diet included meat.
• Neanderthal man (100,000 to 40,000 years ago): lived in Europe and western Asia. Brain size about 1400 cc (similar to modern humans). Buried their dead. Used sophisticated tools. Not directly ancestral to modern humans but lived alongside early Homo sapiens.
• Homo sapiens (modern humans, about 75,000 years ago): developed cave art, cultivated crops, domesticated animals. Brain size about 1350 to 1450 cc. Migrated all over the world. Agriculture began about 10,000 years ago.

Worked NEET Problems

Summary Cheat Sheet

• Universe age: about 13 to 14 billion years. Earth age: about 4.5 billion years. First life: about 3.8 to 4 billion years ago.
• Oparin-Haldane: early Earth had a reducing atmosphere (H2O, CH4, NH3, H2, no free O2). Organic molecules could form spontaneously under UV or lightning energy.
• Miller-Urey (1953): H2O + CH4 + NH3 + H2 + electric sparks → amino acids and organic molecules. Proved chemical evolution is possible.
• Biogenesis (Pasteur): life comes from life. Swan-neck flask experiment disproved spontaneous generation.
• Fossil evidence: preserved remains in rock layers. Older = deeper. Connecting links (e.g. Archaeopteryx: reptile + bird features).
• Homologous organs: same structure, different function. Divergent evolution. Example: forelimbs of human, whale, bat, horse.
• Analogous organs: different structure, same function. Convergent evolution. Example: wings of butterfly and bird.
• Vestigial organs: reduced, non-functional structures. Examples in humans: appendix, wisdom teeth, coccyx.
• Embryology: early vertebrate embryos look similar (gill slits, tail). Supports common ancestry. Haeckel's exact claim is treated with caution.
• Molecular evidence: DNA similarity, universal genetic code, cytochrome c comparisons.
• Industrial melanism: directional natural selection. Dark moths became common after industrialisation because trees became dark. Light moths recovered after clean-air laws.
• Lamarckism: inheritance of acquired characters (use and disuse). Disproved: acquired traits are not encoded in DNA.
• Darwin's natural selection: overproduction + heritable variation + struggle for existence + survival of the fittest. Wallace co-proposed it.
• Hardy-Weinberg equation: p² + 2pq + q² = 1. p + q = 1. Genetic equilibrium.
• 5 disturbing factors: gene flow, genetic drift, founder effect, mutation, natural selection.
• Types of selection: stabilising (intermediate favoured), directional (one extreme favoured), disruptive (both extremes favoured).
• Adaptive radiation: one ancestor → many species, each in a different niche. Darwin's finches (Galapagos), Australian marsupials.
• Human evolution sequence: Dryopithecus/Ramapithecus (15 mya) → Australopithecus (2 mya) → Homo habilis (2 mya, tools) → Homo erectus (1.5 mya, fire) → Neanderthal (100-40 kya) → Homo sapiens (75 kya).

Next: use the interactive learning widgets to explore the six types of evidence for evolution, try the Hardy-Weinberg calculator, and walk through the human evolution timeline, or work through the 16+ NEET PYQs with full solutions. To time yourself, take the free 10-question mock test.