EvolutionNEET Zoology · Class 12 · NCERT Chapter 6

You can expect 1 to 2 questions from Evolution in NEET 2027. The most reliable scoring areas are: the Miller-Urey experiment and what it proved, the Hardy-Weinberg principle and the five factors that disturb it, the difference between homologous and analogous organs, types of natural selection, industrial melanism as an example of natural selection, and the sequence of human ancestors from Australopithecus to Homo sapiens.

Miller and Urey (1953) showed that simple inorganic chemicals can form organic molecules without any living organism. They took a flask with water, methane (CH4), ammonia (NH3), and hydrogen (H2) to simulate early Earth conditions. They passed electric sparks (to simulate lightning) and heated the mixture. After one week, they found amino acids and other organic molecules had formed. This experiment supported Oparin and Haldane's chemical evolution theory: that the building blocks of life could arise spontaneously from non-living matter.

The Hardy-Weinberg principle says that allele and genotype frequencies in a population stay constant from generation to generation if certain conditions are met. The equation is: p² + 2pq + q² = 1, where p = frequency of dominant allele (A), q = frequency of recessive allele (a), p² = frequency of AA genotype, 2pq = frequency of Aa genotype, and q² = frequency of aa genotype. Also, p + q = 1. This equilibrium is called genetic equilibrium. The population must be large, there must be no mutation, no natural selection, random mating, and no gene flow for equilibrium to hold.

Homologous organs are structurally similar (same origin, same bones) but serve different functions. They are evidence of divergent evolution (a common ancestor). Examples: the forelimbs of humans, whales, bats, and horses. All have the same humerus-radius-ulna-carpals-phalanges pattern, but they are used for different things. Analogous organs are structurally different (different origin) but serve similar functions. They are evidence of convergent evolution (different ancestors, similar environment). Examples: the wings of a butterfly and the wings of a bird.

Lamarck proposed that characters acquired during an organism's lifetime are inherited by offspring. His classic example: giraffes stretched their necks to reach leaves, and this longer neck was passed to their children. Darwin proposed natural selection: individuals with heritable variations that suit their environment survive and reproduce more. Over generations, those traits increase in the population. Lamarck's idea was disproved because acquired characteristics (like a bodybuilder's muscles) are not encoded in DNA and cannot be inherited. Darwin's mechanism of natural selection is supported by genetics.

The five factors are: (1) Gene migration or gene flow: movement of alleles into or out of a population. (2) Genetic drift: random changes in allele frequency in small populations. (3) Founder effect: a small group of individuals establishes a new population, taking only a subset of alleles. (4) Mutation: creates new alleles and changes allele frequencies. (5) Natural selection: some alleles improve survival and reproduction, so they increase in frequency. Any one of these factors breaks the conditions for Hardy-Weinberg equilibrium and causes evolution.

The NCERT sequence is: Dryopithecus (15 million years ago, ape-like, walked on all fours) and Ramapithecus (15 million years ago, more man-like) → Australopithecus (2 million years ago, lived in east Africa, walked upright, small brain) → Homo habilis (2 million years ago, first tool user, brain 650 to 800 cc) → Homo erectus (1.5 million years ago, brain about 900 cc, used fire) → Neanderthal man (100,000 to 40,000 years ago, brain size around 1400 cc) → Homo sapiens (modern humans, about 75,000 to 10,000 years ago, cave art, agriculture). Brain size increased and posture became more upright at each stage.