Evolution traces how life began on early Earth from simple molecules and then diversified over billions of years through natural selection and other forces. It covers the origin of life (Oparin-Haldane chemical evolution, the Miller-Urey experiment), the evidences for evolution, the theories of Lamarck, Darwin and Neo-Darwinism, the Hardy-Weinberg principle, speciation and human evolution. It is a high-yield, fact-heavy chapter that consistently contributes several NEET questions every year.
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Origin of Life
Life arose from simple molecules on early Earth, not all at once and not from outer space.
Earth ~4.5 bya; life in water ~3.8 bya.
- Pasteur disproved spontaneous generation (swan-neck flask)
- Oparin–Haldane proposed chemical evolution from inorganic molecules
- Sequence: chemical evolution → coacervates/protobionts → first cell
Miller–Urey Experiment
A lab spark proved organic molecules can form abiotically from the early atmosphere.
Miller & Urey, 1953; experimental support for Oparin–Haldane.
- Closed flask with CH4, NH3, H2 and water vapour
- Electric discharge mimicked lightning/UV energy
- Produced amino acids — building blocks of life
Evidences for Evolution
Several independent lines of proof show that evolution actually happened.
Homology = common ancestry; analogy = common function.
- Fossils show changing forms over rock layers (Eohippus → Equus)
- Homologous organs: same structure, different function (whale, bat, human forelimbs)
- Analogous organs: different structure, same function (butterfly vs bird wing)
Lamarckism vs Darwinism
Two big theories of how evolution happens — one rejected, one foundational.
Fitness = reproductive success. Wallace independently proposed natural selection.
- Lamarck: inheritance of acquired characters + use & disuse (disproved by Weismann)
- Darwin (Origin of Species, 1859): branching descent + natural selection
- Variations favourable to the environment are selected and passed on
Mutation Theory & Neo-Darwinism
Genetics combined with natural selection to complete the modern picture.
de Vries’ mutations: random, large (saltation); Darwin’s variations: small, directional.
- de Vries gave the Mutation theory from Oenothera (evening primrose)
- Variation arises by mutation + recombination
- Selection types: stabilising, directional, disruptive
Hardy–Weinberg Principle
Allele frequencies stay constant across generations when no evolutionary force acts.
p2 = homozygous dominant, 2pq = heterozygous, q2 = homozygous recessive.
- Holds only if: large population, random mating, no mutation, no gene flow, no selection
- Any disturbance changes allele frequencies = evolution
- Deviation from expected frequency measures the extent of evolutionary change
Factors That Cause Evolution
Five forces break Hardy–Weinberg equilibrium and drive allele frequencies to change.
Founder effect = an extreme form of genetic drift in small populations.
- Genetic drift = random change in small populations
- Gene migration (gene flow) shifts alleles between populations
- Natural selection, mutation and recombination also alter frequencies
Adaptive Radiation & Speciation
Many new species evolve from one ancestor by adapting to different niches.
Two radiations giving similar forms in isolated areas = convergent evolution.
- Darwin’s finches (Galapagos): one ancestor → many beak types
- Australian marsupials radiated from a common ancestor
- Drives formation of new species (speciation)
Human Evolution
Hominids evolved with increasing brain size, upright posture and tool use.
Trend: rising cranial capacity + erect bipedal posture.
- H. habilis = first tool-maker (~650–800 cc); H. erectus (~900 cc) probably ate meat
- Neanderthal (~1400 cc) used hides and buried the dead
- H. sapiens arose in Africa; Cro-Magnon is an early modern form
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Frequently Asked Questions
Evolution is the gradual change in the heritable characteristics of populations over successive generations, leading to the diversity of life from common ancestors. It is driven by natural selection acting on variation, along with mutation, genetic drift, gene flow and recombination.
The Hardy-Weinberg principle states that allele frequencies in a population stay constant across generations when no evolutionary force acts. For two alleles with frequencies p and q, p plus q equals 1 and p squared plus 2pq plus q squared equals 1, where p squared is homozygous dominant, 2pq is heterozygous and q squared is homozygous recessive.
Yes, Evolution is part of the NEET syllabus and a scoring chapter, usually giving 2 to 3 fact-based questions per year. Most questions are direct recall, such as Miller-Urey, Hardy-Weinberg, human evolution sequence and homologous versus analogous organs, so it rewards careful NCERT reading.
Homologous organs have the same basic structure but different functions and arise from common ancestry, showing divergent evolution, like the forelimbs of a whale, bat and human. Analogous organs have different structures but the same function and result from convergent evolution, like the wings of a butterfly and a bird.
Lamarckism explains evolution by inheritance of acquired characters and use and disuse of organs, a view later disproved by Weismann. Darwinism explains it by natural selection, where overproduction leads to a struggle for existence and survival of the fittest, with favourable variations being selected and passed on.