Evolution — Short Notes
Evolution = change in gene frequencies of populations over generations, producing new species and adaptations.
Origin of Life
- Big Bang — ~13.7 billion years ago.
- Earth formed ~4.5 bya; primordial atmosphere: NH₃, CH₄, H₂O, H₂ (reducing, no free O₂).
- Oparin & Haldane hypothesis — first life formed from abiotic synthesis of organic molecules.
- Miller & Urey (1953) — recreated primitive atmosphere in a flask with electric discharge → produced amino acids → validated abiotic synthesis.
- First cellular life: ~3.5 bya; anaerobic; photosynthesis evolved by cyanobacteria, adding O₂.
Evidence for Evolution
- Palaeontology (fossils) — record of extinct organisms; e.g. horse evolution (Eohippus → Equus).
- Comparative anatomy:
- Homologous organs — same origin, different function (mammal forelimbs, e.g. human arm & whale flipper) → divergent evolution.
- Analogous organs — different origin, same function (wings of butterfly & bird) → convergent evolution.
- Vestigial organs — leftover, no function (human appendix, wisdom teeth).
- Embryology — Haeckel (later refuted): "ontogeny recapitulates phylogeny"; early embryos of vertebrates similar.
- Molecular biology — DNA/protein sequence similarities show common descent.
- Biogeography — Darwin's finches on the Galapagos.
- Direct observation — bacterial antibiotic resistance, industrial melanism in Biston betularia.
Theories
Lamarck (1809) — Inheritance of acquired characters
- Use and disuse of organs; changes acquired in life pass to offspring. Rejected.
Darwin (1859) — Natural selection
- Voyage of the Beagle; observations of Galapagos finches.
- Key ideas: variation exists; struggle for existence; survival of the fittest; heritable variations accumulate → new species.
- Wallace proposed similar ideas independently.
Modern synthesis (Neo-Darwinism)
- Combines Darwin + Mendel + population genetics + mutations.
- Sources of variation: mutations + recombination + gene flow.
Hardy–Weinberg Principle
- Allele frequencies remain constant in a population if no evolutionary forces act.
- p² + 2pq + q² = 1 (p, q = allele frequencies).
- Assumptions: no mutation, no migration, no selection, no genetic drift, random mating, large population.
- Deviations from HW indicate evolution in progress.
Factors Affecting Allele Frequency
- Gene mutation — new alleles.
- Genetic recombination — during meiosis.
- Genetic drift — random changes; strong in small populations. Bottleneck effect, founder effect.
- Natural selection — differential reproduction.
- Migration (gene flow) — alleles moving between populations.
Types of Natural Selection
- Stabilising — favours average phenotype (e.g. human birth weight).
- Directional — shifts population toward one extreme (e.g. peppered moth in polluted areas).
- Disruptive — favours both extremes over the mean (e.g. finches with big or small beaks).
Speciation
Formation of new species. Requires reproductive isolation.
- Allopatric — geographical separation (Darwin's finches).
- Sympatric — same area, no physical barrier (polyploidy in plants).
Human Evolution (in brief)
- Common ancestor with apes ~15 mya (Dryopithecus, Ramapithecus).
- Australopithecus (~4 mya) — bipedal, small brain (~500 cc), Africa.
- Homo habilis (~2 mya) — 650–800 cc, first stone tools, "handy man".
- Homo erectus (~1.5 mya) — 900 cc, used fire, walked upright.
- Homo neanderthalensis — Europe, 1400 cc, burying dead.
- Homo sapiens — evolved in Africa, spread globally ~75,000–10,000 ya. Brain ~1350 cc.
Adaptive Radiation
Evolution of diverse forms from a common ancestral stock, adapting to different niches:
- Darwin's finches — different beak shapes on Galapagos.
- Australian marsupials — many forms from one ancestor.
Take-aways
- Evolution = descent with modification, driven by variation + differential reproduction.
- Multiple lines of evidence (fossil, anatomical, molecular, biogeographic, observational) support it.
- Hardy-Weinberg gives a null hypothesis; deviations reveal evolutionary pressures.
- Human evolution is a mosaic of tool use, fire, brain expansion, and migration out of Africa.
Evolution — Short Notes
Evolution = change in gene frequencies of populations over generations, producing new species and adaptations.
Origin of Life
- Big Bang — ~13.7 billion years ago.
- Earth formed ~4.5 bya; primordial atmosphere: NH₃, CH₄, H₂O, H₂ (reducing, no free O₂).
- Oparin & Haldane hypothesis — first life formed from abiotic synthesis of organic molecules.
- Miller & Urey (1953) — recreated primitive atmosphere in a flask with electric discharge → produced amino acids → validated abiotic synthesis.
- First cellular life: ~3.5 bya; anaerobic; photosynthesis evolved by cyanobacteria, adding O₂.
Evidence for Evolution
- Palaeontology (fossils) — record of extinct organisms; e.g. horse evolution (Eohippus → Equus).
- Comparative anatomy:
- Homologous organs — same origin, different function (mammal forelimbs, e.g. human arm & whale flipper) → divergent evolution.
- Analogous organs — different origin, same function (wings of butterfly & bird) → convergent evolution.
- Vestigial organs — leftover, no function (human appendix, wisdom teeth).
- Embryology — Haeckel (later refuted): "ontogeny recapitulates phylogeny"; early embryos of vertebrates similar.
- Molecular biology — DNA/protein sequence similarities show common descent.
- Biogeography — Darwin's finches on the Galapagos.
- Direct observation — bacterial antibiotic resistance, industrial melanism in Biston betularia.
Theories
Lamarck (1809) — Inheritance of acquired characters
- Use and disuse of organs; changes acquired in life pass to offspring. Rejected.
Darwin (1859) — Natural selection
- Voyage of the Beagle; observations of Galapagos finches.
- Key ideas: variation exists; struggle for existence; survival of the fittest; heritable variations accumulate → new species.
- Wallace proposed similar ideas independently.
Modern synthesis (Neo-Darwinism)
- Combines Darwin + Mendel + population genetics + mutations.
- Sources of variation: mutations + recombination + gene flow.
Hardy–Weinberg Principle
- Allele frequencies remain constant in a population if no evolutionary forces act.
- p² + 2pq + q² = 1 (p, q = allele frequencies).
- Assumptions: no mutation, no migration, no selection, no genetic drift, random mating, large population.
- Deviations from HW indicate evolution in progress.
Factors Affecting Allele Frequency
- Gene mutation — new alleles.
- Genetic recombination — during meiosis.
- Genetic drift — random changes; strong in small populations. Bottleneck effect, founder effect.
- Natural selection — differential reproduction.
- Migration (gene flow) — alleles moving between populations.
Types of Natural Selection
- Stabilising — favours average phenotype (e.g. human birth weight).
- Directional — shifts population toward one extreme (e.g. peppered moth in polluted areas).
- Disruptive — favours both extremes over the mean (e.g. finches with big or small beaks).
Speciation
Formation of new species. Requires reproductive isolation.
- Allopatric — geographical separation (Darwin's finches).
- Sympatric — same area, no physical barrier (polyploidy in plants).
Human Evolution (in brief)
- Common ancestor with apes ~15 mya (Dryopithecus, Ramapithecus).
- Australopithecus (~4 mya) — bipedal, small brain (~500 cc), Africa.
- Homo habilis (~2 mya) — 650–800 cc, first stone tools, "handy man".
- Homo erectus (~1.5 mya) — 900 cc, used fire, walked upright.
- Homo neanderthalensis — Europe, 1400 cc, burying dead.
- Homo sapiens — evolved in Africa, spread globally ~75,000–10,000 ya. Brain ~1350 cc.
Adaptive Radiation
Evolution of diverse forms from a common ancestral stock, adapting to different niches:
- Darwin's finches — different beak shapes on Galapagos.
- Australian marsupials — many forms from one ancestor.
Take-aways
- Evolution = descent with modification, driven by variation + differential reproduction.
- Multiple lines of evidence (fossil, anatomical, molecular, biogeographic, observational) support it.
- Hardy-Weinberg gives a null hypothesis; deviations reveal evolutionary pressures.
- Human evolution is a mosaic of tool use, fire, brain expansion, and migration out of Africa.