Learning Objectives
- Understand the origin of life and theories of evolution
- Learn about evidences of evolution
- Study mechanisms of evolution and Hardy-Weinberg principle
- Understand human evolution
Key Concepts
Origin of Life
Oparin-Haldane Theory: Life originated from pre-existing non-living organic molecules (chemical evolution/abiogenesis). Primitive Earth had reducing atmosphere (CH4, NH3, H2O, H2) with no free O2. Miller-Urey Experiment (1953): Simulated primitive Earth conditions; electric sparks through gases produced amino acids (glycine, alanine), sugars, and other organic molecules — supported chemical evolution theory. First non-cellular life forms: RNA molecules (RNA world hypothesis).
Theories of Evolution
Lamarckism (Inheritance of acquired characters): Use and disuse of organs causes changes that are inherited. Example: Long neck of giraffe. Disproved by Weismann (cut tails of mice for generations — no effect).
Darwinism (Natural Selection): Charles Darwin — "Origin of Species" (1859). Key ideas: overproduction, struggle for existence, variation, survival of the fittest, natural selection. Nature selects organisms best adapted to the environment. Fitness: Reproductive fitness — ability to produce more offspring.
Hugo de Vries: Mutation theory — evolution by large sudden changes (saltation) based on work with Oenothera lamarckiana (evening primrose).
Evidences of Evolution
Homologous organs: Same structural plan, different functions — indicate divergent evolution. Example: forelimbs of whale, bat, horse, human. Analogous organs: Different structural origin, similar function — indicate convergent evolution. Example: wings of butterfly and bird. Vestigial organs: Non-functional remnants (wisdom teeth, appendix, nictitating membrane in humans). Fossils: Preserved remains/impressions of ancient organisms. Archaeopteryx — link between reptiles and birds. Biogeography, embryology (Ernst Haeckel's recapitulation theory), molecular homology (DNA/protein sequence similarity) also provide evidence.
Mechanisms of Evolution
Hardy-Weinberg Principle: In a large, randomly mating population with no evolutionary forces, allele and genotype frequencies remain constant. p2 + 2pq + q2 = 1 (where p + q = 1). Factors disturbing equilibrium (causing evolution): Gene flow (migration), Genetic drift (random changes in small populations — Founder effect, Bottleneck effect), Mutation, Natural selection, Non-random mating.
Types of Natural Selection: Stabilizing (favours intermediate phenotype), Directional (favours one extreme), Disruptive (favours both extremes — may lead to speciation).
Speciation: Formation of new species. Allopatric (geographic isolation), Sympatric (reproductive isolation without geographic barrier). Adaptive radiation: Single species evolves into multiple species in different habitats. Example: Darwin's finches on Galapagos Islands, marsupials in Australia.
Human Evolution
Progression: Dryopithecus (ape-like) → Ramapithecus → Australopithecus (first hominid, bipedal) → Homo habilis (first tool maker) → Homo erectus (used fire, ate meat) → Homo neanderthalensis (large brain, buried dead) → Homo sapiens (modern humans, ~75,000-10,000 years ago). Migration from Africa to different continents. Brain size increased from 400 cc (Australopithecus) to 1400 cc (modern humans).
Summary
Life originated through chemical evolution on primitive Earth. Darwin's natural selection is the main mechanism of evolution. Evidence comes from homologous organs, fossils, embryology, and molecular biology. Hardy-Weinberg equilibrium describes conditions for no evolution. Human evolution shows progressive increase in brain size and tool use from ape-like ancestors to modern Homo sapiens.
Important Terms
- Natural selection: Differential survival and reproduction based on fitness
- Homologous organs: Same origin, different function — divergent evolution
- Analogous organs: Different origin, same function — convergent evolution
- Adaptive radiation: Evolution of multiple species from single ancestor in different niches
- Genetic drift: Random changes in allele frequency in small populations
- Hardy-Weinberg equilibrium: Constant allele frequencies in absence of evolutionary forces
- Speciation: Formation of new species from existing ones
- Founder effect: Genetic drift when small group colonizes new area
Quick Revision
- Miller-Urey: simulated primitive Earth → amino acids formed
- Darwin: natural selection, survival of the fittest, "Origin of Species" (1859)
- Homologous = divergent; Analogous = convergent evolution
- Archaeopteryx: connecting link between reptiles and birds
- Hardy-Weinberg: p2 + 2pq + q2 = 1; disturbed by mutation, selection, drift, migration
- Darwin's finches: adaptive radiation on Galapagos Islands
- Human: Australopithecus → H. habilis → H. erectus → H. sapiens
- de Vries: mutation theory (saltation) in Oenothera