Evolution
Evolutionary biology is the study of the history of life forms on earth.
Origin of life
- The Big Bang theory explains the origin of the universe (about 13.8 billion years old). Hydrogen and Helium formed, gases condensed into galaxies; earth formed about 4.5 billion years ago.
- Early earth had a reducing atmosphere (water vapour, CH4, CO2, NH3) and no free oxygen.
- Panspermia: idea that life (spores) came from outer space.
- Louis Pasteur disproved spontaneous generation, showing life comes only from pre-existing life.
- Oparin (Russia) and Haldane (England): first life arose from non-living organic molecules, preceded by chemical evolution.
- S.L. Miller (1953): passed electric discharge through CH4, H2, NH3 and water vapour at 800 C and obtained amino acids, supporting chemical evolution.
Theories and Darwin
- Special creation: all species created as such, diversity unchanging, earth about 4000 years old (challenged in the 19th century).
- Charles Darwin (voyage on H.M.S. Beagle) proposed natural selection and branching descent. Fitness = reproductive fitness.
- Alfred Wallace reached similar conclusions independently. Thomas Malthus (populations) influenced Darwin.
- Lamarck: evolution by use and disuse and inheritance of acquired characters (giraffe neck) - no longer believed.
- Hugo deVries: mutations (saltation) cause evolution; mutations are random and directionless, Darwinian variations are small and directional.
Evidences for evolution
- Paleontological: fossils in dated rock sediments.
- Embryological: Heckel's vestigial gill slits in vertebrate embryos (von Baer noted embryos never pass through adult stages of other animals).
- Comparative anatomy: homologous organs (same structure, different function; divergent evolution; e.g., forelimbs of whale, bat, cheetah, human; thorn/tendril of Bougainvillea/Cucurbita) vs analogous organs (different structure, same function; convergent evolution; e.g., wings of butterfly and bird, eye of octopus and mammal, flippers of penguin and dolphin, sweet potato and potato).
- Molecular: similar genes/proteins point to common ancestry.
- Natural selection in action: industrial melanism in peppered moths; antibiotic/pesticide resistance (evolution by anthropogenic action). Evolution is a stochastic, not directed, process.
Adaptive radiation
- Evolution of many species from one ancestral stock in a geographical area: Darwin's finches (Galapagos) and Australian marsupials. Two radiations producing similar forms (placental wolf vs Tasmanian wolf) = convergent evolution.
Hardy-Weinberg principle
- Allele frequencies are stable and constant generation to generation (genetic equilibrium). Sum of allele frequencies = 1.
- p2 + 2pq + q2 = 1 (binomial expansion of (p+q)2): AA = p2, Aa = 2pq, aa = q2.
- Five factors disturb equilibrium (cause evolution): gene flow (gene migration), genetic drift, mutation, genetic recombination, natural selection. Founder effect is drift in a small founding population.
- Types of natural selection: stabilising (favours mean), directional (shifts away from mean), disruptive (favours both extremes).
Human evolution
- Dryopithecus (ape-like) and Ramapithecus (man-like) ~15 mya -> Australopithecus -> Homo habilis (650-800 cc) -> Homo erectus (~900 cc) -> Neanderthal man (~1400 cc) -> Homo sapiens.