Evolution and Adaptation
Evolution and Adaptation tests the evidence for evolution, the mechanisms that change allele frequencies, and the adaptations organisms show to their environments.
Origins and evidence
- Chemical evolution is associated with Oparin and Haldane; the Miller–Urey experiment produced amino acids from simple gases.
- Evidence includes fossils, homologous structures (same origin, different function), analogous structures (different origin, same function), embryology, biogeography and molecular comparisons such as cytochrome c.
- Lamarck proposed use and disuse and inheritance of acquired characters; Darwin and Wallace proposed natural selection acting on heritable variation.
Population genetics
- The modern synthesis combines genetics with natural selection.
- Hardy–Weinberg equilibrium: p + q = 1 and p² + 2pq + q² = 1, where p and q are allele frequencies. It holds only with no mutation, no migration, random mating, a large population and no selection.
- Selection may be directional, stabilising or disruptive. Industrial melanism in Biston betularia is a classic directional example; sickle-cell heterozygotes have an advantage where malaria is common.
- Speciation may be allopatric or sympatric and requires reproductive isolation. Adaptive radiation produced Darwin's finches.
Human evolution
Australopithecus, Homo habilis, Homo erectus and Homo sapiens show increasing brain size and tool use.
Worked example
A recessive phenotype appears in 16% of a population. Then q² = 0.16, so q = 0.4 and p = 0.6. Heterozygote frequency = 2pq = 2 × 0.6 × 0.4 = 0.48, or 48%. The reasoning is that the recessive phenotype frequency gives q² directly, not q.
Traps
Homologous and analogous structures are often confused; Lamarck is not Darwin; the recessive phenotype frequency is q²; heterozygote advantage maintains a harmful allele; 'fitness' means reproductive success, not physical strength.