Natural Selection
What you must understand
Evolution is change in allele frequencies in a population over time. Natural selection acts on heritable variation; individuals with favorable traits reproduce more, shifting allele frequencies. Selection acts on phenotypes, not directly on genotypes, and populations-not individuals-evolve.
Mechanisms of evolution include natural selection, mutation, genetic drift (bottleneck and founder effects), and gene flow. Hardy-Weinberg equilibrium describes a non-evolving population; deviations indicate evolution. Conditions: no mutation, no migration, large population, random mating, no selection.
Speciation occurs when populations become reproductively isolated (geographic or behavioral). Evidence for evolution includes fossils, homologous structures, and molecular (DNA/protein) similarities.
Traps
- Individuals do not evolve; populations do.
- Natural selection acts on existing variation; it does not create variation on demand.
- Genetic drift is random and strongest in small populations.
- Hardy-Weinberg describes a hypothetical non-evolving population, not a real one.
Rule of thumb
If a question mentions a small population or a disaster reducing population size, suspect genetic drift (bottleneck).
Worked example
If p = 0.6, then q = 0.4. Heterozygote frequency = 2pq = 2(0.6)(0.4) = 0.48.