Evolution and Biodiversity
Evolution and Biodiversity
This domain tests the mechanisms that change allele frequencies, the evidence for common descent, and how species arise and diversify.
Core rules
- Natural selection acts on heritable variation: individuals with traits that improve survival or reproduction leave more offspring, so allele frequencies shift.
- The main mechanisms of evolution are mutation, gene flow, genetic drift, non-random mating, and selection. Only selection consistently adapts populations to their environment.
- Hardy–Weinberg equilibrium describes a non-evolving population: p + q = 1 and p² + 2pq + q² = 1.
- Speciation can be allopatric (geographic isolation) or sympatric, for example through polyploidy or behavioural isolation.
- Homologous structures indicate common ancestry; analogous structures indicate convergent evolution.
Worked example
A recessive condition affects 4% of a population at equilibrium. q² = 0.04, so q = 0.2 and p = 0.8. Heterozygote frequency = 2pq = 2 × 0.8 × 0.2 = 0.32, or 32%. Reasoning: take the square root to find q before calculating carriers. The affected frequency is not q unless the trait is dominant.
Traps
- Individuals do not evolve; populations do.
- Hardy–Weinberg requires no mutation, no migration, no selection, random mating, and a large population.
- Genetic drift is random and has stronger effects in small populations.
- A trait being common does not prove it is adaptive; it may be neutral or correlated with another selected trait.