Key ideas
Evolution is a change in allele frequencies over generations. Natural selection acts on heritable variation in fitness, while drift, gene flow and mutation change frequencies without regard to fitness.
- Natural selection needs variation, inheritance and differential survival or reproduction. Fitness is relative reproductive success, and the selection coefficient s is the fitness disadvantage.
- Genetic drift is a random change in allele frequency that is strongest in small populations. Founder effects and bottlenecks are special cases.
- Gene flow moves alleles between populations and reduces differences between them. Mutation is the original source of new alleles.
- A molecular clock estimates divergence time from the number of sequence differences and a known substitution rate.
Key equations
q′ = q(1 − sq) ÷ (1 − sq²)w = 1 − sdivergence time = difference ÷ (2 × rate)F = 1 − H_obs ÷ H_exp
Worked example
An allele has frequencies p = 0.6 and q = 0.4, and the recessive homozygote has a selection coefficient s = 0.5. After one generation, q′ = q(1 − sq) ÷ (1 − sq²) = 0.4 × (1 − 0.2) ÷ (1 − 0.08) = 0.32 ÷ 0.92 = 0.35. Two species differing at 6 % of sites with a divergence rate of 2 % per million years split about 3 million years ago.
Common mistakes
- Saying individuals evolve. Populations evolve, individuals do not change their alleles.
- Thinking selection creates the needed variation. It only acts on variation that is already there.
- Treating drift as selection. Drift is random with respect to fitness.
Practice
Test yourself with practice questions on this topic: Evolution and natural selection