Hardy-Weinberg tells us what happens when nothing changes allele frequencies. Four forces do change them. Natural selection is only one.
Key idea
Allele frequencies change by mutation (new alleles), genetic drift (chance, strongest in small populations), gene flow (alleles carried in or out by migrating individuals or pollen) and natural selection (alleles that help survival and reproduction become more common).
Why it works
Drift is the law of small numbers. Flip a coin 10 times and you may get 7 heads; flip it 10,000 times and you will be very close to half. In a small population, which individuals happen to leave offspring is partly luck, so allele frequencies wander. In a large one, the luck averages out.
- Bottleneck: a disaster leaves a few survivors, whose alleles are a random sample. Hunting cut northern elephant seals to perhaps 20 to 100 animals by the 1890s. Under protection they now number well over 100,000, but they have very little genetic variation.
- Founder effect: a few individuals start a new population, such as on an island, carrying only some of the old population's alleles.
- Gene flow makes neighboring populations more alike; cutting it off lets them drift and adapt apart.
Worked example: A founder population
Ten birds blown to an island start a new population. Among their 20 alleles for a gene, 6 are a. What is q in the new population, if the mainland has q = 0.1?
- q = 6 ÷ 20 = 0.3 on the island.
- The founders were a small, chance sample of the mainland birds.
Answer: q = 0.3, three times the mainland value, by chance alone.
Worked example: Naming the force
Pollen from a field of one kind of corn blows into a neighboring field, bringing alleles the second field lacked. Which mechanism is this?
- Alleles are moving between populations.
Answer: Gene flow.
Common mistake
It is natural to think every evolutionary change is an adaptation. Drift changes frequencies by chance, with no regard to whether an allele helps. In a small population, drift can even make a harmful allele common, or fix it so every individual has it. Ask: is this change better explained by advantage, or by small numbers and luck?