Tiny population, neutral trait → drift. Tracks the environment → selection. Migrants → gene flow. Count copies, not individuals.
A population evolves when its allele frequencies change across generations. Four forces do that: natural selection, genetic drift, gene flow, and mutation. Most questions come down to which one is at work.
Key terms
- Allele frequency — the share of all copies of a gene in a population that are one allele. Each diploid carries two copies.
- Genetic drift — change in allele frequencies from chance alone: which alleles happen to get passed on. Small populations swing the most.
- Bottleneck and founder effect — drift in a crashed population or a new colony. A few individuals carry a chance sample of alleles.
- Gene flow — alleles moving between populations in migrants, pollen, or seeds. It makes the populations more alike.
- Mutation — the only source of brand-new alleles. It's random and rare, so on its own it shifts frequencies slowly.
- Negative frequency-dependent selection — the rarer form has higher fitness, so neither form can take over. Both persist.
Name the force
- Count alleles, not individuals. N diploids carry 2N copies. A heterozygote adds one copy of each allele; a homozygote adds two of one.
- Compare generations. If allele frequencies hold still, the population isn't evolving, even if genotype frequencies shift.
- Does the change track the environment? An allele that rises under a pressure and falls once it lifts is under selection, and costly without it.
- Is the population small and the trait neutral? Random swings, fixation, or loss point to drift.
- Are individuals, pollen, or seeds moving between populations? Gene flow pulls their allele frequencies toward each other.
Why it matters
This topic sets up Hardy-Weinberg. When none of these forces act and mating is random, allele and genotype frequencies stay put from one generation to the next. That is Hardy-Weinberg equilibrium. Many Hardy-Weinberg questions come down to which condition broke. Expect tables of allele frequencies over time next to a clue: spraying, predators, migrants, or a tiny population. Match the pattern of change to the force.
Exam traps
| Trap | Actually |
|---|
| Large populations drift more because they hold more alleles. | Drift is sampling error. Small samples miss their source by more, so small populations drift faster. |
| Gene flow makes populations more different by adding new variants. | Gene flow mixes alleles between populations, so their frequencies converge. It can add variation within one population but opposes divergence between them. |
| Selfing that halves the heterozygotes each generation changes allele frequencies. | A selfed heterozygote makes AA and aa offspring in equal numbers. Genotypes shift toward homozygotes; allele frequencies stay put. |
| An allele that rose under a pesticide will keep rising. | Resistance often carries a cost. Stop spraying and selection can reverse, pulling the allele back down. |
Worked example
On a mainland, a huge lizard population has held the throat-color allele b at a frequency of 0.50 for decades. A storm carries 20 lizards to an empty island on floating debris: 12 BB, 6 Bb, and 2 bb. Throat color doesn't affect survival or mating. What is the frequency of b among the founders, and what explains the difference from the mainland?
- Count copies. 20 diploid lizards carry 40 alleles.
- Copies of b: 6 from the Bb lizards + 2 × 2 from the bb lizards = 10. Frequency = 10/40 = 0.25.
- The founders differ from the mainland (0.25 vs 0.50), and throat color played no part in who rode the debris.
- A small random sample that misses its source's frequencies → founder effect, a form of genetic drift.
b = 0.25 among the founders. The drop from 0.50 is a founder effect: genetic drift from sampling only 20 individuals.
Small samples miss their source's proportions by chance, and nothing about throat color decided which lizards were carried off. The near miss is natural selection, but selection needs a trait that changes survival or reproduction, and the prompt rules that out. The other slip is counting lizards: 2 bb out of 20 is 0.10, a genotype frequency, not the allele frequency.