Mutation makes new alleles but barely moves their frequency
- The idea
- Predict
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- Why
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The idea
“When an allele gets more common, stays common, or differs between populations, mutation is the cause.”
Every new allele starts as a mutation, so it's easy to credit mutation when one gets common. Mutation is just very slow.
The short version
The mistake: When an allele gets more common, stays common, or differs between populations, mutation is the cause.
What's actually true: Mutation supplies new alleles, but at one gene it's too slow to move their frequency much. Selection, drift and gene flow do that.
- Mutation is the only source of new alleles. At one gene, it changes only about 1 in 100,000 to 1 in 1,000,000 copies per generation.
- So mutation alone takes thousands of generations to move a frequency a few percentage points.
- Fast changes come from natural selection, genetic drift in small populations, and gene flow. With selection, carriers leave more offspring and that edge compounds.
- Mutation can keep a harmful allele rare by replacing copies selection removes. If a harmful allele stays common, selection holds it there, as with heterozygote advantage.
On the exam: Mutation is slow. A big frequency change in tens of generations points to selection, drift or gene flow.