An allele that loses sometimes doesn't have to vanish
- The idea
- Predict
- See it
- Why
- Check
The idea
“If an allele does badly in some genotypes or some years, selection will wipe it out.”
Watching aa individuals die off makes it easy to picture the a allele fading out. Most copies of a rare allele aren't in aa individuals at all.
The short version
The mistake: If an allele does badly in some genotypes or some years, selection will wipe it out.
What's actually true: Selection counts every copy of an allele. When heterozygotes do best, the allele stays even if its homozygote does badly.
- Selection on an allele averages over all its copies. When a is rare, nearly every copy is paired with A, in an Aa heterozygote.
- When heterozygotes survive best, called heterozygote advantage, each allele rises when it is below a balance point and falls when above it. So both alleles stay.
- Changing conditions can also keep an allele around. A bad year lowers its frequency without erasing it, and good years can bring it back.
On the exam: Before you call an allele doomed, ask how its heterozygotes do and whether conditions change.