Nib asked
How does natural selection change allele frequencies?
Selection changes no individual. When carriers of one allele survive and breed more, that allele becomes a bigger share of the population's gene pool each generation.
6 steps, drawn by Nib. Read it here, or watch him draw it. Updated .
1the real claim
Here's what natural selection really does. No single moth changes. The mix of moths does. Evolution is a shift in how common each allele is.
a common mix-upA moth does not adapt or change color during its life; selection changes how common each allele is across generations.
2allele frequency
Say a gene has two alleles, light d and dark D. The share of all copies that are D is its allele frequency: here, one percent.
Allele frequency = the share of all copies of a gene in the population that are one allele.
3selection
On sooty trees, birds spot light moths first. Say dark ones always survive to breed, light ones only eighty percent. That gap is selection.
4the climb
Survivors pass on more D each generation. From one percent it climbs slowly, then fast: near half by generation twenty-four, and still rising.
With dark dominant and light moths 80 percent as fit, D rises from 1 percent to about 47 percent in 24 generations.
5real moths
In Britain, dark moths rose from a few percent in eighteen forty-eight to nearly all by eighteen ninety-eight, then fell back once clean-air laws cleared the soot.
6the whole idea
So here's the whole idea: the variation was already there, the environment favors some alleles, and the population's allele frequencies shift. Individuals never change.
Selection is a filter, not a teacher: it never changes a moth, it changes which alleles the next generation inherits.