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how does natural selection change allele frequencies
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.
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.
On sooty trees, birds spot light moths first. Say dark ones always survive to breed, light ones only eighty percent. That gap is selection.
Survivors pass on more D each generation. From one percent it climbs slowly, then fast: near half by generation twenty-four, and still rising.
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.
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.