CoachA population is a group of individuals of one species that interbreed; its gene pool is the set of all their alleles. For a locus with two alleles and in a diploid population of individuals, the allele frequencies are and , and the genotype frequencies are the fractions of , and . Evolution, in the narrowest sense, is a change of allele frequencies between generations; the question of this chapter is what changes them and by how much.
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Example 22.3 (Reading a frequency)
Cystic fibrosis, recessive, affects one European child in 2500: , , and the carrier frequency is — one person in 25 carries an allele that kills its homozygotes, and of the copies of the allele sit in healthy carriers where selection cannot see them. That is why a lethal recessive is removed so slowly, and why eugenic schemes to eliminate recessive diseases by sterilising the affected never could have worked: the arithmetic of the next section shows how slowly.
Example 22.12 (The moth’s arithmetic)
The melanic allele of the peppered moth is dominant. From a frequency of about in 1848 to of moths (a frequency near ) by 1895 is fifty generations; the selection equation reproduces it with between and against the pale form — and Kettlewell’s release experiments of the 1950s, in which birds took pale moths from sooty trunks and dark ones from clean trunks in about that ratio, measured the coefficient directly. The return after clean-air laws, from melanic in 1960 to under by 2000, ran at the same speed the other way. The whole episode — a large population, a dominant allele, a strong and reversible selective agent — is population genetics happening in public.