fraction

The passage around this formula

A basic case illustrates the kind of prediction this apparatus makes possible. For a single locus with two alleles under simple directional selection with selection coefficient s favoring an allele at frequency p , one classical discrete-generation recursion is $p_{t+1} = \frac{p_t(1+s)}{1 + s\,p_t}$. This is a deliberately simplified model — haploid, one locus, constant s , no drift, no linkage — and real populations violate several of its assumptions routinely. Its value is not as a literal forecast of any specific population but as a baseline against which departures (frequency-dependent selection, epistasis, drift-dominated dynamics in small populations) can be identified and measured.

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