[_ Old Earth _] So, how do you actually know when natural selection is acting on a population?

[Barbarian]

The answer was independently found by a mathematician G.H. Hardy, and a biologist, Wilhem Weinberg. The Hardy-Weinberg principle says that in the absence of any selective pressure, and with no appreciable immigration or emigration, the next generation of genotypes for alleles with frequencies A and a, will have the distribution of: (AxA) + 2 Axa + axa.

The assumptions of Hardy-Weinberg are:

• organisms are diploid
• only sexual reproduction occurs
• generations are non overlapping
• mating is random
• population size is infinitely large
• allele frequencies are equal in the sexes
• there is no migration, mutation or selection

In population genetics, there are a good number of mathematical models that deal with cases where these assumptions do not hold. If those are controlled, or very close to the assumptions, then a deviation from the predicted distribution indicates natural selection is at work.