Fitness: reproduction, not strength
Of all the forces in this track, natural selection is the only one that consistently pushes frequencies in a particular direction. Its engine is fitness — and fitness is not about being big or fierce. It is simply the expected number of surviving, reproducing offspring a genotype leaves behind, relative to others. An allele that even slightly raises that number becomes more common over time.
We measure selection with a selection coefficient s: a genotype with relative fitness 1 − s leaves a fraction (1 − s) as many offspring as the best genotype. A small s (say 0.01) is a gentle, slow pressure; a large s is harsh. Even tiny coefficients, compounded over many generations, can drive an allele to high frequency or to fixation.
Directional selection against a recessive aa, one generation
Start: p = freq(A) = 0.5, q = freq(a) = 0.5
Fitness: AA = 1, Aa = 1, aa = 1 - s, with s = 0.2
Genotype freq fitness contribution
AA 0.25 1.0 0.250
Aa 0.50 1.0 0.500
aa 0.25 0.8 0.200
------------------------------------------
mean fitness (w-bar) = 0.950
New allele freq of a:
q' = [ (0.50)*(1.0)*0.5 + (0.25)*(0.8) ] / 0.950
= [ 0.25 + 0.20 ] / 0.950
= 0.45 / 0.950
= 0.4737
q fell from 0.500 to 0.474 in a single generation.When selection preserves variation
Selection doesn't always purge alleles. Balancing selection actively *maintains* a polymorphism. The classic case is heterozygote advantage: when the Aa heterozygote is fitter than either homozygote, selection keeps both A and a in the pool, because eliminating either would reduce the supply of the favored heterozygotes.
The textbook example is sickle-cell: in regions where malaria has been common, carriers (one sickle allele, one typical) gain partial malaria resistance, while having two sickle alleles causes serious disease. Selection therefore keeps the sickle allele at a moderate frequency rather than removing it — a sobering example of how an allele harmful in one form can persist because another form helps survival.
Mutation–selection balance
If selection removes a harmful allele every generation, why does any harmful allele still exist? Because mutation keeps regenerating it. Mutation–selection balance is the steady state where the rate at which new harmful alleles appear by mutation exactly offsets the rate at which selection removes them. The allele never disappears; it settles at a low equilibrium frequency.