What is Fitness?

The fitness $w$ of a genotype is its relative contribution of offspring to the next generation — survival to reproduction times fecundity, scaled so that the best genotype has $w = 1$. The selection coefficient $s$ of a genotype is its shortfall, $w = 1 - s$. Fitness is a property of a genotype in an environment, not of an allele in itself: the melanic moth’s allele had $w > 1$ relative to the pale one on sooty bark and $w < 1$ on lichen.