Fitness (biology) - Fitness Is A Propensity

Fitness Is A Propensity

Fitness is often defined as a propensity or probability, rather than the actual number of offspring. For example, according to Maynard Smith, "Fitness is a property, not of an individual, but of a class of individuals — for example homozygous for allele A at a particular locus. Thus the phrase ’expected number of offspring’ means the average number, not the number produced by some one individual. If the first human infant with a gene for levitation were struck by lightning in its pram, this would not prove the new genotype to have low fitness, but only that the particular child was unlucky." Equivalently, "the fitness of the individual - having an array x of phenotypes — is the probability, s(x), that the individual will be included among the group selected as parents of the next generation."