Fitness and its role in evolutionary genetics

Abstract

Although the operation of natural selection requires that genotypes differ in fitness, some geneticists may find it easier to understand natural selection than fitness. Partly this reflects the fact that the word 'fitness' has been used to mean subtly different things. In this Review I distinguish among these meanings (for example, individual fitness, absolute fitness and relative fitness) and explain how evolutionary geneticists use fitness to predict changes in the genetic composition of populations through time. I also review the empirical study of fitness, emphasizing approaches that take advantage of recent genetic and genomic data, and I highlight important unresolved problems in understanding fitness.

Figure 1. Allele frequency versus time

The x-axis represents time in generations and the y-axis represents the frequency of the A₁ allele in a haploid species. A₁ is favored by natural selection: in the plot shown, w₁ = 1 and w₂ = 0.8. Given this fitness difference, natural selection will push A₁ to progressively higher frequencies. The curve shown is sigmoidal.

Figure 2. A three-dimensional Wrightian fitness landscape

The two axes making up the “floor” of the plot represent allele frequencies at two different loci and the z-axis rising out of the plot represents mean fitness. The fitness landscape shown has two peaks. As Wright emphasized, evolution by natural selection can get stuck on a local adaptive peak that may not represent the highest adaptive peak on the landscape.