Fitness and Selection

Abstract

Selection affects the genetic composition of a population through differences in fitness amongst genotypes. With discrete generations, fitnesses are usually measured relative to the value for some standard genotype because only relative fitnesses matter. For an autosomal locus with two alleles, when the highest fitness is associated with homozygotes for a particular allele, this allele will spread through the population. When the heterozygote has the highest fitness, a polymorphism is maintained by selection. Variation can also be maintained by frequency‚Äźdependent selection, temporal and spatial variation in fitnesses and a balance between the input of deleterious mutations and their elimination by selection. Selection theory can be extended to more complex situations such as overlapping generations. Selection can act at different levels, including selection among gametes produced by the same individual or amongst groups of individuals. Selection at one site in the genome can also influence evolution and variation at linked sites.

Key Concepts:

  • Evolution by natural or artificial selection involves differences in fitnesses amongst genotypes.

  • The fitness of a genotype measures its expected genetic contribution to the next generation.

  • Directional selection at a single diploid genetic locus results in the replacement of one allele at the locus by the alternative allele associated with the fittest homozygote.

  • Balancing selection, such as heterozygote advantage, results in the maintenance within a population of two or more alleles at a locus.

  • Mutations to deleterious alleles occur throughout the genome and contribute to variability before the alleles are removed from the population by selection.

  • Genetic drift in finite populations can cause the loss of beneficial mutations and spread of deleterious mutations.

  • Selection can act at levels of organisation either below or above that of the individual, which may generate conflicts between its effects at different levels.

  • Altruistic behaviour may evolve by kin or group selection.

  • Selection at one site in the genome influences evolution and variation at sites that are genetically linked to it.

Keywords: balancing selection; genetic conflict; heterozygote advantage; hitchhiking; kin selection; mutation rate; overlapping generations; polymorphism; selection coefficient

Figure 1.

Heterozygote advantage at a single locus with two alleles, A1 and A2. The Y axis shows the change in frequency (Δq) of A2, as a function of its frequency in the population, q (X axis). The red arrows show that the change in frequency is always in the direction of the equilibrium frequency, q*, whose position is indicated by the vertical blue line.

Figure 2.

The loss of a rare beneficial mutation from a population, as result of random sampling events. Each red circle indicates a copy of the initial mutation, which arose in generation 0. The numbers at the bottom indicate the numbers of generations after the origin of the mutation.

Figure 3.

The effects of a selective sweep (a) and background selection (b) on linked variants in the absence of recombination. Each horizontal line represents a different copy of the same chromosome in the population. Black circles indicate neutral variants, white circles beneficial mutations and red circles deleterious mutations.

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Charlesworth, Brian(Mar 2013) Fitness and Selection. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0005444.pub3]