Weak selection

Weak selection in evolutionary biology is when individuals with different phenotypes possess similar fitness, i.e. one phenotype is weakly preferred over the other. Weak selection, therefore, is an evolutionary theory to explain the maintenance of multiple phenotypes in a stable population. Weak selection can only be used to explain the maintenance of mutations in a Moran process. A Moran process is one in which birth and death are paired events, and therefore population size remains constant. If the population size was increasing, both wild type and mutant phenotypes can proliferate and the weak selection for one phenotype results in no particular selection for either. Hence weak selection requires a finite population to operate. Otherwise there would be no expectation of fixation and hence no selection. The result of weak selection is two phenotypes with similar fixation probabilities. Weak selection works to elongate fixation time for two competing alleles. Consequently, weak selection provides a model for describing how evolution can occur in large steps in a population in which multiple alleles are maintained. There are two basic reasons that two phenotypes could have very similar fitness. One reason could be that the phenotypic differences between wild type and mutant are large but the significance of the mutation is minor. An example could be a change in pigmentation. Another reason could be that the phenotypic differences between wild type and mutant are actually small, such as tail length variation. In either case, the significance of the mutation, which is determined by the environment creating the selective pressure, is low in comparison to other mutations. Hence, almost near neutral mutations result in phenotypes that are weakly selected. Weak selection creates a situation in which the evolutionary dynamics governing the phenotype frequencies in a population are mainly driven by random fluctuations. Hence weak selection increases the impact of stochastic processes on the evolutionary dynamics of the trait being weakly selected.