Niche evolution under competition

Whitlock's model invokes competition between species. A suite of other models have investigated how competition affects niche evolution (e.g. Slatkin 1980; Taper and Case 1985). In many of these models, a species is assumed to display a character that affects its niche use, and that this is under genetic control. The extent of competition between individuals depends on the overlap in niche use. The models run in two phases: first competition is applied and the fitness of the different phenotypes is calculated. Then quantitative genetics is applied to calculate the new phenotypes after reproduction has taken place. A carrying capacity is also specified for each environment that determines how the level of competition relates to the number of individuals using that part of the niche.

When only one species is present, it evolves so that its niche gives the highest carrying capacity under intraspecific competition (Figure 9.2). When two species are present, displacement of the trait occurs through intraspecific competition. Two general outcomes are possible: character divergence and character convergence. Character divergence occurs when the mean phenotypes of the species move away from each other, because the

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Fig. 9.2 Reduction of niche width by interspecific competition. (a) When just a single species uses a resource, the average niche position is determined by the most abundant resource type. b) A competitor species will tend to displace the resident, reducing the resident's niche width.

Type of resource Type of resource

Fig. 9.2 Reduction of niche width by interspecific competition. (a) When just a single species uses a resource, the average niche position is determined by the most abundant resource type. b) A competitor species will tend to displace the resident, reducing the resident's niche width.

fitness of the phenotypes that overlap is reduced. However, more rarely character convergence can occur. Two circumstances that favour this are (1) when one resource is essential for both species, such that it pays to dominate that resource, and (2) if one resource is particularly abundant while others are limiting.

Thus, competition between species can lead to character divergence among sympatric species, and subsequent specialization, or convergence depending on the circumstances. Of course, there is no reason why competition need be the only interspecific interaction that can affect niche evolution; organisms might, for example, become specialized because their fitness is reduced in some habitats via predation or parasitism. Historically though, such interactions have yet to receive their due share of theoretical attention.

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