Niche Shape and Genetic Aspects of Character Displacement

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Abstract
Character displacement involving a single, diallelic locus in each of two competing species is investigated via computer simulation. The niche, as the sum of the utilization efficiencies of the population for combinations of environmental variables, is distinguished from the resource peak, which is a set of combinations of environmental variables actually present at exploitable frequencies in the environment. The two populations compete for a single resource peak made up of five subunits. Each genotype in each species has an optimum peak subunit, but survives in all of them. The intermediate subunit is optimal for a genotype in each species. Movement is restricted between subunits, which are thus treated here as microhabitats. The simulation covers a geographical range with each species occurring alone where its birth rate is high, with the birth rate declining as the range of the other species is approached; sympatry occurs in a region of intermediate birth rates for both species. The character displacement involves characters determined by the genotypes at the competition locus. The shape of the resource peak has a profund effect on the form taken by character displacement. A “steep” peak (one with the intermediate subunit much more frequent than the rest) results in a sharp step in the morphoclines, with two stable equilibrium points at the point of equal birth rates. A “bimodal” peak (one with equally frequent subunits, each optimum for a genotype for one species only) leads to a gradual change. Other peak types also have characteristic effects, showing that ecological inferences can be drawn from the form taken by character displacement.

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