Invasion of vacant niches and subsequent sympatric speciation.

An individual-based simulation study was conducted to examine the population dynamics of 'invasion of a vacant niche' and subsequent speciation (by reproductive isolation) when food resources are randomly distributed spatially within the habitat and the frequencies of different food types are bimodally distributed (i.e. smaller and larger sizes of food being most abundant). The initially vacant niche was that of unused larger sizes of food. When phenotypic variation for resource use (i.e. food sizes) was small in the initial population, and each female could choose a mate from anywhere in the habitat, the population could not invade the vacant niche. But when the dispersal distance of the offspring and the area within which a female could choose a mate were small (i.e. the genetic neighbourhood size was small), the population could, in most cases, evolve to use both smaller and larger food sizes and form sister species sympatrically, with each species utilizing one of the two niches (small and large sizes of food). When phenotypic variation in resource use in the initial population was large, the population could, in most cases, invade the vacant niche by evolving to use both smaller and larger sizes of food. The probability of speciation increased as the dispersal distance of offspring decreased. The results indicate that populations whose individuals have small Wright's genetic neighbourhoods may often exploit a vacant niche and diversify sympatrically in the process.