Models for Peripheral Populations: The Role of Immigration
The movement of organisms over space has manifold consequences for both the ecology (Levin, 1976; McMurtrie, 1978) and genetics (Endler, 1977; Karlin, 1982) of populations. In this paper I examine how the rate of immigration influences the size, stability and genetic composition of a peripheral population. I contrast two classes of discrete-generation population models. (Comparable continuous-time models have been discussed elsewhere (Holt, 1983).) In the first, immigrants are genetically identical to residents, whereas in the second, immigrants differ at a haploid locus and are less fit in the local environment. For both we can ask how the rate of immigration affects equilibrial population density, N*, local stability, and the pattern of fluctuations around N* in unstable populations. For the genetic model we must also determine conditions for the persistence of a polymorphism. The study of the maintenance of pockets of local adaptation in the face of gene flow is a classical problem in population genetic theory (Haldane, 1930; Nagylaki, 1977). Here I place this problem into an ecological context. If selection coefficients are functions of population density, and immigration can change population size, I show that we cannot understand the ecological consequences of immigration without also understanding the genetic consequences.
KeywordsPeripheral Population Unique Positive Equilibrium Polymorphic Population Population Genetic Theory Unstable Population
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