ABSTRACT
The dispersal of organisms is clearly an important aspect of many ecological processes. It drives biological invasions, allows populations to colonize empty habitats, and allows individuals to track resources and avoid predators or competitors. It plays a significant role of the life histories of many organisms. Yet, despite the fact that
dispersal is ubiquitous, our understanding of its evolutionary causes and ecological effects is still quite limited. In their introduction to the book “Dispersal” (Clobert et al., 2001), the editors remark that “dispersal is probably the most important life history trait involved in both species persistence and evolution” and that “One of the most studied yet least understood concepts in ecology and evolutionary biology is the movement of individuals, propagules, and genes.” There are a number of factors that can influence the evolution of dispersal, and correspondingly there are a number of different modeling approaches that have been used to study it. Factors that are commonly invoked to explain the evolution of dispersal can be either genetic or ecological (Gandon and Michalakis, 2001). Genetic factors include kin selection, i.e., reduction of competition between related individuals (Hamilton and May, 1977), and avoidance of inbreeding (Gandon, 1999). The main ecological factors involve environmental heterogeneity in time and/or space (McPeek and Holt, 1992). In the present article we will focus our attention on ecological factors, especially spatial heterogeneity. Most of the analysis of the ecological aspects of the evolution of dispersal has been based on ecological models rather than explicitly evolutionary models. Evolutionary conclusions typically have been drawn from ecological models by means of the notion of evolutionarily stable strategies. A strategy is said to be evolutionarily stable if a population using it cannot be invaded by a small population using any other strategy. The idea is that the strategies observed in natural systems are those that are evolutionarily stable, because they can resist invasion. If two strategies are compared and the first is found to be evolutionarily stable relative to invasion by the second while the second is not evolutionarily stable with respect to the first then the interpretation is that the first should be able to invade and displace the second. On the other hand, if neither strategy is evolutionarily stable with respect to the other then each can invade the system when rare and hence they may be expected to coexist in some sort of stable polymorphism. (The theory of uniform persistence or permanence gives a rigorous mathematical formulation for this idea; see Hutson and Schmitt (1992).) Most of the analysis we will describe in this article is motivated by the idea of evolutionary stability.
