Adaptive Dynamics of Speciation: Ecological Underpinnings

Speciation occurs when a population splits into ecologically differentiated and reproductively isolated lineages. In this chapter, we focus on the ecological side of nonallopatric speciation: Under what ecological conditions is speciation promoted by natural selection? What are the appropriate tools to identify speciation-prone ecological systems? For speciation to occur, a population must have the potential to become polymorphic (i.e., it must harbor heritable variation). Moreover, this variation must be under disruptive selection that favors extreme phenotypes at the cost of intermediate ones. With disruptive selection, a genetic polymorphism can be stable only if selection is frequency dependent (Pimm 1979; see Chapter 3). Some appropriate form of frequency dependence is thus an ecological prerequisite for nonallopatric speciation. Frequency-dependent selection is ubiquitous in nature. It occurs, among many other examples, in the context of resource competition (Christiansen and Loeschcke 1980; see Box 4.1), predator–prey systems (Marrow et al. 1992), multiple habitats (Levene 1953), stochastic environments (Kisdi and Meszéna 1993; Chesson 1994), asymmetric competition (Maynard Smith and Brown 1986), mutualistic interactions (Law and Dieckmann 1998), and behavioral conflicts (Maynard Smith and Price 1973; Hofbauer and Sigmund 1990). The theory of adaptive dynamics is a framework devised to model the evolution of continuous traits driven by frequency-dependent selection. It can be applied to various ecological settings and is particularly suitable for incorporating ecological complexity. The adaptive dynamic analysis reveals the course of long-term evolution expected in a given ecological scenario and, in particular, shows whether, and under which conditions, a population is expected to evolve toward a state in which disruptive selection arises and promotes speciation. To achieve analytical tractability in ecologically complex models, many adaptive dynamic models (and much of this chapter) suppress genetic complexity with the assumption of clonally reproducing phenotypes (also referred to as strategies or traits). This enables the efficient identification of interesting features of the engendered selective pressures that deserve further analysis from a genetic perspective.