Abstract. Geographic variation may ultimately lead to the splitting of a subdivided population into reproductively isolated units in spite of migration. Here, we consider how the waiting time until the first split and its location depend on different evolutionary factors including mutation, migration, random genetic drift, genetic architecture, and the geometric structure of the habitat. We perform large-scale, individual-based simulations using a simple model of reproductive isolation based on a classical view that reproductive isolation evolves as a by-product of genetic divergence. We show that rapid parapatric speciation on the time scale of a few hundred to a few thousand generations is plausible even when neighboring subpopulations exchange several individuals each generation. Divergent selection for local adaptation is not required for rapid speciation. Our results substantiates the claims that species with smaller range sizes (which are characterized by smaller local densities and reduced dispersal ability) should have higher speciation rates. If mutation rate is small, local abundances are low, or substantial genetic changes are required for reproductive isolation, then central populations should be the place where most splits take place. With high mutation rates, high local densities, or with moderate genetic changes sufficient for reproductive isolation, speciation events are expected to involve mainly peripheral populations.Key words. Centrifugal speciation, holey adaptive landscapes, mathematical modeling, parapatric, peripatric.Received March 19, 1999. Accepted January 27, 2000.The geographic range sizes of most species are much larger than the typical dispersal distances of individuals (or gametes). This creates an opportunity for the generation and maintenance of extensive genetic differences among geographic populations of the same species in spite of migration. Several factors contribute to these processes. Because each specific mutation is a very rare event, whereas the number of possible mutations is enormous, different mutations will appear in different geographic areas. Mani and Clarke (1990) refer to this factor as ''mutational order.'' Additional divergence will be created by stochastic factors affecting survival and reproduction, which are commonly referred to as ''genetic drift.'' Finally, variation in abiotic and biotic conditions can result in systematic differences in selection regimes that operate in different parts of the species range to augment geographic differentiation. Extensive geographic variation is well documented for most species (e.g., Endler 1977;Avise 1994). A classical example of extreme geographic differentiation are ''ring species'' (e.g., Mayr 1942Mayr , 1963Wake 1997), where genetic differences between some neighboring populations result in strong reproductive isolation.Generation of geographic variation is a necessary step in most scenarios of speciation. Here, we concentrate on parapatric speciation, that is, speciation with some gene flow between neighboring su...
A classical view of speciation is that reproductive isolation arises as a by-product of genetic divergence. Here, individual-based simulations are used to evaluate whether the mechanisms implied by this view may result in rapid speciation if the only source of genetic divergence are mutation and random genetic drift. Distinctive features of the simulations are the consideration of the complete process of speciation (from initiation until completion), and of a large number of loci, which was only one order of magnitude smaller than that of bacteria. It is demonstrated that rapid speciation on the time scale of hundreds of generations is plausible without the need for extreme founder events, complete geographic isolation, the existence of distinct adaptive peaks or selection for local adaptation. The plausibility of speciation is enhanced by population subdivision. Simultaneous emergence of more than two new species from a subdivided population is highly probable. Numerical examples relevant to the theory of centrifugal speciation and to the conjectures about the fate of "ring species" and "sexual continuums" are presented.
Sexual conflict has been suggested as a general cause of genetic diversification in reproductive characters, and as a possible cause of speciation. We use individual-based simulations to study the dynamics of sexual conflict in an isolated diploid population with no spatial structure. To explore the effects of genetic details, we consider two different types of interlocus interaction between female and male traits, and three different types of intra-locus interaction. In the simulations, sexual conflict resulted in at least the following five regimes: (1) continuous coevolutionary chase, (2) evolution toward an equilibrium, (3) cyclic coevolution, (4) extensive genetic differentiation in female traits/genes only, and (5) extensive genetic differentiation in both male and female traits/genes. Genetic differentiation was hardly observed when the traits involved in reproduction were determined additively and interacted in a trait-by-trait way. When the traits interacted in a component-by-component way, genetic differentiation was frequently observed under relatively broad conditions. The likelihood of genetic differentiation largely depended on the number of loci and the type of within-locus dominance. With multiple loci per trait, genetic differentiation was often observed but sympatric speciation was typically hindered by recombination. Sympatric speciation was possible but only under restrictive conditions. Our simulations also highlight the importance of stochastic effects in the dynamics of sexual conflict.
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