Ecological Speciation

This chapter examines how populations in different environments can fall at different stages along a continuum of progress toward ecological speciation. It traces how variation can be used to infer ecological speciation through either of two general approaches: (1) integrated signatures of reproductive isolation based on measures of gene flow, and (2) confirmation of the ecological basis of reproductive barriers. The chapter also addresses the rapidity of ecological speciation (rapid speciation), at what point progress toward ecological speciation becomes irreversible (speciation reversal), to what extent ecological speciation is driven by competitive (adaptive speciation) or reproductive (reinforcement) interactions, and how many traits (magic traits) and selective pressures (dimensionality) are involved.

Non-adaptive speciation of snails by left-right reversal is facilitated on oceanic islands

The nearly neutral theory of molecular evolution predicts that small population size is essential for non-adaptive evolution. Evolution of whole-body left-right reversal in snails is generally a compelling example of non-adaptive speciation, because variants with reversed chirality would suffer from reduced mating opportunities within a population. Despite this reproductive disadvantage, sinistral snail species have repeatedly originated from dextral ancestors in terrestrial pulmonates. Here I show that snail speciation by reversal has been accelerated on oceanic islands. Analysing the global biogeography of 995 genera across 84 stylommatophoran families, I found that the proportion of sinistral snail genera was enhanced in genera endemic to oceanic islands. Oceanic islands are relatively small land masses offering highly fragmented habitats for snails. Thus, the upper limit of population size would probably have been small for a long time there. Oceanic islands may have facilitated the fixation of the nonadaptive allele for speciation by reversal, allowing subsequent ecological divergence of sibling species. This study illustrates the potential role of genetic drift in non-adaptive speciation on oceanic islands.

Adaptive Diversification Due to Resource Competition in Sexual Models

This chapter examines evolutionary branching in sexual populations. As sexual populations converge to what would be a branching point in clonal models, splitting obviously becomes a problem, because mating between different marginal phenotypes generally creates intermediate phenotypes. Through segregation and recombination, sexual reproduction can prevent the establishment of diverging phenotypic clusters in randomly mating populations. To allow for a phenotypic split, mating needs to be assortative with respect to the ecological trait that is under disruptive selection. Thus, the question of evolutionary branching in sexual populations, that is, of adaptive speciation, is intimately tied to questions about the evolution of assortative mating. If evolutionary branching occurs in sexual populations due to the presence of assortative mating mechanisms, the diverging phenotypic clusters will show prezygotic reproductive isolation at least to some extent, and hence they can be viewed as representing incipient species.