Speciation by local adaptation and isolation by distance in extended environments

Abstract

Speciation is often associated with geographical barriers that limit gene flow. However, species can also emerge in continuous homogeneous environments through isolation by distance. When the environment is not homogeneous, natural selection contributes to differentiation by local adaptation and tends to facilitate speciation. To explore how isolation by distance and adaptation combine to determine species diversity, we implemented a model regulated by these two components. The first is implemented via mating restrictions on spatial proximity and genetic similarity. The second is realized by an ecological phenotype subjected to adaptation by natural selection. We consider scenarios where the environment is either homogeneous, with a single ecological optimum, or heterogeneous with two distinct optima. We show that the interplay between selection and isolation by distance affect not only species formation but also phenotypic distributions and speed of speciation. In homogeneous environment, speciation occurs only under restrictive mating, but it takes longer if selection is weak. In contrast, in heterogeneous environments with two local optima and strong selection, species well adapted to each of the optima emerge along the spatial structure, leading to the formation of groups with distinct phenotypes. Permissive mating leads to the formation of only two species, each occupying one of the optima; restrictive mating leads to several species per optimum, in a much faster speciation process. Interestingly, when selection is weak and mating is restrictive, several species form, but the process is slow. Moreover, species average phenotypes do not remain constant over generations, causing the phenotypic distribution to oscillate, never reaching a stationary pattern.