Assortative mating, sexual selection, and their consequences for gene flow inLittorina

Samuel Perini,Anja M Westram,Roger K Butlin,Kerstin Johannesson,Marina Rafajlović

doi:10.1111/evo.14027

Abstract

When divergent populations are connected by gene flow, the establishment of complete reproductive isolation usually requires the joint action of multiple barrier effects. One example where multiple barrier effects are coupled consists of a single trait that is under divergent natural selection and also mediates assortative mating. Such multiple-effect traits can strongly reduce gene flow. However, there are few cases where patterns of assortative mating have been described quantitatively and their impact on gene flow has been determined. Two ecotypes of the coastal marine snail, Littorina saxatilis, occur in North Atlantic rocky-shore habitats dominated by either crab predation or wave action. There is evidence for divergent natural selection acting on size, and size-assortative mating has previously been documented. Here, we analyze the mating pattern in L.saxatilis with respect to size in intensively sampled transects across boundaries between the habitats. We show that the mating pattern is mostly conserved between ecotypes and that it generates both assortment and directional sexual selection for small male size. Using simulations, we show that the mating pattern can contribute to reproductive isolation between ecotypes but the barrier to gene flow is likely strengthened more by sexual selection than by assortment.

Highlights

Evolution of reproductive isolation under gene flowBiodiversity is tightly connected with ecosystem functions and it can be understood by studying how species form, migrate and go extinct (BUTLIN et al 2009; CARDINALE et al 2012)
Using single-species contact zones, it is possible to provide important details about the mechanisms implicated during divergence while using multispecies contact zones, where more species show divergent populations that are in contact over the same environmental gradient, it is possible to address the generality of patterns of reproductive isolation
Contact zones are geographic areas where differentiated populations of the same taxon or of closely related taxa meet and exchange genes. They are expected to coincide with habitat transitions, especially when two populations have diverged in situ due to local adaptation, but they can be found in the absence of environmental gradients when two populations, prior to contact, have undergone independent evolution (BARTON AND HEWITT 1985; SWENSON AND HOWARD 2005)

Summary

Introduction

Evolution of reproductive isolation under gene flowBiodiversity is tightly connected with ecosystem functions (e.g., primary production and nutrient cycling) and it can be understood by studying how species form, migrate and go extinct (BUTLIN et al 2009; CARDINALE et al 2012). This variation in copulation duration and potentially in sperm transfer, might impact on the contributions of assortative mating and sexual selection to the barrier to gene flow between the Crab and Wave ecotypes.

Results

Conclusion