Interactions between mechanisms of reproductive isolation

Speciation is responsible for the diversity of species observed today and corresponds to the build-up of reproductive isolation between populations. Reproductive isolation can be generated by different mechanisms that have been extensively characterized, yet how their interactions affect speciation remains largely unknown. Here, we explicitly model the interaction of three key mechanisms (local adaptation, mate choice and genetic hybrid incompatibilities) quantifying their relative contribution to the evolution of reproductive isolation. We modeled two populations exchanging migrants using Fishers Geometric Model for local adaptation, phenotype matching for mate choice, and multiple pairs of Bateson-Dobzhansky-Muller Incompatibilities (DMI). All three mechanisms were determined by the same set of loci, creating conditions for interactions between barriers both at the genetic and population levels. We found very few cases where the three barriers evolved together. Instead, two barriers could evolve depending on the migration rate: either local adaptation and genetic incompatibilities for limited migration, or local adaptation and mate choice for higher migration. Our results showed that local adaptation due to ecological differentiation was the first to evolve and by far the most effective reproductive barrier. Finally, we demonstrated that in a polygenic model, populations could become locally adapted and evolve strict mate choice, yet they would not accumulate incompatibilities provided that there was sufficient gene flow.