Evolutionary branching and speciation

Understanding speciation, that is, the split of an ancestral lineage into two or more descendant lineages, is a fundamental challenge in evolutionary biology. Classical theories explain speciation as a consequence of geographical isolation. However, recent theories have emphasized that frequency-dependent ecological interactions can induce a bimodal split in phenotype distributions, and hence speciation, under sympatric conditions, i.e. in the absence of geographical barriers. In this talk I review some of these recent developments, which are based on the phenomenon of evolutionary branching. Evolutionary branching is a type of evolutionary dynamics that occurs when frequency-dependent selection first drives a population toward a fitness minimum in phenotype space and then splits the population into two distinct phenotypic dusters. After briefly reviewing the underlying theoretical framework of adaptive dynamics and the basic mechanisms leading to evolutionary branching in asexual populations, I will first show how incorporation of multi-locus genetics and assortative mating leads to a theory of evolutionary branching in sexual populations, and hence to models for sympatric speciation. Next I will illustrate that selection regimes leading to evolutionary branching readily arise from a wide variety of different ecological interactions within and between species, including competition, predation, and mutualism. I will end the talk by discussing some recent results about evolutionary branching in spatially structured populations, showing that spatially localized ecological interactions enhance the drive towards diversification through evolutionary branching and can lead to spatial segregation of the newly emerging species.