When is sympatric speciation a possible evolutionary outcome?

Abstract The process of speciation is the source of biodiversity. The most popularly accepted mode of speciation is allopatric speciation, where geography imposes the initial barrier to gene flow, and then biological barriers come up. On the other hand, sympatric speciation, which was not accepted as a possibility for long, requires that the process of speciation happen in the absence of a geographical barrier, in a well-mixed population. Several attempts have been made to theoretically identify the conditions in which speciation can occur in sympatry, but have several problems associated with them. We propose a model for sympatric speciation based on adaptation for resource utilization. We use this genetics- based model to investigate the relative roles of prezygotic and postzygotic barriers, from the context of ecological disruptive selection, sexual selection, and genetic architecture, in causing and maintaining sympatric speciation. We show that sexual selection that acts on secondary sexual traits does not play any role in the process of speciation in sympatry, and that assortative mating based on an ecologically relevant trait forces the population to show an adaptive response. We also demonstrate that understanding the genetic architecture of the trait under ecological selection is very important, and that it is not required for the strength of ecological disruptive selection to be very high in order for speciation to occur in sympatry. With this, we provide an insight into the kind of scenarios in which sympatric speciation can be demonstrated in lab.