Assortative mating counteracts the evolution of dispersal polymorphisms

Polymorphic dispersal strategies are found in many plant and animal species. An important question is how the genetic variation underlying such polymorphisms is maintained. Numerous mechanisms have been discussed, including kin competition or frequency-dependent selection. In the context of sympatric speciation events genetic and phenotypic variation is often assumed to be preserved by assortative mating. Thus, recently, this has been advocated as a possible mechanism leading to the evolution of dispersal polymorphisms. Here, we examine the role of assortative mating for the evolution of trade-off driven dispersal polymorphisms by modelling univoltine insect species in a metapopulation. We show that assortative mating does not favour the evolution of polymorphisms. On the contrary, assortative mating favours the evolution of an intermediate dispersal type and a uni-modal distribution of traits within populations. As an alternative mechanism dominance may explain the occurrence of two discrete morphs.