  Recent theoretical studies have shown that demographic stochasticity can
greatly increase the tendency of asexually reproducing phenotypically diverse
organisms to spontaneously evolve into localised clusters, suggesting a simple
mechanism for sympatric speciation. Here we study the role of demographic
stochasticity in a model of competing organisms subject to assortative mating.
We find that in models with sexual reproduction, noise can also lead to the
formation of phenotypic clusters in parameter ranges where deterministic models
would lead to a homogeneous distribution. In some cases, noise can have a
sizeable effect, rendering the deterministic modelling insufficient to
understand the phenotypic distribution.
