  We introduce an analytical model for population dynamics with intra-specific
competition, mutation and assortative mating as basic ingredients. The set of
equations that describes the time evolution of population size in a mean-field
approximation may be decoupled. We find a phase transition leading to sympatric
speciation as a parameter that quantifies competition strength is varied. This
transition, previously found in a computational model, occurs to be of first
order.
