Preferring the fittest mates: an analytically tractable model.

We develop an analytically tractable model of female preference for fit mates. Our population-genetic model allows to trace the dynamics at both the individual and the population level. The preference for fit mates links ecological adaptation and mating success is individually advantageous and causes polymorphic subpopulations. This polymorphism is a strong and stable clustering in genotype and phenotype space. The alleles coding for the mating preference spread rapidly through the population, thereby increasing the selection pressure between different habitats. The resulting polymorphism exceeds the expected selection-migration equilibrium by several orders of magnitude. The evolution of preference for fit mates can, thus, act as the trigger for parapatric speciation because it initiates prezygotic isolation and divergence.