Résumé : Assortative mating is a mating pattern in which individuals with similar phenotype mate with one another more frequently than would be expected under a random mating pattern. It may influence the evolutionary fate of a population, by provoking for instance the stopping of gene flow between subpopulations. But which evolutionary forces lead to the appearance of mating preferences in a population ? In this talk, by means of a stochastic individual based model of a varying size population, we will study under which conditions a population of individuals mating uniformly at random (without sexual preference) may acquire a sexual preference : assortative mating.