Published models of sympatric speciation through habitat specialization require balanced polymorphism at loci affecting fitness in different habitats, but the conditions for maintenance of such polymorphism are stringent. Here I suggest a complementary mechanism favouring the evolution of habitat races, regarded as a prelude to sympatric speciation. It rests on the assumption that beneficial mutations with habitat-specific effects frequently arise and become incorporated into gene pools. When a mutation with fitness effects differing between habitats spreads in the population, there is indirect selection favouring habitat choice over random dispersal. I studied this process in a two-locus, two-habitat computer simulation. The results suggest that the conditions favouring sympatric speciation are less stringent than previously thought.