A subpopulation begins to exploit resources not used by the parent population. Example includes the cichlids in Lake Victoria which is less than a million years old yet there are some 200 species of cichlids in the lake. A balanced polymorphism coupled with assortative mating could result in sympatric speciation in animals. Dimorphic bills in birds, mating between individuals of the same bill type could result in eventual speciation. The reproductive barriers may evolve without being favored directly by natural selection.
Reproductive isolation is usually a secondary consequence of divergence.
Postzygotic and prezygotic barriers may evolve as a consequence of divergence. Reproductive barriers may evolve more directly as a consequence of sexual selection . Again, the reproductive barriers evolve to enhance reproductive success not as a safeguard against interspecies breeding. Biological species concept emphasizes the reproductive isolating mechanisms rather than the recognition of a mate in one's own species- recognition concept of species . The recognition concept of species is based on a number of molecular, morphological and behavioral characteristics to define good mates- mates in the same species.