Sexually reproducing species must have evolved specific mechanisms enabling mutual recognition of sexual partners. Only in this way is it possible to ensure that the members of a single species will recognize one another in nature and reproduce together. Traits according to which the members of one sex recognize the members of the other sex of the same species or according to which the members of a single hermaphroditic species recognize one another can be subject to evolutionary changes through the action of genetic drift and selection. As soon as a certain part of the population creates a new means of recognizing sexual partners, preconditions are created for the branching off of a new species by ethological speciation. The differences in these mechanisms that could evolve, for example allopatrically, can form very effective, internal, prezygotic reproduction barriers that are capable of ensuring the coexistence of two evolving species even if they secondarily meet in a single territory.