Origin of sexuality
The primary cause of the emergence of sexuality, i.e. differentiation of individuals of a single species into males and females, was apparently morphological differentiation of gametes into two types, microgametes and macrogametes, i.e. the emergence of morphological anisogamy. This differentiation is a phenomenon that is very old in evolution; however, it was preceded by functional differentiation, i.e. functional anisogamy. The formation of two or more mating types of cells that cannot reproduce sexually within the group and can reproduce only with the members of another mating type occurs in organisms in which specialized sex cells, gametes, are not formed and where their function is fulfilled by a relatively unspecialized somatic cell. This situation is encountered, e.g., yeasts and ciliates.