Speciation is the formation of new species. A requirement of speciation is that subpopulations are prevented from interbreeding. This is called reproductive isolation. When subpopulations are reproductively isolated, natural selection and genetic drift can result in evolution taking a different course in each subpopulation. Reproductive isolation can occur in different ways.
Premating isolation prevents mating from taking place. For example, impenetrable barriers, such as rivers or mountain ranges, may separate subpopulations. Other forms of premating isolation are more subtle. If courtship behavior patterns of two animals are not mutually appropriate, mating does not occur. Similarly, individuals with different breeding periods or that occupy different habitats are unable to breed with each other.
Postmating isolation prevents successful fertilization and development, even though mating may have occurred. For exampie, conditions in the reproductive tract of a female may not support the sperm of another individual, which prevents successful fertilization. Postulating isolation also occurs because hybrids are usually sterile (e.g., the mule produced from a mating of a male donkey and a mare is a sterile hybrid). Mismatched chromosomes cannot synapse properly during meiosis, and any gametes produced are not viable. Other kinds of postulating isolation include developmental failures of the fertilized egg or embryo.