An isolating mechanism is any property of two species that stops them from interbreeding. They are fundamental to the biological species concept.
Biologists distinguish between prezygotic and postzygotic mechanisms: mechanisms that prevent the formation of hybrid zygotes and those that reduce the viability or fertility of hybrid zygotes.
The table gives Dobzhansky's classification of the main types of isolating mechanisms. In particular cases, isolation is not likely to be due completely to only one factor from Dobzhansky's list. It may be caused by a mix of several prezygotic and postzygotic factors.
Hybrid sterility: This is a type of a postzygotic isolating mechanism. For example, the mule is a hybrid between a donkey and a horse or pony. The diploid chromosome number in the donkey is 62 and in the horse or pony it is 64. The mule therefore has 63 chromosomes and is infertile.
Table: Dobzhansky's classification of reproductive isolation mechanisms. From Dobzhansky (1970).