As we saw in Speciation, the major factor in defining species is reproductive isolation, a lack of interbreeding with other groups or reproductive isolation. This crucial step can occur in many different ways. Some are as dramatic as mountain ranges growing up between populations and others are as subtle as the chemical incompatibility of gametes.

There are two main categories of reproductive isolation: prezygotic and postzygotic. Prezygotic isolation occurs before the formation of a zygote can take place. In most cases mating does not even occur. Forms of prezygotic isolation include spatial, behavioral, mechanical and temporal isolation. Postzygotic isolation occurs after members of two different species have mated and produced a zygote. The offspring of such a mating is called a hybrid. Hybrids are frequently unable to reproduce themselves, so the production of a hybrid is not considered a successful mating. Hybrids are prevented from reproducing by developmental abnormalities that keep them from sexually maturing, ill health that causes most to die before reaching sexual maturity, or by sterility.