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.
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