Post-zygotic isolating factors involve mechanisms that come into play after fertilization of the egg. Typically, they involve some kind of genetic incompatibility. One example, which we saw earlier in Fig. 22.3 and will explore later in the chapter, is the case of two organisms with different numbers of chromosomes.

In some instances, the effect can be extreme. For example, the zygote may fail to develop after fertilization because the two parental genomes are sufficiently different to prevent normal development. In others, the effect is less obvious. Some matings between different species produce perfectly viable adults, as in the case of the horse–donkey hybrid, the mule. As we have seen, though, all is not well with the mule from an evolutionary perspective. The horse and donkey genomes are different enough to cause the mule to be infertile. As a general rule, the more closely related—and therefore genetically similar (Chapter 21)—a pair of species, the less extreme the genetic incompatibility between their genomes.