What is an example of postzygotic isolation?

Postzygotic isolation refers to mechanisms that occur after fertilization, preventing the hybrid offspring from developing into a fertile adult. An example of this is hybrid sterility, where hybrids are produced from the mating of two different species but are unable to reproduce themselves. Mules, which are the sterile offspring of a horse and a donkey, illustrate this concept well. Although the mating of these two species can result in a viable offspring, the mule cannot reproduce due to an uneven number of chromosomes, leading to infertility. This demonstrates how postzygotic mechanisms can ensure that even when reproduction occurs, the gene flow between the original species is limited.

Temporal isolation, mechanical isolation, and ecological isolation are all prezygotic mechanisms that prevent mating or fertilization altogether, which means they occur before zygote formation. This distinction is crucial in understanding the different types of reproductive barriers in the speciation process.