Chapter 22 - Speciation - 22.4 - Reproductive Isolation Is Reinforced When Diverging Species Come into Contact - 22.4 Recap - Learning Outcomes - Page 480: 2

Though natural selection does not directly favor the emergence of postzygotic isolating mechanisms, individual organisms that breed only within their own species demonstrate higher fitness than those that mate with another species. As a result, individuals that avoid interbreeding with other species will have a selective advantage, and any trait that contributes to such avoidance will be favored. This also explains how postzygotic isolating mechanisms can be described to reinforce prezygotic isolating mechanisms.

Though natural selection does not directly favor the emergence of postzygotic isolating mechanisms, individual organisms that breed only within their own species demonstrate higher fitness than those that mate with another species. As a result, individuals that avoid interbreeding with other species will have a selective advantage, and any trait that contributes to such avoidance will be favored. This also explains how postzygotic isolating mechanisms can be described to reinforce prezygotic isolating mechanisms.