Final answer:
Natural selection favors prezygotic mechanisms because they prevent the expenditure of resources on offspring that will not survive or be infertile, thus conserving energy and ensuring only favorable traits are passed on.
Step-by-step explanation:
Natural selection tends to favor prezygotic mechanisms because they are a more efficient form of reproductive isolation compared to postzygotic mechanisms. Prezygotic barriers work by preventing the formation of a zygote altogether, which means that there is no expenditure of energy or resources on offspring that may not survive or may be infertile due to genetic incompatibilities. Examples of prezygotic barriers include behavioral differences that prevent courtship or mating, physiological barriers where gametes are unable to fuse, and differences in reproductive anatomy that prevent successful mating. In contrast, postzygotic mechanisms occur after zygote formation and include issues that cause the offspring to be inviable or sterile, such as the mule, which is the infertile offspring of a female horse and a male donkey.
Given the investment required to produce offspring, natural selection favors reproductive strategies that conserve energy and maximize the chances of survival and reproduction of individuals with favorable traits. Since prezygotic barriers prevent the production of less-viable or nonviable offspring from the start, they are generally more advantageous and therefore more likely to be strengthened over time through the process of reinforcement.