All of the following are examples of postzygotic reproductive isolating mechanisms EXCEPT Select one: a. two plant species flowering at different times. b. the hybrid offspring of two species dying before birth. c. a sheep adapted to mountains mating with a sheep adapted to desert producing offspring maladapted to either habitat. d. the hybrid offspring of two species being fertile but their offspring being inviable or sterile. e. the hybrid offspring of two species being sterile.

a. two plant species flowering at different times.

Explanation:

The only option that is not a post-zygotic reproductive isolation mechanism would be option a, when two plant species flower at different times.

Post-zygotic reproductive isolation mechanisms are those mechanisms that act to ensure that the product of fertilization remains invalid. This is as opposed to pre-zygotic isolation mechanisms that act to prevent fertilization and hence, formation of zygotes.

In option b, c, d and e; the zygote has been formed already and all the forces are just acting to ensure that it remains invalid. whereas in option a, there is no fertilization yet.

Two plant species flowering at different times will ensure that there is no cross pollination between the two species. Without pollination, there cannot be a zygote formation.

Hence, the correct option is a.