Reproductive isolating mechanisms prevent reproduction between members of a species that may have been isolated geographically and when they come together, they can no longer mate. Or, they may have accumulated enough differences that even if they are together, reproductive isolating mechanisms prevent fertilization and production of offspring. Eventually, this can lead to the production of a new species over time. An example is a temporal mechanism. The two species may become fertile at different times of the year, thus rendering them unable to reproduce. Different bull frogs- cold adapted races and warm adapted can successfully be bred in the lab, but in the wild, due to different times of the year that they breed, would never successfully produce offspring. Two different plants may flower at different times of the year, preventing them from cross pollinating. Physically they may have changed enough so that it is impossible for their different reproductive organs to successfully mate. Mating or courtship rituals may be performed incorrectly preventing mating between animals. This is known as a behavioral isolating mechanism. Pheromones which are the sexual hormones that attract mates may be the wrong type and prevent mating. All of these are pre-zygotic mechanisms. Post- zygotic mechanisms include failure of the hybrid to thrive or hybrid sterility. An example is that the two animals successfully mate and the hybrid is born sterile--like a donkey mated with a horse produces a sterile mule. Natural selection selects the most "fit" for the environment to live and reproduce. Perhaps since so many barriers to successful reproduction exist, is the way nature weeds out the least fit!