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MCAT Question A Day - 5/13/13 - Answer!

A mating between a female horse (mare) and a male donkey produces a mule. All mules are sterile. This is an example of

A.  mutation.
B.  post-zygotic reproductive isolation.
C.  pre-zygotic reproductive isolation.
D.  speciation.

The correct answer is (B). Pre-zygotic isolation refers to any conditions that prevent fertilization from taking place (breeding at different times, breeding in different geographic regions, reproductive structures are incompatible, etc.). Since the horse and donkey have successfully mated and produced an offspring, this is an example of a post-zygotic isolating mechanism (hybrid sterility). A new species (mules) would not arise in such a situation (choice D) because the mules themselves cannot produce offspring.