A horse and a donkey can mate to produce a mule, but mules are sterile. This is an example of which reproductive isolating mechanism?
Correct answer A. Hybrid sterility (post-zygotic isolation)
Explanation
Hybrid sterility acts after fertilisation. The mule's mismatched chromosome sets (63 in total) cannot pair properly in meiosis, so no gene flow occurs between horses and donkeys.
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