What is the primary consequence of mechanical isolation?

The primary consequence of mechanical isolation is indeed related to difficulty in fertilization due to structural differences between the reproductive organs of different species. Mechanical isolation occurs when two species are physically unable to mate with one another, often because their reproductive structures are incompatible. This means that even if they may attempt to mate, the anatomical differences prevent successful copulation, leading to a failure in the transfer of sperm. As a result, fertilization cannot occur, ensuring that these species remain distinct and do not produce hybrid offspring.

In contrast, the other choices imply various outcomes that do not accurately describe what mechanical isolation entails. The inability to find mates pertains more to behavioral isolation rather than mechanical barriers. Complete reproductive success would suggest that mating and fertilization occur successfully, which is not the case with mechanical isolation. Lastly, increased hybrid viability refers to situations where hybrids can survive and thrive, which is contrary to the implication of mechanical isolation that prevents hybrid formation altogether.