USA Biology Olympiad (USABO) Practice Exam

Which of the following is not considered a mechanism of natural selection?

1. Directional selection

2. Stabilizing selection

3. Disruptive selection

4. Sexual selection

The correct answer is: Sexual selection

Natural selection refers to the processes by which certain traits become more or less common in a population based on their impact on the survival and reproductive success of individuals. Directional selection, stabilizing selection, and disruptive selection are all recognized mechanisms of natural selection that describe different ways that traits can be favored or selected against in a population. Directional selection occurs when one extreme of a trait distribution is favored, leading to a shift in the population's traits over time. Stabilizing selection promotes average traits and selects against extremes, thus maintaining the status quo in traits within the population. Disruptive selection, on the other hand, favors individuals at both extremes of a trait spectrum while selecting against the average, which may lead to increased diversity within the population. Sexual selection, while often considered a form of natural selection, differs slightly as it specifically relates to the reproductive success stemming from preference for certain traits in mates. This mechanism explains how characteristics that may not necessarily enhance survival can become more prevalent due to reproductive advantages. Given this context, the distinction here lies in the fact that, while sexual selection is a form of selection, it is primarily concerned with mate choice and reproductive success rather than survival in the typical environmental context associated with the other three mechanisms.