USA Biology Olympiad (USABO) Practice Exam

The development of a thicker coat of hair in mountain goats is most likely the result of directional selection. This type of natural selection occurs when environmental pressures favor one extreme phenotype over others. In this case, mountain goats living in cold, mountainous regions experience harsh winter conditions. Goats with thicker coats are better insulated and more likely to survive and reproduce compared to those with thinner coats. Over generations, the trait for thicker hair becomes more prevalent in the population because those individuals are better adapted to their environment.

Stabilizing selection, in contrast, would favor intermediate traits, reducing variation around a mean, which is not what is observed in this scenario since thick coats provide a clear survival advantage in extreme environments. Disruptive selection involves favoring two or more extreme phenotypes, which is not applicable here as it is the adaptation to a single, specific environmental challenge (cold temperatures) that's crucial. Sexual selection focuses on traits that improve mating success rather than survival in harsh conditions, which is not relevant to the adaptation of coat thickness for temperature regulation.