Final answer:
Stabilizing selection favors average phenotypes and selects against extremes, leading to reduced genetic variance within a population. This can drive alleles for beneficial traits like optimal fur color in mice to become more common, potentially leading to fixation over time.
Step-by-step explanation:
Stabilizing Selection and Genetic Variation
One process that reduces variation and may lead to fixation is stabilizing selection. This form of natural selection favors an average phenotype and selects against extreme variants within a population. In the case of a mouse population that lives in the woods, stabilizing selection would likely favor individuals whose fur color closely matches the brown of the forest floor, increasing their chances of survival by avoiding predation. Mice with fur colors that deviate significantly from this average shade would be more visible to predators and, thus, less likely to survive and reproduce. Over time, the alleles for the beneficial average fur color become more common, reducing genetic variance within the population.
While stabilizing selection leads to decreased genetic variation, it is essential to note other selection types. For example, directional selection moves the population toward one extreme phenotype, while diversifying selection favors multiple extreme phenotypes. Frequency-dependent selection and sexual selection are also essential mechanisms that can increase or decrease variation. Additionally, small initial population sizes can lead to a decline in genetic diversity, making populations more susceptible to changes in environmental conditions or to recessive lethal alleles.