Final answer:
In populations where a rare allele can be more fit, the type of selection that occurs is known as negative frequency-dependent selection. This selection favors rare phenotypes, increasing genetic variance and contributing to a balance of multiple alleles.
Step-by-step explanation:
In some populations, a rare allele may be more fit than common variants; this situation can arise through a process known as negative frequency-dependent selection.
During this type of selection, rare phenotypes have a selective advantage simply because they are rare. This can lead to a balancing effect where multiple alleles are maintained in the population because each has its own advantage when they are at a low frequency.
This dynamic is in contrast with positive frequency-dependent selection, where common phenotypes are favored.
An example of negative frequency-dependent selection is seen in some animal populations with multiple male forms. Large alpha males and smaller sneaky males each have distinct mating strategies that succeed due to their relative rarity.
In environments where there are distinctly different habitats, such as a beach with both light-colored sand and patches of dark grass, light-colored mice and dark-colored mice may each have survival advantages in their respective habitats, while intermediate colored mice have a survival disadvantage, leading to diversifying selection.
In summary, negative frequency-dependent selection can maintain a rare allele in a population by giving it a fitness advantage when it is less common, contributing to the overall genetic variance within the population.