Because natural selection acts to increase the frequency of beneficial alleles and traits while decreasing the frequency of deleterious qualities, it is adaptive evolution. Natural selection acts at the level of the individual, selecting for those that have a higher overall fitness compared to the rest of the population. If the fit phenotypes are those that are similar, natural selection will result in stabilizing selection, and an overall decrease in the population’s variation. Directional selection works to shift a population’s variance toward a new, fit phenotype, as environmental conditions change. In contrast, diversifying selection results in increased genetic variance by selecting for two or more distinct phenotypes.
Other types of selection include frequency-dependent selection, in which individuals with either common (positive frequency-dependent selection) or rare (negative frequency-dependent selection) are selected for. Finally, sexual selection results from the fact that one sex has more variance in the reproductive success than the other. As a result, males and females experience different selective pressures, which can often lead to the evolution of phenotypic differences, or sexual dimorphisms, between the two.
increase in frequency of beneficial alleles and decrease in deleterious alleles due to selection
selection that favors phenotypes at one end of the spectrum of existing variation
selection that favors two or more distinct phenotypes
(also, Darwinian fitness) individual’s ability to survive and reproduce
selection that favors phenotypes that are either common (positive frequency-dependent selection) or rare (negative frequency-dependent selection)
good genes hypothesis
theory of sexual selection that argues individuals develop impressive ornaments to show off their efficient metabolism or ability to fight disease
theory of sexual selection that argues only the fittest individuals can afford costly traits
trait that gives a truthful impression of an individual’s fitness
individual’s ability to survive and reproduce relative to the rest of the population
phenotypic difference between the males and females of a population
selection that favors average phenotypes