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Directional selection can be observed in finch beak size, peppered moth color, African cichlid mouth types, and sockeye salmon migration periods. If there is continuous allele frequency change as a result of directional selection generation from generation, there will be observable changes in the phenotypes of the entire population over time.
The McDonald–Kreitman test [1] is a statistical test often used by evolutionary and population biologists to detect and measure the amount of adaptive evolution within a species by determining whether adaptive evolution has occurred, and the proportion of substitutions that resulted from positive selection (also known as directional selection).
Selection can be divided into three classes, on the basis of its effect on allele frequencies: directional, stabilizing, and disruptive selection. [103] Directional selection occurs when an allele has a greater fitness than others, so that it increases in frequency, gaining an increasing share in the population.
The first and most common function to estimate fitness of a trait is linear ω =α +βz, which represents directional selection. [1] [10] The slope of the linear regression line (β) is the selection gradient, ω is the fitness of a trait value z, and α is the y-intercept of the fitness function.
Natural selection within a population for a trait that can vary across a range of values, such as height, can be categorised into three different types. The first is directional selection , which is a shift in the average value of a trait over time—for example, organisms slowly getting taller. [ 80 ]
The peacock tail in flight, the classic example of an ornament assumed to be a Fisherian runaway. Fisherian runaway or runaway selection is a sexual selection mechanism proposed by the mathematical biologist Ronald Fisher in the early 20th century, to account for the evolution of ostentatious male ornamentation by persistent, directional female choice.
This phenomenon can cause a tropical storm to escalate into a hurricane or a hurricane to jump one or more categories in less than a day.
Balancing selection is rare compared to purifying selection. [1] It can occur by various mechanisms, in particular, when the heterozygotes for the alleles under consideration have a higher fitness than the homozygote. [2] In this way genetic polymorphism is conserved. [3]