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However, it is possible for directional selection to take a very long time to find a local optimum on a fitness landscape. [29] A possible example of long-term directional selection is the tendency of proteins to become more hydrophobic over time, [30] and to have their hydrophobic amino acids more interspersed along the sequence. [31]
Stabilizing selection is the most common form of nonlinear selection (non-directional) in humans. [13] There are few examples of genes with direct evidence of stabilizing selection in humans. However, most quantitative traits (height, birthweight, schizophrenia) are thought to be under stabilizing selection, due to their polygenicity and the ...
The existence of limits in artificial selection experiments was discussed in the scientific literature in the 1940s or earlier. [1] The most obvious possible cause of reaching a limit (or plateau) when a population is under continued directional selection is that all of the additive-genetic variation (see additive genetic effects) related to that trait gets "used up" or fixed. [2]
Selection can be divided into three classes, on the basis of its effect on allele frequencies: directional, stabilizing, and disruptive selection. [102] 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.
These charts depict the different types of genetic selection. On each graph, the x-axis variable is the type of phenotypic trait and the y-axis variable is the amount of organisms. Group A is the original population and Group B is the population after selection. Graph 1 shows directional selection, in which a single extreme phenotype is favored.
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. Here, the function indicates either an increase ...
The Price equation can describe any system that changes over time, but is most often applied in evolutionary biology. The evolution of sight provides an example of simple directional selection. The evolution of sickle cell anemia shows how a heterozygote advantage can affect trait evolution. The Price equation can also be applied to population ...
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] Secondly, disruptive selection is selection for extreme trait values and often results in two different values becoming most common, with selection against the average value.