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Stabilizing selection (not to be confused with negative or purifying selection [1] [2]) is a type of natural selection in which the population mean stabilizes on a particular non-extreme trait value. This is thought to be the most common mechanism of action for natural selection because most traits do not appear to change drastically over time ...
Disruptive selection is a specific type of natural selection that actively selects against the intermediate in a population, favoring both extremes of the spectrum. Disruptive selection is inferred to oftentimes lead to sympatric speciation through a phyletic gradualism mode of evolution.
Group A is the original population and Group B is the population after selection. Top (Graph 1) represents directional selection with one extreme favored. Middle (Graph 2) represents stabilizing selection with the moderate trait favored. Bottom (Graph 3) represents disruptive selection with both extremes being favored.
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. This would be when ...
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.
In a study of the fresh-water Eurasian perch, a change in fitness was reported with a change in their density. An estimate of the selection gradient by linear and quadratic regression indicated a shift of the selection regime between stabilizing and directional selection at low density to disruptive selection at higher density. [11]
Mathematical models, laboratory studies, and observational evidence supports the existence of parapatric speciation's occurrence in nature. The qualities of parapatry imply a partial extrinsic barrier during divergence; [2] thus leading to a difficulty in determining whether this mode of speciation actually occurred, or if an alternative mode (notably, allopatric speciation) can explain the data.
The first to propose what is now the most pervasive hypothesis on how sympatric speciation may occur was John Maynard Smith, in 1966. He came up with the idea of disruptive selection. He figured that if two ecological niches are occupied by a single species, diverging selection between the two niches could eventually cause reproductive ...