What distinguishes disruptive and directional selection pressures when both select for extreme genetic traits?
Disruptive selection pressures select for extreme genetic traits at both ends of the phenotype distribution, while directional selection pressures select for extreme genetic traits in one direction only. Disruptive selection, also known as diversifying selection, results in the creation of two or more distinct phenotypic classes, while directional selection results in the gradual shift of the population towards one extreme trait. In other words, disruptive selection increases genetic diversity, while directional selection reduces it. Additionally, disruptive selection may occur in stable environments with no selective pressures, while directional selection occurs in response to changing environmental conditions or due to differences in reproductive success.
Disruptive and directional selection pressures are two different types of natural selection that can both lead to the selection of extreme genetic traits. However, there are some key differences between them.
1. Directional selection: This occurs when individuals with a trait at one extreme of a phenotypic range have the highest fitness and are favored by the environment, leading to a shift in the frequency of that trait over time. In directional selection, there is a clear shift towards one extreme of the trait distribution, while the other extreme is selected against.
2. Disruptive selection: In this type of selection, individuals with extreme traits at both ends of a phenotypic spectrum have higher fitness compared to individuals with intermediate traits. Disruptive selection can occur when different environments or selective pressures favor individuals with different extremes of a trait.
So, what distinguishes the two types of selection pressures?
1. Direction: In directional selection, there is a clear favoring of one extreme trait value over the other, resulting in a shift in the population towards that extreme. In contrast, disruptive selection favors extreme traits at both ends of the spectrum, leading to the population splitting into two distinct groups.
2. Intermediate traits: In directional selection, individuals with intermediate traits have lower fitness and are selected against, leading to a decrease in their frequency in the population over time. In contrast, individuals with intermediate traits may have lower fitness in disruptive selection, but they do not disappear entirely because the extreme trait values at both ends of the spectrum are favored.
In summary, while both disruptive and directional selection pressures can select for extreme genetic traits, the main difference lies in the direction of selection and the fate of individuals with intermediate traits.