Chapter 15 AP Biology: Types of Natural Selection

Directional selection

a mode of natural selection in which a single phenotype is favored, causing the allele frequency to continuously shift in one direction

Disruptive selection

a mode of natural selection in which extreme values for a trait are favored over intermediate values

Sexual selection

a mode of natural selection in which individuals of one sex mate preferentially with particular individuals of the opposite sex rather than at random

Stabilizing selection

a type of natural selection in which genetic diversity decreases as the population stabilizes on a particular trait value

An example of stabilizing selection:

in a population of mice that live in the woods, natural selection will tend to favor individuals that best blend in with the forest floor and are less likely to be spotted by predators.

An example of disruptive selection:

in many populations of animals that have multiple male mating strategies, such as lobster, large, dominant alpha males obtain mates by brute force, while small males can sneak in for furtive copulations with the females in an alpha male's territory. In this case, both the alpha males and the "sneaking" males will be selected for, but medium-sized males, which cannot overtake the alpha males and are too big to sneak copulations, are selected against.

An example of sexual selection:

in many populations of birds, bigger tails on male birds mean that the overall health of the bird is better, and females prefer to mate with the bird with the largest feathers for this reason.

An example of directional selection:

the evolution of the peppered moth in eighteenth- and nineteenth-century England. Prior to the Industrial Revolution, the moths were predominately light in color, which allowed them to blend in with the light-colored trees and lichens in their environment. As soot began spewing from factories, the trees darkened and the light-colored moths became easier for predatory birds to spot.