Directional Selection

When the environment changes, populations will often undergo directional selection (see the figure below), which selects for phenotypes at one end of the spectrum of existing variation. A classic example of this type of selection is 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. But as soot began spewing from factories, the trees became darkened, and the light-colored moths became easier for predatory birds to spot.

Over time, the frequency of the melanic form of the moth increased because they had a higher survival rate in habitats affected by air pollution because their darker coloration blended with the sooty trees. Similarly, the hypothetical mouse population may evolve to take on a different coloration if something were to cause the forest floor where they live to change color. The result of this type of selection is a shift in the population’s genetic variance toward the new, fit phenotype.

Part (a) shows a robin clutch size as an example of stabilizing selection. Robins typically lay four eggs. Larger clutches may result in malnourished chicks, while smaller clutches may result in no viable offspring. A wide bell curve indicates that, in the original population, there was a lot of variability in clutch size. Overlaying this wide bell curve is a narrow one that represents the clutch size after natural selection, which is much less variable. Part (b) shows moth color as an example of directional selection. Light-colored pepper moths are better camouflaged against a pristine environment, while dark-colored peppered moths are better camouflaged against a sooty environment. Thus, as the Industrial Revolution progressed in nineteenth-century England, the color of the moth population shifted from light to dark, an example of directional selection. A bell curve representing the original population and one representing the population after natural selection only slightly overlap. Part (c) shows rabbit coat color as an example of diversifying selection. In this hypothetical example, gray and Himalayan (gray and white) rabbits are better able to blend into their rocky environment than white ones. The original population is represented by a bell curve in which white is the most common coat color, while gray and Himalayan colors, on the right and left flank of the curve, are less common. After natural selection, the bell curve splits into two peaks, indicating gray and Himalayan coat color have become more common than the intermediate white coat color.

Different types of natural selection can impact the distribution of phenotypes within a population. In (a) stabilizing selection, an average phenotype is favored. In (b) directional selection, a change in the environment shifts the spectrum of phenotypes observed. In (c) diversifying selection, two or more extreme phenotypes are selected for, while the average phenotype is selected against.

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In science, sometimes things are believed to be true, and then new information comes to light that changes our understanding. The story of the peppered moth is an example: the facts behind the selection toward darker moths have recently been called into question. Read this article to learn more.

This lesson is part of:

Evolution of Populations

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