Types of Natural Selection - Free Worksheet

Natural selection is the gradual process by which heritable traits become either more or less common in a population as a result of any or all of the following items

Overpopulation

Competition for resources

Changes in the environment

Predators

There are three types of natural selection

Directional

Stabilizing

Disruptive

Directional selection occurs when one or two extreme traits are selected A classic example is the peppered moth during the industrial revolution. In the mid 1800’s most peppered moths were white and black peppered moths were rare to see. As the industrial revolution moved forward factories were fueled by coal and became dark. By 1900 the population of peppered moths were mostly black. The black color on the dark tree was more favorable than the white color.

Stabilizing selection occurs when the environment selects for the average of a trait. One example is the number of eggs a bird population lays. Many birds may lay two eggs but this may not be enough for the birds to survive and four eggs may be too many birds to feed. Over time the population will stabilize around three eggs.

Disruptive selection is the opposite of stabilizing. The average is not favorable and the extremes become favorable. For example, a population of rabbits live among black and white rocks. The fur color of the rabbits range from white to gray to black. The white and black are the extremes. Because the white and black colors can camouflage on the white and black rocks. The extremes will be selected for and the average color gray will be selected against. Over time this type of selection can lead to speciation.

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