Question

3. This graph shows the bell curve depicting the variation of traits within a population before and after natural selection. 1) Identify and describe this type of selection. 2) Give an example of a population and an environmental pressure that would shift this population as shown in the graph -- you can use the rock pocket mice, peppered moth example, or an organism of your choice.

Answer 1

The type of selection depicted by the graph is directional selection.

In directional selection, one extreme phenotype is favored over other phenotypes, causing the allele frequency to shift over time in the direction of that phenotype. This type of selection is common when the environment changes in a consistent direction.

An example of a population and an environmental pressure that would shift this population as shown in the graph is the peppered moth in industrial England.

Before the Industrial Revolution, the light-colored form of the moth was more common because it could blend in with the light-colored lichens and trees.

However, during the Industrial Revolution, soot from factories darkened the trees and killed the lichens, making the light-colored moths more visible to predators.

As a result, the dark-colored form of the moth, which was previously rare, became more common because it could blend in with the darkened trees. This is an example of directional selection because the environment changed in a way that favored one extreme phenotype (dark-colored moths) over the other (light-colored moths).