Question

Describe each of the types of natural selection given in this unit. Include the role that each plays in increasing or decreasing variation within a population.

Answer 1

3 Types of natural selection can be observed.

1. Stabilizing selection

2. Directional selection

3. Disruptive selection

Stabilizing selection

Selective pressure select against the two extremes of a trait. ie, both the extremes are eliminated and the middle class is selected and stabilized. Example if plant height is the character that undergoes a stabilizing selection, plants which are too short will be eliminated by reasons like lack of sunlight. At the same time, too long plants are also may eliminated by the problems like wind damage. So the medium sized plants are selected and stabilized. Results in a curve with a center peak with middle selection.

This increases the variations in a population

Directional selection

In this case the population will be shifting towards one extreme and eliminates the other extreme. So it is a goal oriented selection. One of the extremes are fixed and stabilized and other one is eliminated. Example: The length of neck of Giraffe. Individuals with short neck are not selected because they can't reach to the branches but the ones with longer necks are been selected since they can reach to the branches. This results in a curve with peak shifted towards any of the extremes.

This favors a particular trait to dominate and others to eliminate

Disruptive selection

In this case, both the extremes are selected and the individuals in the middle of the trait are eliminated. This results in a curve with 2 peaks in total each extremes each.

This fixes both extremes and eliminates middle class