Selection may act on a trait to produce three different patt

Selection may act on a trait to produce three different patterns of evolutionary change (stabilizing selection, directional selection and disruptive selection). Explain each pattern (how trait mean and variation are affected) and give a example of each THAT WE DID NOTDISCUSS IN CLASS. Please make it clear from your descriptions how selection leads to the pattern you describe, with specific references to each example.

Solution

Three types of selection are there:

1. Stabilizing selection: In this the average phenotype is favored. In this, natural selection selects against the extremes and imparts best fitness to the average phenotypes.

Example 1: Suppose there is a species of mice which lives in a forest. The ground coloration is brown (Bb). A cross between black (BB) and white (bb) mice will result in this phenotype. So stabilizing selection will favor the mice having average coloration i.e. brown. Rest of the mice will be visualized and killed by predators.

Example 2: Robins lay four eggs which is average. Small clutch size is not favored because it may result in no viable offspring. Large clutch size is also not favored because it results in competition.

2. Directional selection: If there is an environmental change, then the pattern of stabilizing selection changes to one or the other end. This is called as directional selection.

Example 1: A good example is industrial revolution of Europe. Before the revolution, moths were predominantly light in color. Later on, after the revolution, those moths survived more who were dark in color. So, there is a shift in selection from average (light color) to extremes having dark color.

3. Disruptive or diversifying selection: This selection pattern favors the extreme phenotypes only. Average individuals do not survive much, i.e. their fitness is very less.

Example: In Himalayan rabbits the grey rabbits (RR) and white rabbits (rr) have greater fitness than white rabbits (Rr) who do not blend with the surroundings and are tracked by the predators.