Q23.4-3CC - Campbell Biology

Question

Consider a population in which heterozygotes at a certain locus have an extreme phenotype (such as being larger than homozygotes) that confers a selective advantage. Compare this description to the models of selection modes shown in Figure 23.13. Does this situation represent directional, disruptive, or stabilizing selection? Explain your answer.

Step-by-Step Solution

Answer

A population with heterozygotes at a certain locus with an extreme phenotype would represent a directional selection. The modes of natural selection are formulated in terms of phenotype. Thus, a population that has the heterozygotes advantage depends on its phenotype.

Step 1: Modes of Natural Selection

Natural selection occurs in three modes based on the phenotype of the population as per the environment. The modes are directional, disruptive, and stabilizing selection. The modification of allele frequencies through natural selection leads to adaptive evolution. 

Step 2: Disruptive selection

Disruptive selection occurs when a population has two types of phenotypic individuals. Natural selection favors both phenotypes but selects against intermediate individuals. In the given case, the population has an extreme phenotype. Thus, disruptive selection does not occur.

Step 3: Stabilizing selection

When a population has two phenotypic individuals, nature selects the intermediate individuals and picks against both extreme phenotypes, and such mode of natural selection is called stabilizing selection. It does not represent the population with an extreme phenotype. 

Step 4: Directional selection

As per the given case, a population with extreme heterozygotes represents a directional selection. Directional selection occurs in a population that shows the extreme distribution in its phenotype. Depending on the phenotypic expression of the organism, the most suitable one, capable of living in its surrounding environment, go through positive natural selection.