Question

In a population, surrounded with dark gray rocks, there lived 2 types of beetles, a gray beetle and a white beetle. In the starting population, there were 5 gray beetles and 5 white beetles, until it was populated with birds surrounding areas above the dark gray rocks. Due to this incident, all white beetles were eaten by the birds and all gray beetles survived.
Using information from the passage, what mechanism of evolution is described and what genotype frequency of AA, Aa, and aa for the next generation is accurate?

A. mutation; 0.40, 0.20, 0.40

B. genetic drift; 0.20, 0.40, 0.40

C. natural selection; 0.40, 0.40, 0.20

D. migration; 0.20, 0.40, 0.40

Answer 1

Directional selection favors one of the extreme phenotypes over the other one. Option C is correct. Natural selection is the evolutive force, and 0.40, 0.40, and 0.20 are the new genotypic frequencies.

What is directional selection?

Directional selection is one type of natural selection. Let us remember that natural selection is an evolutive force.

Directional selection increases the proportion of individuals with an extreme phenotypic trait.

There must be a selective pressure or environmental pressure acting on populations to lead the species to increase the number of individuals expressing that extreme phenotype.

This selection presents more frequently in those cases in which interactions between living organisms and the environment modify in the same direction.

In the exposed example,

• two original phenotypes ⇒ gray and white
• substrate color ⇒  dark gray rocks
• environmental pressure ⇒ birds preying on beetles

- Grey beetles managed to camouflage with rocks since their gray color was similar.

- White beetles were exposed on the dark gray rocks, so they were easily seen by birds.

- Predation by birds was an environmental pressure that had a stronger influence on the white phenotype.

Natural selection favored gray-colored beetles, who could survive and reproduce.

Assuming a diallelic gene expressing complete dominance codes for beetles color, we can suggest that the frequency of one of the homozygous genotypes increased, while the other decreased to near cero. Heterozygous individuals kept the allele coding for the white phenotype.

Among the options, C is the correct one. Natural selection is the evolutive force, and 0.40, 0.40, and 0.20 are the new genotypic frequencies.

You can learn more about natural selection and  directional selection at

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