Reddish Egret populations in the U. S. Were hunted to near extinction in the early 1900s and the majority of survivors were the dark-feathered morph. This would be an example of:

This would be an example of Directional Selection

Explanation:    

Natural selection is the result of the phenotype-environment interaction which determines gene destiny in space and time, selecting beneficial alleles and increasing their frequency in the population.  

Directional selection increases the proportion of individuals with an extreme phenotypic trait. 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, Reddish Egret populations were under hunting pressure, which drove them to increase the frequency of dark-feathered individuals, as well as the frequency of the allele codifying for this trait. Dark individuals were the ones that got to survive hunting, they could reproduce and increase in number.

answer: a. a gene that causes a specific trait.

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