Toxic substances often have a bitter taste that causes animals who try to eat such substances to spit them out rather than swallow them. Additional data suggest that gorilla populations have a very low frequency of nontasters. Which of the following best describes the likely evolution of the TAS2R38 locus in the gorilla population? (A) Gorillas who could taste bitter toxins were more likely to survive and reproduce than nontasters
(B) Gorillas who could taste bitter toxins became resistant to the toxins, so the taster gene was no longer under selective pressure
(C) Gorillas acquired the taster allele by horizontal gene transfer from chimpanzces.
(D) Gorillas who could taste bitter toxins could utilize food resources that nontasters could not.

(A) Gorillas who could taste bitter toxins were more likely to survive and reproduce than nontasters.

Explanation:

Natural selection is the result between the individual phenotype and the environment that determines the destiny of genes. It results in adaptation, which means the increase of the aptitude phenotype.    

Aptitude (or fitness) is reflected by the phenotype that results in survival, fertility, and mating capability. Aptitude is the contribution of each genotype to the next generation. It is a way of measuring the individual ability to leave fertile offspring. Aptitude must be significant to the natural selection act in its favor.  

Natural selection can act favoring an allele or against it, according to how it affects the fitness of individuals.

When many organisms in a population sharing the same trait die, this is because they did not have good fitness, so they were not adapted to the environment and its pressures. The alleles coding for that trait were not good for the fitness of the animals, so they do not get to survive.

These individuals die before reproducing, so they can not transfer their genetic charge to the following generation. Eventually, the alleles coding for the trait will decrease in the population, probably near zero. Natural selection is acting against this phenotype. Other alleles will be beneficiated, and their frequency in the population will increase.  

This change in alleles frequency is what we call adaptation.                

In the exposed example:

So, in this population, gorillas that could taste bitter toxins were more likely to survive and reproduce than non-tasters.

b.