The natural selection can cause the change in the frequencies of alleles being the ones with higher fitness more common in the population than the others. There is some types of natural selection, like directional selection (individual traits on ane side in the population survive better with the time than others), sexual selection (some traits have greater sucess in obtaining mates, like strinking adornments), disruptive selection (the extreme values for a trait are favored over intermediated values), and stabilizing selection (the genetic diversity decrease as the population stabilizes on a particular trait). The case in the question is a example of directional selection (a), since the phenotype is favored because of the higher fitness of the trait (of the individuals with the alleles in the population).