Natural selection can therefore mean differential reproduction among members of the sarne gene pool. We earlier said that selection would promote those alleles that give rise to phenotypes that are better adapted to a given environment. In other words, the frequencies of 'such alleles would tend to increase and those of ill-adapted ones would tend to decrease. What we are sayingis that natural selection is a matter of changes in gene frequencies. Differential reproduction has a dual role to perform. One it leads to or maintains adaptations and therefore determines whether individuals of a population or species will survive; and two, it is a process which determines what changes will occur in the frequencies of alleles in a gene pool of sexually reproducing individuals.