Allele frequency refers to the fraction of individuals with a particular version of a given gene. what effect does natural selection have on the allele frequency of a population? a. it causes the allele frequency to resemble that of a small number of individuals that became separated from the rest of the population. b. it increases the frequency of alleles that improve a species' survival in a particular environment. c. it causes random changes and the allele frequency of certain traits may increase or decrease. d. it greatly reduces the total population, which increases the effects of genetic drift on allele frequency.