B. Natural selection usually causes a species to change gradually. Evolution is a change in gene frequency over long periods of time, with natural selection playing a major role in evolution. In natural selection, phenotypic advantages are selected for survival. For example, white moths that rest on brown tree trunks will be more easy spotted and eaten by birds than brown moths of the same species. Over time, as the birds eat the white moths with the genes that code for the white phenotype, there will be more brown moths in the gene pool.
