In a population with brown and green alleles for color, genetic drift View Available Hint(s) In a population with brown and green alleles for color, genetic drift has more effect on the evolution of a small population. occurs when individuals move into or out of a population, changing the allele frequencies in the population. causes populations to become better adapted to their environments. has more effect on the evolution of a large population. always increases the frequency of brown alleles in the population.