In a fish population in a shallow stream, the genotypic frequencies of yellowish-brown fish and greenish-brown fish changed significantly after a flash flood randomly swept away individuals from that stream. this change in genotypic frequency is most likely attributable to:

Genetic drift.

Genetic drift is when the frequency of a certain allele, in a population, is chosen due to random sampling of organisms. It may alleles to disappear completely and thereby reduce genetic variation. This is way happen in the fish population after the flash flood