In a laboratory population of flies, the female flies are gray and the males are yellowish gray. biologists observed that all the male flies had an equal chance for reproduction, but the male flies with the brightest colors were more likely to successfully reproduce. what phenomenon could explain such a change?

The correct answer is sexual selection.

Explanation:

Sexual selection is illustrated as a medium of natural selection that comprises a selection of mates of the contrary sex by the individual of one biological sex, and at the same time competing with the individuals of the similar sex for the members of the contrary sex.  

As in the given question, the male flies exhibiting brightest colors are more effective in reproducing in comparison to the male flies exhibiting light colors, it shows that the phenomenon of sexual selection is taking place.