In one butterfly species, the colors of individuals range from white to black, with many shades of gray in between. If the butterflies in a mountain population become more and more similar in color over several generations (e. g., if most butterflies are the same shade of gray), what kind of evolutionary force is likely acting on the population?

Answer: Incomplete dominance

Explanation:

Incomplete dominance describes the force behind the emergence of an heterozygous phenotype (gray-coloured butterflies) as offspring of two contrasting homozygous phenotypes (white-coloured and black-coloured parent butterflies)

Furthermore, incomplete dominance set in when neither of the parent traits is dominant over the other.

Thus, gray-colour appear as an intermediate of the white-colour and black-colour