Why is the melanistic trait not easily removed from the gene pool in warmer regions, even if dark fur might cause thermal stress?

Answer

The trait is not easily eliminated if it is only slightly detrimental

Since the gene is dominant, even if it imposes a slight disadvantage like overheating stress in warm areas, it persists in the gene pool unless the disadvantage is strong enough to eliminate carriers quickly, or if breeding pairs are limited.

Why is the melanistic trait not easily removed from the gene pool in warmer regions, even if dark fur might cause thermal stress?
colorvariationsquirrelMelanismeastern gray squirrel