What historical interaction between species might have reinforced the melanistic gene in specific geographic locations?

Answer

Interspecies breeding between Eastern Gray Squirrels and Fox Squirrels

Historical interspecies breeding between the closely related Gray Squirrel and Fox Squirrel is sometimes cited as a reason why the melanistic gene was introduced or strengthened in certain locales.

What historical interaction between species might have reinforced the melanistic gene in specific geographic locations?
colorvariationsquirrelMelanismeastern gray squirrel