@db @_Seraphim_ Genetic Drift is the term for the quick changes from circumstance. There's a chance that some gene for "make more melanin" was present already, probably bundled with a other gene (they like to travel in groups). How it would be expressed is beyond me, but perhaps immune system signals saying [damage from radiation] flip the switch or perhaps the ones that didn't have that died and it was a genetic bottleneck. How such a gene would be turned on (do animals have radiation receptors besides knowing there's damage?) or is it simply the immune system jannies cleaning up the damage?