This whole gradualism bit can be carried much too far.
Decades ago I read an article about Eskimos and how they moved into the Arctic and over centuries developed their tools and garments needed to survive under such condition.
Even though I was just a kid, I realized that if they didn’t have most of that stuff the first winter, they wouldn’t live through it.
http://www.museum.state.il.us/exhibits/changing/journey/
Photographs from late 18 and early 19 hundreds of Siberian peoples.
LOL Stupid Eskimos selected against.