This is called genetic isolation, and it's the origin of many species. But when it happens you don't get a gradient. You get two populations separated by (usual

This is called genetic isolation, and it's the origin of many species. But when it happens you don't get a gradient. You get two populations separated by (usually) a geographic barrier. The closest case I could imagine for this in humans are the australian aboriginals and papuans. You don't have a continuous gradient south-north between them and asian populations (assuming the Out Or Africa as true), but a sharp cut dated 50K years ago. Europeans are genetically closer to an asian than asians to papuans or australians. Lets imagine a small group of engineers that develope a bad software pattern in full isolation. They don't know better, so they work in that way for years. Now suppose they get in contact again with the rest of the world. How could their bad pattern expand to the rest of the world engineers if it is inferior? The ones that addopt it would be selected against, the ones who doesn't would be selecter positively. For it to expand it had to have at least something positive (unless you do a Chengis Khan). This is what happened to sickle cell disease, which is a bad mutation but gives you resistence to malaria, so the sickle cell disease is common in Africa. Meanwhile, African Americans have much less of the mutation, because in the USA there is no malaria, thus the gene is being slowly removed from the population.