I suspect that the ones following national boundaries are due to surveys done
That's why I've written it's possible.
]quote]
in country and horizontal lines just approximate generalities (i.e. you can say that southern Europe has more than Northern Europe)[/QUOTE]
If there's an "approximate" generalisation on the scale of entire continent, shown by a simple horizontal line, I find it strange. Especially since northern South America has more people of african or indigenous descent than southern one does, so it should be the other way round probably, but someone making this map did a simple "the more south, the bigger lactose intolerance" assumption.
I think most of it is of mixed European and Amerindian descent, like me. A lot of Brazilians are of Japanese descent, though, especially in São Paulo.
Yes, that is true. I've once read a book which discussed these matters, a history of South America (a pretty big one, 3 tomes), and I base my knowledge on it. It claimed, however, that Argentina, for example, was primarily a mestizzo country, but only till big white immigration in XIX century, which rendered this country, and some others, much more "white" than they were before. In general, if I recall correctly, Peru and Bolivia have the highest number of amerindian / mestizzo population, and I think (I'm not sure about it) that Venezuela, Colombia had more black people than other hispanic colonies. I'll check it, but my general opinion is that there were developed and established cultures existed prior to european advent, Indians and mestizzos are obviously more present, both because they were more numerable there to start with, and because it was easier for them to merge with Europeans than for some jungle or steppe nomadic tribe.
South America also has a lot more Southern European descent (namely Spanish and Portuguese), which are shown to have more lactose intolerance.
Also a lot of arabic immigration. But when it comes to Argentina or SE Brazil, immigration of Poles or Germans was very big as well. So I doubt they should be on the same level of lactose intolerance as subsaharean Africa...