For a long time, the fact that immigrants from eastern Europe, Asia, and Latin America tended to be shorter than native-born Americans was taken as evidence that there were inherent racial differences between these populations. However, the immigrants had children who grew up to be as tall as anyone else. Geneticists now tell us that, although there can be considerable variation in height within a group due to genetics, height differences between groups are largely caused by diet. What does this demonstrate?