In Population Genetics, when part of a population moves to a new location and has a massive expansion in size, then it will differ genetically from the original population because all the offspring come from that small group. This is known as the Founder Effect and important when studying the biology of islands: think Darwin's finches. As you say, this is true of descendents of European settlers in the US when compared genetically to the European population as a whole. I'm interested in genetic genealogy and it is a very clear effect. I didn't actually understand what Chris Carter meant here though.There is a gentic, i'd like to say condition, but it is a result of colonisation by a select group, we'd call it founders syndrome. This acualy occured so far as certain genetic dissorders were more prevelant due to random chance in the small sample size of the originall european settlers to the US. I wouldn't be surprised if Chris Carter is playing on that idea.
It also true of those who originally** left Africa, so while those living on other continents have more varied appearances (due to adaptation to the environment), they are less genetically varied than those who remained in Africa.As you say, this is true of descendents of European settlers in the US when compared genetically to the European population as a whole.