With the advent of gene-sequencing technology, scientists have confirmed Dobzhanksy’s discovery of variability between populations. But it turns out that some species have another sort of variation as well, variation within populations rather than between them. Humans are a spectacular example. Human races were once thought to be distinct, so much so that some even went so far as to claim that they represented separate species. Research on human genetics shows that it is indeed possible to trace back the ancestry of different ethnic groups for thousands of years. But genes have also managed to cross the boundaries of these groups so often that there is much more variability within the people in any given population than between populations. The distinctions that we conventionally use to divide the species into races — skin color, hair, and the shape of faces — are controlled only by a few genes, while most other variable genes do not respect so-called racial boundaries. If all the humans on Earth were wiped out except a single tribe in a remote New Guinea valley, the survivors would still preserve 85% of the genetic variability of our entire species.