For those who want to argue human races are a scientific category, they have to use scientific standards to prove their case. That is precisely the problem. The only way to argue for scientific human races is by defining them differently than for other species, but there is no scientific justification for defining them differently.
One of the factors that makes the human species unique from other similar species is that we lack much genetic diversity. We are a bottleneck species. Twice in human evolution the entire species originated from a single common ancestor. On top of that, human populations have never been isolated for long enough periods to form separate lines of evolution. Humans all around the world have moved around and mixed together almost ceaselessly. Even island people are known to have traveled great distances.
There simply isn’t enough genetic diversity to form separate human races.
Why Your Race Isn’t Genetic
by Michael White
from Pacific Standard magazine
“Templeton examined two genetic definitions of race that are commonly applied by biologists to vertebrate species. In both cases, races clearly exist in chimpanzees, our nearest relatives, but not in humans.
“One natural definition of race is a group whose members are genetically much more similar to each other than they are to other groups. Putting a number on what counts as “much more” is a somewhat arbitrary exercise, but Templeton found that the genetic differentiation between populations of chimpanzees is over seven times greater than the genetic differentiation between broad geographical populations of humans. Furthermore, the level of genetic differentiation between human populations falls well below the threshold that biologists typically use to define races in non-human species.
“Races could also be defined by genetic branches on the family tree. For most of us, this is the most intuitive definition of race. It’s one that, at first glance, is consistent with recent human evolution: After originating in Africa, part of our species branched out first into Asia and Europe, and then to the rest of the world. We should thus expect different geographical populations to be distinct genetic limbs on our species’ recent evolutionary tree.
“But as it turns out, our species’ family history is not so arboreal. Geneticists have methods for measuring the “treeness” of genetic relationships between populations. Templeton found that the genetic relationships between human populations don’t have a very tree-like structure, while chimpanzee populations do. Rather than a family tree with distinct racial branches, humans have a family trellis that lacks clear genetic boundaries between different groups.
“These findings reflect our unusual recent evolutionary history. Unlike the distinct populations of chimps, humans continued to exchange both goods and genes with each other even as they rapidly settled an enormous geographical range. Those ongoing contacts, plus the fact that we were a small, genetically homogeneous species to begin with, has resulted in relatively close genetic relationships, despite our worldwide presence. The DNA differences between humans increase with geographical distance, but boundaries between populations are, as geneticists Kenneth Weiss and Jeffrey Long put it, “multilayered, porous, ephemeral, and difficult to identify.” Pure, geographically separated ancestral populations are an abstraction: “There is no reason to think that there ever were isolated, homogeneous parental populations at any time in our human past.””