By the measure of genes though, humans are amazingly uniform. Humans are genetically less diverse than chimps, and both chimps and humans are much less diverse than a common species of fruit fly. Given our species' long history of racial conflict, our genetic uniformity may come as a surprise. Not too long ago people in polite company would debate whether different human races really all belonged to one species. Our DNA tells us that our genetic differences don't even come close to matching the variety found within a single, apparently monotonous fruit fly species.
Measuring Genetic Diversity
How do we measure genetic diversity? The basic concept is simple. Pick a few dozen or a few hundred genes and tally the differences in those genes in a sample of a few dozen or a few hundred people. Thus at gene A, person 1 and person 2 may differ at 5 DNA bases, while person 1 and person 3 may differ at 7 bases. After making all of the possible comparisons for gene A, you compute the average number of differences - in the case of gene A let's say we find an average of 6 differences between any two people. If gene A is 6000 bases long, we say any two people would differ on average once in every 1000 DNA bases.
But when we look at gene B, we may get a different answer - maybe only an average of one difference every 2000 DNA bases. To get a good measure of how genetically different humans tend to be, it's obviously best to get results from many different genes, or even entire genomes. Most recent studies have found that humans differ on average once in every 1000-1500 DNA bases. Given that we have 3 billion DNA bases in our genome (just counting one copy - either your maternal or paternal version), any two people are likely to differ at about 2-3 million positions in their genomes. That number applies to differences within yourself as well - there are about 2-3 million differences between the DNA you inherited from your mother and the DNA you inherited from your father.
(There exist more sophisticated ways of measuring genetic differences, and we could include genetic variants that involve more than just a difference at a single DNA base pair - we could count segments of DNA that have been inserted or deleted, called copy number variants. However the basic approach to measuring genetic difference that I've described is all we need here.)
How does this genetic variation stack up against variation in other animals? While we tend to differ at 1 position in every 1000 bases or so, chimps differ at about 1 in every 750 bases, and gorillas differ at roughly one in every 650 bases. The fly species D. melanogaster is much more genetically diverse, and tends to differ at one out of every few hundred bases. Keep in mind that these estimates are just averages - genetic variation is not spread out evenly across an entire genome. Some portions of our genomes are highly variable, while others are relatively uniform.
The bottom line is this: although fruit flies and gorillas may look largely the same to us, they beat us hands down in genetic diversity.
Genetic Differences Among Human Populations
Of course the genetic differences that do exist among humans are enough to generate much of the biological diversity we see around us - differences in skin, hair, and eye color, our voices, our physical stature, and our personalities. Obviously environment plays a big role in many traits, but as the differences between Samoans and Japanese illustrate, genetics can account for a great deal even when there is a large environmental influence.
At first guess, you might think that most of our genetic diversity would fall along racial lines. Race differences often seem to be the most obvious differences among different human groups, so it wouldn't be surprising if genetic differences fell along racial lines as well. Genetically, a white guy like me should be much more similar to my white neighbor than my black one, right?
Before we get to the answer, let's fill in some background. In any discussion of genes and diversity, it is critical to keep in mind one obvious but nevertheless subtle point: only genes that exist in different versions are responsible for our biological diversity. There may be a gene that is critical for intelligence, but if that gene is absolutely 100% identical in all humans, then it does not account for differences in intelligence among humans. We may one day identify all of the genes involved in producing intelligence (defining intelligence may actually be a harder problem), but we won't understand the genetic basis of differences in intelligence until we discover the human genetic variation in the genes responsible. This narrows our species' genetic palette - not all of the genes responsible for a particular trait will be a source of diversity in that trait.
With the availability of genome-scale tools, researchers have devoted a significant amount of effort in the last eight years to finding those genes that exist in different versions among humans. We haven't mapped all of this genetic diversity yet, but there is enough to start looking at genetic variation among humans in different parts of the world. With this new data, we are developing a much more detailed picture of how races differ genetically.
'Race' though, is a horribly sloppy term. Geneticists prefer to speak about populations, not out of political correctness but because race is extremely imprecise. We've all filled out some form or another asking whether we are Black, White, Hispanic, or 'none of the above.' It's obvious that this is much less informative than knowing whether someone's ancestry is African, Australian Aborigine, European, or East Asian.
Thus researchers like Richard Lewontin have argued that "As a biological rather than a social construct, 'race' has ceased to be seen as a fundamental reality characterizing the human species." Race may be too imprecise to be biologically meaningful, but there has to be some biological reality behind the obvious physical differences in different human populations, right?
Yes, there are genetic differences between different human populations, but the big surprise is this: genetic differences between human populations are few compared to the differences within human populations.
Here is what that means:
If you compare my genome with that of a Chinese grad student down the hall from me, you'll find that only tiny fraction of the 2-3 million differences between us tells you much about our ancestry. Among Chinese, there may be a tendency to have a DNA base 'G' at position XYZ in gene ABC on chromosome 12, while among Europeans (where my ancestors came from), there is a tendency to have an 'A' at that same position. What we find though, in almost all cases, is that these tendencies are not absolute: 90% of Chinese may have base 'G', while the other 10% have base 'A'. And maybe 70% of Europeans have base 'A' at position XYZ, while 30% have base 'G'.
So in other words, the fact that my Chinese friend has base 'G' at position XYZ in gene ABC does not tell you with certainty that he's Chinese. In fact both my Chinese friend and I may have the base 'G' at that same position, even though it is less likely in my case. If you look at any one gene, you don't get enough information to make an accurate call.
In order to really see differences among human populations, you have to look at many genes (or any place in the genome where humans vary - it doesn't have to be a gene). In the Chinese population, base 'G' may be common at position XYZ on chromosome 12, base 'T' may be more common at position TUV on chromosome 6, etc., etc. So once you look at dozens or hundreds of informative positions, you can say with high confidence, 'this person is Chinese, and that one is European.' (And of course we could all be American or Canadian or British by birth - we're obviously talking about ancestral populations here.)
By examining enough genes, we can reliably use just DNA to correctly assign people to ancestral geographical populations. Researchers can do it blindly - they can look at DNA sequence for 1,000 people whose identities are hidden, and use DNA information to assign those people to geographical populations. Once the assignments are made, the researchers take a peek at the true identities of their sample group, and it turns out that their assignments are extremely accurate.
But here is where it gets paradoxical: while there are enough genetic differences among human populations to make accurate classifications, those genetic differences make up only 5-15% of the total amount of genetic variation. Most of the genetic variation among humans has nothing to do with differences in populations. The genetic differences between 'races' are minor compared to the differences between people in general.
Medicine and Race
So what does this mean about the biological differences we can observe directly? One key area where this has implications is medicine. This result suggests that race is not actually a very good predictor of medical outcomes (at least those based on genetics, not lifestyle). Let's take a classic case: There is a genetic variant in an angiotensinogen gene that substantially increases one's risk for developing high blood pressure. This genetic variant shows up fairly frequently among some African populations, in up to 90% of the population in some cases. But, like most medically important genetic variation, it shows up in other populations as well. 30% of Europeans also have this genetic variant.
Suppose there is a blood pressure drug that is ineffective in people with this particular angiotensinogen genetic variant. If you simply make treatment decisions on the basis of race, you will frequently make the wrong call - some Africans who would benefit wouldn't get the drug, and a fair number of Europeans who don't benefit would be given a useless prescription. As University of Utah geneticist Lynne Jorde argues (PDF):
[Genetic] variation tends to be shared widely among populations, so race will often be an inaccurate predictor of response to drugs or other medical treatments. It would be far preferable to test directly the responsible alleles [genetic variants] in affected individuals.
When it comes to medically important traits, we need to avoid typological thinking - when we consider genetic variation as a whole, humans don't fall neatly into racial or population categories. Yes, a minor fraction of genetic variation enables scientists to accurately categorize people by ancestral population, but the vast majority of the variation that produces diversity in our susceptibility to disease, response to drugs, and even our behavior does not fall neatly along racial lines. All human populations overlap substantially in their genetic diversity.
Intelligence and Race
The debate over race and intelligence has a long and tarnished history, although that doesn't mean it's not a legitimate scientific question to address. However, the debate has taken place almost entirely outside modern genetics, falling instead within the realm of psychology (such as work done by Arthur Jensen). Some writers would have you believe that science is converging on a consensus that the 'IQ' gap between various races is genetic (and that liberal conspirators are trying to cover it up). That claim is false. Researchers have not identified a single genetic variant with an impact on intelligence that falls along population lines. In fact several studies have recently tested variants in genes that appear to be involved in controlling brain size. No correlation with intelligence was found. Yes, genetics does play a significant role in intelligence, and many other traits. But there is simply no genetic evidence (and I mean real genetics, not psychology) for genetic differences in intelligence between human populations.
Why is this so? Other traits, like skin color, obviously fall along population lines. While skin color is obviously not a 100% reliable predictor, skin color is a major indicator of race. Irish, Kenyans, Pakistanis, and Chinese populations all have clearly different skin tones.
It turns out, not surprisingly, that the genetic variation for some (but not all) skin color genes does in fact follow population divisions, in contrast with most other genetic variation. This is most likely because skin color differences end up being relatively simple - a single variant of a gene (causing lighter skin, for example) can easily become common in a population through natural selection. The result is that you have different human populations with dramatic differences in skin color.
Other traits, however, are much more complex than skin color. Physical differences which are determined not by one, but many different genetic variants, are unlikely to split neatly by population. Intelligence is probably one of the most complex traits humans possess. It is almost certainly affected by variants in many different genes, and many of those genes have other important functions in the body. That means this: two different human populations could have easily developed differences in skin color between them, but differences in intelligence would have been extremely hard to develop, by chance or by natural selection.
Racial conflict has long been a part of human societies. Along with that conflict has come frequent speculation (most famously, but not exclusively among whites with European ancestry) that one race is inferior to another. Some have been worried that modern genetics would substantiate that belief, but our best genetic evidence to date shows those worries unfounded. Genetics does play a large role in the diversity we find among human beings. That diversity, in spite of some dramatic but superficial exceptions like skin color, is shared in common among all races.