Why Evolution Is True (36 page)

In response to these distasteful episodes of racism, some scientists have overreacted, arguing that human races have no biological reality and are merely sociopolitical “constructs” that don’t merit scientific study. But to biologists, race—so long as it doesn’t apply to humans!—has always been a perfectly respectable term. Races (also called “subspecies” or “ecotypes”) are simply populations of a species that are both geographically separated and differ genetically in one or more traits. There are plenty of animal and plant races, including those mouse populations that differ only in coat color, sparrow populations that differ in size and song, and plant races that differ in the shape of their leaves. Following this definition,
Homo sapiens
clearly does have races. And the fact that we do is just another indication that we don’t differ from other evolved species.

The existence of different races in humans shows that our populations were geographically separated long enough to allow some genetic divergence to occur. But how much divergence, and does it fit with what the fossils indicate about our spread from Africa? And what kind of selection drove those differences?

As we would expect from evolution, human physical variation occurs in nested groups, and in spite of valiant efforts by some to create formal divisions of races, exactly where one draws the line to demarcate a particular race is completely arbitrary. There are no sharp boundaries: the number of races recognized by anthropologists has ranged from three to more than thirty. Looking at genes shows even more clearly the lack of sharp differences between races: virtually all the genetic variation uncovered by modern molecular techniques correlates only weakly with the classical combinations of physical traits such as skin color and hair type commonly used to determine race.

Direct genetic evidence, accumulated over the last three decades, shows that only about 10 to 15 percent of all genetic variation in humans is represented by differences
between
“races” that are recognized by differences in physical appearance. The remainder of the genetic variation, 85 to 90 percent, occurs
among individuals within races.

What this means is that races don’t show all-or-none differences in the forms of genes (alleles) that they carry. Instead, they usually have the same alleles, but in different frequencies. The ABO blood group gene, for example, has three alleles: A, B, and O. Almost all human populations have these three forms, but they are present in different frequencies in different groups. The O allele, for example, has a frequency of 54 percent in Japanese, 64 percent in Finns, 74 percent in South African !Kung, and 85 percent in Navajos. This is typical of the kind of differences we see in DNA: you can’t diagnose a person’s origin from a single gene alone, but must do so from looking at a combination of many genes.

At the genetic level, then, human beings are a remarkably similar lot. That is just what we would expect if modern humans left Africa a mere 60,000 or 100,000 years ago. There has been little time for genetic divergence, although we have spread to all corners of the world, breaking up into various far-flung populations that were genetically isolated until recent decades.

So does this mean that we can ignore human race? No. These conclusions don’t mean that races are merely mental constructs or that the small genetic differences between them are uninteresting. Some racial differences give us clear evidence of evolutionary pressures that acted in different areas, and can be useful in medicine. Sickle-cell anemia, for example, is most common in blacks whose ancestors came from equatorial Africa. Because carriers of the sickle-cell mutation have some resistance to falciparium malaria (the deadliest form of the disease), it’s likely that the high frequency of this mutation in African and African-derived populations resulted from natural selection in response to malaria. Tay-Sachs disease is a fatal genetic disorder that is common among both Ashkenazi Jews and the Cajuns of Louisiana, probably reaching high frequencies via genetic drift in small ancestral populations. Knowing one’s ethnicity is a tremendous help in diagnosing these and other genetically based diseases. Moreover, the differences in allele frequencies between racial groups mean that finding appropriate organ donors, which requires a match between several “compatibility genes,” should take race into account.

Most of the genetic differences between races are trivial. And yet others, like those physical differences between a Japanese individual and a Finn, a Masai and an Inuit, are striking. We have the interesting situation, then, that the overall differences in gene sequences between peoples are minor, yet those same groups show dramatic differences in a range of visually apparent traits, such as skin color, hair color, body form, and nose shape. These obvious physical differences are not characteristic of the genome as a whole. So why has the small amount of divergence that has occurred between human populations become focused on such visually striking traits?

Some of these differences make sense as adaptations to the different environments in which early humans found themselves. The darker skin of tropical groups probably provides protection from intense ultraviolet light that produces lethal melanomas, while the pale skin of higher-latitude groups allows penetration of light necessary for the synthesis of essential vitamin D, which helps prevent rickets and tuberculosis.
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But what about the eye folds of Asians, or the longer noses of Caucasians? These don’t have any obvious connection to the environment. For some biologists, the existence of greater variation between races in genes that affect physical appearance, something easily assessed by potential mates, points to one thing:
sexual selection.

Apart from the characteristic pattern of genetic variation, there are other grounds for considering sexual selection as a strong driving force for the evolution of races. We are unique among species for having developed complex cultures. Language has given us a remarkable ability to disseminate ideas and opinions. A group of humans can change their culture much faster than they can evolve genetically. But the cultural change can also
produce
genetic change. Imagine that a spreading idea or fad involves the preferred appearance of one’s mate. An empress in Asia, for example, might have a penchant for men with straight black hair and almond-shaped eyes. By creating a fashion, her preference spreads culturally to all her female subjects, and, lo and behold, over time the curly-haired and round-eyed individuals will be largely replaced by individuals with straight black hair and almond-shaped eyes. It is this “gene-culture coevolution”—the idea that a change in cultural environment leads to new types of selection on genes—that makes the idea of sexual selection for physical differences especially appealing.

Moreover, sexual selection can often act incredibly fast, making it an ideal candidate for driving the rapid evolutionary differentiation of physical traits that occurred since the most recent migration of our ancestors from Africa. Of course, all this is just speculation, and nearly impossible to test, but it potentially explains certain puzzling differences between groups.

Nevertheless, most controversy about race centers not on physical differences between populations, but behavioral ones. Has evolution caused certain races to become smarter, more athletic, or cannier than others? We have to be especially careful here, because unsubstantiated claims in this area can give racism a scientific cachet. So what do the scientific data say? Almost nothing. Although different populations may have different behaviors, different IQs, and different abilities, it’s hard to rule out the possibility that these differences are a nongenetic product of environmental or cultural differences. If we want to determine whether certain differences between races are based on genes, we must rule out these influences. Such studies require controlled experiments: removing infants of different ethnicity from their parents and bringing them up in identical (or randomized) environments. What behavioral differences remain would be genetic. Because these experiments are unethical, they haven’t been done systematically, but cross-cultural adoptions anecdotally show that cultural influences on behavior are strong. As the psychologist Steven Pinker noted, “If you adopt children from a technologically undeveloped part of the world, they will fit in to modern society just fine.” That suggests, at least, that races don’t show big innate differences in behavior.

My guess—and this is just informed speculation—is that human races are too young to have evolved important differences in intellect and behavior. Nor is there any reason to think that natural or sexual selection has favored this sort of difference. In the next chapter we’ll learn about the many “universal” behaviors seen in all human societies—behaviors like symbolic language, childhood fear of strangers, envy, gossip, and gift-giving. If these universals have any genetic basis, their presence in every society adds additional weight to the view that evolution hasn’t produced substantial psychological divergence among human groups.

Although certain traits like skin color and hair type have diverged among populations, these appear to be special cases, driven by environmental differences between localities or by sexual selection for external appearance. The DNA data shows that, overall, genetic differences among human populations are minor. It’s more than a soothing platitude to say that we’re all brothers and sisters under the skin. And that’s just what we’d expect given the brief evolutionary span since our most recent origin in Africa.

ALTHOUGH SELECTION doesn’t seem to have produced major differences between races, it has produced some intriguing differences between
populations
within ethnic groups. Since these populations are quite young, it is clear evidence that selection has acted in humans within recent times.

One case involves our ability to digest lactose, a sugar found in milk. An enzyme called lactase breaks down this sugar into the more easily absorbed sugars glucose and galactose. We are born with the ability to digest milk, of course, for that’s always been the main food of infants. But after we’re weaned, we gradually stop producing lactase. Eventually, many of us entirely lose our ability to digest lactose, becoming “lactose intolerant” and prone to diarrhea, bloating, and cramps after eating dairy products. The disappearance of lactase after weaning is probably the result of natural selection: Our ancient ancestors had no source of milk after weaning, so why produce a costly enzyme when it’s not needed?

But in some human populations, individuals continue to produce lactase throughout adulthood, giving them a rich source of nutrition unavailable to others. It turns out that lactase persistence is found mainly in populations that were, or still are, “pastoralists”—that is, populations who raise cows. These include some European and Middle Eastern populations, as well as Africans such as Masai and Tutsi. Genetic analysis shows that the persistence of lactase in these populations depends on a simple change in the DNA that regulates the enzyme, keeping it turned on beyond infancy. There are two alleles of the gene—the “tolerant” (on) and “intolerant” (off) form—and they differ in only a single letter of their DNA code. The frequency of the tolerant allele correlates well with whether populations use cows: it’s high (50 to 90 percent) in pastoralist populations of Europe, the Middle East, and Africa, and very low (1 to 20 percent) in Asian and African populations that depend on agriculture rather than milk.

Archaeological evidence shows that humans began domesticating cows between 7,000 and 9,000 years ago in Sudan, and the practice spread into sub-Saharan Africa and Europe a few thousand years later. The nice part of this story is that we can, from DNA sequencing, determine when the “tolerant” allele arose by mutation. That time, between 3,000 and 8,000 years ago, fits remarkably well with the rise of pastoralism. What’s even nicer is that DNA extracted from 7,000-year-old European skeletons showed that they were lactose-intolerant, as we’d expect if they weren’t yet pastoral.

The evolution of lactose tolerance is another splendid example of gene-culture coevolution. A purely cultural change (the raising of cows, perhaps for meat) produced a new evolutionary opportunity: the ability to use those cows for milk. Given the sudden availability of a rich new source of food, ancestors possessing the tolerance gene must have had a substantial reproductive advantage over those carrying the intolerant gene. In fact, we can calculate this advantage by observing how fast the tolerance gene increased to the frequencies seen in modern populations. It turns out that tolerant individuals must have produced, on average, 4 to 10 percent more offspring than those who were intolerant. That is pretty strong selection.
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Anybody who teaches human evolution is inevitably asked: Are we still evolving? The examples of lactose tolerance and duplication of the amylase gene show that selection has certainly acted within the last few thousand years. But what about right now? It’s hard to give a good answer. Certainly many types of selection that challenged our ancestors no longer apply: improvements in nutrition, sanitation, and medical care have done away with many diseases and conditions that killed our ancestors and removed previously potent sources of natural selection. As the British geneticist Steve Jones notes, five hundred years ago a British infant had only a 50 percent chance of surviving to reproductive age, a figure that has now risen to 99 percent. And for those who do survive, medical intervention has allowed many to lead normal lives who would have been ruthlessly culled by selection over most of our evolutionary history. How many people with bad eyes, or bad teeth, unable to hunt or chew, would have perished on the African savanna? (I would certainly have been among the unfit.) How many of us have had infections that, without antibiotics, would have killed us? It’s likely that, due to cultural change, we are going downhill genetically in many ways. That is, genes that once were detrimental are no longer so bad (we can compensate for “bad” genes with a simple pair of eyeglasses or a good dentist), and these genes can persist in populations.