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PDF FormatEnter Cochran and Harpending with The 10,000 Year Leap. They argue for the existence of troublesome traits, but they do so in a new way. Instead of inferring underlying genetic differences from controversial tests, they claim that the laws of population genetics predict exactly the sort of differences we see in the test data. They don’t make any lengthy arguments for the tests as a valid measure of genetic differences. They simply state that if what they propose regarding population genetics and human history is true, then we should expect to see precisely those differences we do see in the test results.
To understand this line of argument, a short primer on population genetics is in order. Population genetics is a well-established sub discipline of evolutionary biology that studies gene frequencies in populations. A standard question in this discipline would be why gene X is common in population A but rare in population B, even though A and B are of the same species. There are a number of ways such differences can arise. Sometimes they are the result of pure chance, but the most important cause is natural selection. If natural selection is the reason for the unequal distribution of gene X, geneticists would say that gene X likely increased the reproductive success—or “fitness”—of A, whereas it decreased that of B. In population A, gene X was “selected for” and therefore spread; among B it was “selected against” and therefore disappeared or became very rare.
Broadly speaking, the environment is the “selector” referred to by the term “natural selection.” Population geneticists characterize environments according to their “selection pressures,” that is, the many forces at work that determine fitness. Geneticists would say that gene X worked for population A and not B because A was situated in an environment that placed it under different selection pressures than B. In other words, population A’s environment selected for gene X because it improved fitness in that environment, and population B’s environment selected against it because it didn’t. The important point is this: When two genetically similar populations are subjected to significantly different selection pressures, significant genetic differences between the populations will nearly always appear.
Cochran and Harpending apply this theory to human beings, and make the following claims as a result: Humans live in populations, that is, groups whose members breed more often among themselves than with outsiders. Typically, all these populations taken together are at genetic equilibrium, meaning that a) the forces of natural selection have genetically optimized all of them for life under common selection pressures; and b) as a result of this optimization, they are relatively homogeneous genetically. Occasionally, however, some disruptive event—a change in the environment, a pandemic disease, etc.—upsets the equilibrium within particular populations. In other words, these changes affect only some of these populations, not all of them. For the affected groups, the traits selected under the old selection pressures are no longer advantageous under the new ones. At this point, two things tend to occur: sudden evolutionary acceleration and genetic division.
What this means is that as the affected populations adapt to new selection pressures, the rate of genetic change—measured by shifting gene frequencies—speeds up, exceeding the long-term average. This process, however, is usually temporary. Assuming that both selection pressures and population size remain stable, the rate of genetic change necessarily decelerates as natural selection adapts the affected populations to the new selection pressures. As the process approaches a new equilibrium between genes and pressures—meaning that nearly all new genetic mutations are deleterious—evolutionary change slows to a crawl, and the new equilibrium will remain mostly stable until the next disruptive event.
It should be immediately apparent, however, that this process causes something else to occur as well: It divides humankind into two different genetic types. The affected population has adapted to new selection pressures and reached a new equilibrium, while the unaffected populations have simply remained as they were. However, even when they do occur, divisions such as these have usually proven temporary, mainly because human populations are almost never completely isolated for long periods of time. For a variety of reasons, one population’s genes tend to spread to others and the result is a species-wide equilibrium. But this return to a common equilibrium is not a given. If two populations remain isolated long enough and are subject to sufficiently different selection pressures, then they can evolve into separate species. This happened, for example, with chimpanzees and humans some six million years ago.
