Few villages today are as geographically isolated as Limone: Most have other villages fairly close to them, and generally there's traffic between them. People make frequent visits to these places close to home. The simplest and oldest mechanism of gene flow—marrying someone from the next village over— therefore still prevails. More often than not, this has meant women leaving their homes to join their husbands' communities, an ancient pattern that we still see in chimpanzees. This village-to-village contact has been a factor in gene flow ever since people settled in villages, and it has been one of the most important ways in which favorable alleles have spread. Given time, neighborhood marriages can carry an allele thousands of miles. A beneficial allele originating in one band or village could, through such intermarriage, gradually spread to neighboring populations, and then to neighbors' neighbors, and so on. Alleles with a big advantage would spread more rapidly than alleles with a small advantage.
In any case, with some simplifying assumptions, it's possible to model the spread of an adaptive allele with a mathematical formula. In that model, the frequency of the favored allele spreads in the form of a wave with a constant speed. The speed depends on the selective advantage and the root-mean-square distance separating the parents' and the child's birthplaces. If we call that marital distance a and the selective advantage of the allele s, the speed of advance is approximately a X (2s)% miles per generation.
Hunter-gatherers can be amazingly mobile, and since most recent hunter-gatherers were spread very thinly, there often were no girls next door. So hunter-gatherers, especially in sparsely settled areas, had to find mates at a considerable distance. A generation ago, when many Bushmen were still wandering freely, their average marital distance was over 40 miles. This may not have been typical in prehistory. In the days before agriculture, when everybody and his brother was a hunter-gatherer, most lived in choice territories, not in the marginal habitats like the Kalahari Desert where that way of life has persisted. Population density would have been higher in those conditions than among Bushmen today, and people may not have had to search so far for a mate. However, it is clear that agriculture eventually led to crowding. Peasant farmers usually marry people living nearby, not least because there are plenty of people living nearby to choose from. In an example discussed by Alan Fix, based on census records from a densely settled part of rural England about 150 years ago, the average marital distance was only 6 or 7 miles.7
Consider a new allele that has an advantage of 5 percent. In a well-mixed population it would rise to high frequency in about 8,000 years. Among hunter-gatherers like the Bushmen, it might spread 9 miles per generation, on average, and among farmers, about 1 or 2 miles per generation. Since the preponderance of recent evolution seems to have been driven by the changes associated with agriculture, the 1.5 miles per generation you'd expect in farmers would be a good estimate. So in the 400 generations since the birth of agriculture (at twenty-five years per generation), a gene with a 5 percent advantage would have moved out about 600 miles.
Although this way of spreading genes is simple, universal, and easy to understand, it's not the only way, and it's slow. When you run the numbers, it's hard to see how it can carry al-leles as far as they have actually traveled in the time available. There is a similar problem in understanding the spread of oak trees in England. Some 15,000 years ago, back in the Ice Age, oak trees were extinct there, or nearly so—oak trees just don't grow very well under thousands of feet of ice. Possibly a few hung on in protected southern river valleys. Yet today they're all over the island—a fine thing, no doubt, but how did they manage it? Oaks shouldn't spread very fast for the simple reason that the acorn doesn't fall far from the tree.
The answer is that unusual events took place often enough for oak trees to spread faster than would have been possible if all the acorns stayed near their trees. Occasionally a bird carried an acorn far ahead of the main wave of advance and started a new oak forest. An acorn might have floated a long way down a river and sprouted. Maybe some of the first humans who resettled Britain after the ice melted took some acorns with them on a long hunting trip north and dropped a few. The speed of advance is determined more by these rare, long-distance events than by gravity and squirrels.
So although local marriage is a big part of the story, rarer, weirder, more complicated events most likely determined the speed of advance of new favorable alleles in humans.
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