Wednesday, November 16, 2005


"But of course all species have family trees. All species inherit genetic material. All species with two sexes have an Adam and an Eve. Genes and gene trees are an ubiquitous feature of life on Earth. The techniques that we apply to recent human history can also be applied to the rest of life.

"Cheetah DNA reveals a 12,000-year-old population bottleneck important to feline conservationists. Maize DNA has stamped upon it the unmistakable signature of its 9,000-year Mexican domestication. The coalescence patterns of HIV strains can be used by epidemiologists and medical doctors to understand and contain the virus. Genes and gene trees reveal the history of the flora and fauna of Europe: the vast migrations driven by ice ages whose waxing pushed temperate species into southern-European refuges, and whose waning stranded Arctic species on isolated mountain ranges. All these events and more can be traced in the distribution of DNA around the globe, a historical reference book which we are only just learning to read.

"We have seen how different genes have different stories to tell, which can be pieced together to reveal something of our history, both modern and ancient. How ancient? Amazingly, our oldest MRCA [Most Rccent Common Ancestor] genes can even date back before we were human at all. This is especially so when natural selection favors variety in the population for its own sake. Here's how it works.

"Suppose there are two blood types called A and B, which confer immunity to different diseases. Each blood type is susceptible to the disease against which the other type has immunity. Diseases flourish when the blood type that they can attack is abundant, because an epidemic can get going. So if B people, say, happen to be common in the population, the disease that hurts them will enjoy an epidemic. Consequently, B people will die until they cease to be common, and the A people increase—and vice versa. Whenever we have two types, the rarer of which is favored because it is rare, it is a recipe for polymorphism: the positive maintenance of variety for variety's sake. The ABO blood group system is a famous polymorphism which has probably been maintained for this kind of reason.

"Some polymorphisms can be quite stable—so stable that they span the change from an ancestral to a descendant species. Astonishingly, our ABO polymorphism is present in chimpanzees. It could be that we and chimps have independently 'invented' the polymorphism, and for the same reason. But it is more plausible that we have both inherited it from our shared ancestor, and independently kept it going during our six milIion years of separate descent, because the relevant diseases have been continuously at large throughout that time. This is called trans-specific polymorphism, and it may apply to far more distant cousins than chimpanzees are to us.

"A stunning conclusion is that, for particular genes, you are more closely related to some chimpanzees than to some humans. And I am closer to some chimpanzees than to you (or to 'your' chimpanzees). Humans as a species, as well as humans as individuals, are temporary vessels containing a mix of genes from different sources. Individuals are temporary meeting points on the crisscrossing routes that genes take through history. This is a tree-based way to express the central message of The Selfish Gene, my first book. As I put it there, 'When we have served our purpose we are cast aside. But genes are denizens of geological time: genes are forever.' " —Richard Dawkins, THE ANCESTOR'S TALE, pp. 59-61

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