From New Scientist:
Luis Barreiro of the University of Montreal in Quebec, Canada, and his colleagues identified 16 regions of the genome associated with short stature in the Batwa pygmies of Uganda. They then compared these regions in 169 Batwa and 74 Baka pygmies from West Africa.
“In both groups, there was greater variation in those regions associated with being short, but no overlap between them,” says Barreiro. This suggests they evolved their stature independently instead of inheriting the same “pygmy genes” from a common ancestor.
The team thinks “pygmyism” offers and evolutionary advantage to those living in a rainforest because they can duck under branches more easily.
But pygmy societies (men under five feet tall) exist in many parts of the world, and
Besides the differences within pygmy populations, there are also some non-pygmy populations that face some of the same physical challenges as pygmies but haven’t evolved a short stature. For example, many human populations live in dense forests and experience regular food shortages, and yet these populations have larger body sizes.
Other theories include adaptation to low levels of ultraviolet light or to heat and humidity, and need to reach sexual maturity more quickly due to harsh living conditions.
In any event, the use of the term “evolution” suggests that the change is permanent. But is it? An interesting article in Scientific American in 1998 noted that humans as a group are getting taller:
Let’s use this basic operating principle of evolution to predict, retrospectively, the direction of change in human height if evolution were the cause of the change. We know from studies conducted in industrial England that children born into lower socioeconomic classes were shorter, on average, than children born into wealthy families. We also know that poorer families had larger numbers of children.
Given those initial conditions, what would evolution predict? The average population should have become shorter because the shorter individuals in the population were, from an evolutionary fitness perspective, more successful in passing on their genes. But this did not happen. Instead, all segments of the population–rich and poor, from small and large families–increased in height. Thus, natural selection, the process whereby differences in reproductive success account for changes in the traits of a population, does not explain why we are taller.
The change is generally accounted for by improved quality of life, which allows most people to reach their potential maximum height.
Question: Do pygmies who leave the rain forest start to grow taller within a few generations? If so, is it meaningful to talk about their stature a form of evolution, as opposed to an adaptation within a range of possible heights? One that varies with conditions because it confers no permanent change?
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