The Plausibility of Life

The Plausibility of Life makes the exciting argument that phenotypic plasticity, both in development and in adult life, greatly facilitates evolution. When trying to explain it, the easiest example for me is the change resulting from moving a population to high altitudes. The red blood cell count and breathing rate increase to compensate for the lower oxygen content; those changes are phenotypic variation, without genetic change. Those changes create other stresses on the organism, though, so the population undergoes selection for genetic changes that relieve the stress. The result is that a population can colonize a new habitat without having to wait for fortunate mutations, and once in the new habitat can evolve to fit better, with the existing plasticity of phenotypic expression facilitating rapid evolution.

This article exactly matches the example, and there are two other interesting aspects. First, the mutation described has clearly happened in three separate populations. Even assuming direct descent, the Andean founding population would be separated from the Tibetan population by at least several thousand years, during which time the high altitude adaptation would have been unneeded. Random mutation soon wrecks anything not selected for, so it seems much more likely to have evolved separately several times. Second, the speed of evolution is astounding. The ability to achieve very pronounced changes in the phenotype by altering the degree to which a gene is expressed (also part of The Plausibility of Life's argument) makes evolution very rapid in cases like this. Even assuming humans have lived in the Andes for 10,000 years with a generation every 20 years, that is still only 500 generations in which to achieve a remarkable adaptation. It is interesting though that the adaptation has not completely swept through the population. I know too little about population biology to guess why that might be.