Courtesy of Douglas C. Wallace
Researchers constructed a phylogenetic tree based upon human mitochondrial DNA (mtDNA) variation. A branch bifurcates whenever they found an additional polymorphism. At the top of the inverted tree is mitochondrial "Eve"; the illustration shows two mtDNA sub-branches, or lineages, found in Europe and the Middle East. The J1- and J2-branch polymorphisms in the cytochrome b gene might have spread because they were climatically advantageous. (Reprinted from E. Ruiz-Pesini, et al., Science, 303:223–6, 2004.)
Many selective forces must have influenced human evolution, but the only one that all population geneticists seem to agree upon is malaria. Time and again, studies have identified certain DNA polymorphisms – most famously, the β-globin variant underlying red-cell sickling – that helped people resist this mosquito-borne disease. The reproductive success of such individuals spread these polymorphisms throughout regions where malaria is endemic.
Geneticists have been much more reluctant, in contrast, to ...
