The Washington Post today carried this article detailing what, exactly, caused certain sections of humanity to evolve white skin. While the natural forces behind the selection have been understood for some time*, the specific cause was unknown until now. Considering the energy expended over it throughout history, it's a little ironic that the change itself is very, very small.

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* Bones need vitamin D to form properly. In pre-industrial societies, vitamin D comes almost exclusively from the sun via an interaction between (as I recall) ultraviolet light and fatty tissues near the skin. Even a mild form of ricketts (bone deformation due to lack of vitamin D) will render a woman incapable of surviving childbirth.

Dark skin filters much of the vitamin D-producing ultraviolet sunlight light out, which is not a problem in the tropics, but becomes a big deal in the further north you go. Hence a powerful positive selection pressure for light skin.