|MadSci Network: Evolution|
Dear Hannah: You ask a number of related questions. Let me try to explain the situation in as parsimonious a way as possible. Evolution by means of natural selection implies that those individuals with more advantageous traits are selected for survival more often than those with less advantageous traits and thus more often survive to pass on these advantageous traits to offspring. Advantageousness is decided by the particular environmental factors present during the lifetimes of the individuals involved.
I believe that the amount and placement of hair in modern human anatomy in part represents the natural selection for the advantageous quantity and placement of this trait on the body. This selection probably coincided with the dispersion of modern humans into areas with different environmental pressures. In addition, it needs to be understood that much of the genetic makeup of humans as with other biological creatures is neutral. In other words, natural selection does not work in favor or against these neutral traits unless and until the specific environment pressures specific organisms to change evolutionarily because of physical advantageousness or disadventageousness. Please read over Darwin's "On the Origin of Species..." as well as numerous articles on natural selection in recent articles in such publications as the magazine "Natural History" and popular books by such authors as Stephen Jay Gould.
Try the links in the MadSci Library for more information on Evolution.