PLEIOTROPY, NATURAL SELECTION, AND THE EVOLUTION OF SENESCENCE

form the much simpler task of merely maintaining what is already formed. It is true, of course, that some parts of organisms do literally wear out. Human teeth, for instance, show wear similar to that of any tool subjected to friction, but this wear is no more a part of senescence than is the wearing away of replaceable epidermal cells. The senescence of human teeth consists not of their wearing out but of their lack of replacement when worn out. August Weismann (1891) was the first biologist of the evolutionary era to advance a theory of senescence. He believed that organisms must inevitably show a decline analogous to that of mechanical devices, but that, in addition, there was a specific death-mechanism designed by natural selection to eliminate the old, and therefore wornout, members of a population. He did not clearly indicate how such a mechanism could be produced by natural selection. He was likewise dubious about the exact nature of the deathmechanism, but indicated that it might involve a specific limitation on the number of divisions that somatic cells might undergo. Weismann's theory is subject to a number of criticisms, the most forceful of which are: 1) The fallacy of identifying senescence with mechanical wear, 2) the extreme rarity, in natural populations, of individuals that would be old enough to die of the postulated death-mechanism, 3) the failure of several decades of gerontological research to uncover any deathmechanism, and 4) the difficulties involved in visualizing how such a feature could be produced by natural selection.