Why Should Senescence Evolve? An Answer Based on a Simple Demographic Model
Why age? Many single-celled organisms do not undergo either individual or clonal senescence. Moreover there are multicellular organisms which show no aging because lost cells, subcellular organelles, and smaller constituents are continually replaced. It would appear that evolution has simultaneously progressed in two divergent directions. On the one hand, highly evolved species tend, in general, to live longer than primitive ones. On the other hand, most higher forms, at least in the animal kingdom, exhibit aging. Other things being equal, it is clear that long life confers selective advantage on a species. It is much less obvious that aging does so.
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