One way of studying mortality is to record the age at death in a population of similar individuals. Thus it is possible to plot out survivorship curves for a group of organisms all born at the same (a cohort). For example, curve I of Fig. 8.1 shows a survivorship curve for a natural population of ramshorn snails. Here the probability of death reduced with age, and this can be ascribed to the fact that in nature small, young snails are more often susceptible to predation, disease or accident than are older organisms. Sometimes these agents of ‘unnatural’ death act in an age-independent manner as is shown for another species of snail in curve II of Fig. 8.1. Take ‘unnatural’ death away, however, as in domestication and a welfare state, and there is still mortality — a mortality which strikes the old more than the young. This sort of process is illustrated in curve III of Fig. 8.1 for the ramshorn snail under laboratory conditions, which provided plenty of food but excluded predation. If the possibility that curve III was caused by a gradual deterioration of the environment can be excluded, then it must have been brought about by deterioration intrinsic to the organisms themselves. This is the process of ageing or senescence. The survivorship curve for the human population has probably shifted from a Type I–II curve in ancient times to a Type III curve in recent times.
KeywordsAgeing Process Catastrophe Theory Ageing Phenomenon Base Analogue Gompertz Equation
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