To close this discussion of development, let's contemplate briefly the termination of the life cycle. Development from fertilized egg to adult is just one end of a series of biological changes that all organisms experience. For more than a century, development has represented a central challenge to experimental biologists, and it is currently a major focus of research activity. The inevitability of aging and death haunts the human consciousness and would seem to pose an equivalent intellectual challenge, yet we seem to have little in the way of explanation for why this happens to us. We plant annuals and perennials in the garden and we stand in awe of trees that have lived for centuries. We know that many insects may live for only days or weeks, a dog or cat for little more than a decade, while we and elephants and sea turtles may hope for several score years. But whether an individual is claimed by accident or infirmity, the end is inevitable. Why?
Sometimes the answer to this question is sought in terms of proximate cause. Perhaps our parts wear out, and if we just knew more about repair we could greatly extend our lives. And so we put a modest amount of money into research on aging, or geron tology, as the field is called. Interestingly, medical advances have increased the average life expectancy, but this has been accomplished solely by preventing death during younger years. Nothing in medical science has extended significantly the maximum age of about a century that individuals can reach; we simply increase the number of individuals who live to approach that maximum.
As we get along in years, a number of unpleasant things happen to us, including susceptibility to a host of ailments such as coronary disease, cancer, dementia, broken bones that do not heal, and so forth, and it becomes just a matter of time until one or another of these afflictions carries us away. We may not like to think about it this way, but we are programmed to age and die.
If that is the correct way to view the matter, then perhaps we can find some meaning, or at least explanation, if we wonder about ultimate cause. But to arrive at a sensible answer I think we have to recognize again the great variety of life-history strategies that living forms have adopted. Different forms of life cycles represent alternative ways genetic information has of propagating itself through time. It is not that one strategy is better than another; all that now exist have simply proved to be adequate to the challenges that they have met in their evolutionary history. Nothing will be gained by looking for the advantage of one compared to another, although we can make some sensible statements about ecological conditions that may either favor or compromise success of one or another strategy. To find the answer to our riddle, we should instead look for arguments that can be applied more generally.
It matters little where we start, so let's begin with ourselves. In our life history, our young take time to develop strength and experience, and it would be pointless for reproductive capacity to mature so early that novice parenting would not be successful. Within this constraint, imposed by the particular life history of our species, there is nevertheless reason for individuals to start reproducing as early as is consistent with a successful outcome. One reason why this is so is that the longer an individual postpones reproduction, the greater is the probability that he or she will either succumb to an accident before being able to reproduce or that there will be unwanted mutations in the germ line. If for any reason the probability of successful reproduction declines with age, regardless of physical condition, then those individuals that make an effort to reproduce early will, on average, leave more and healthier off spring than those that defer. In our species the risks of pregnancy are in fact greater for older women. Furthermore, earlier reproduction hastens the contribution of the bearer's genes into succeeding generations. Everything else being equal, selection will favor early reproduction, but not so early as to jeopardize success. In itself, this is a compromise.
Two other considerations bear on the argument. If the act of reproduction carries with it sufficient cost—for example, either direct risk to life or indirect risk through weakening or added exposure to danger—there may be an effective limit to an individual's capacity for reproduction. Perhaps it is for this reason that human females produce but several hundred eggs in their lifetime. If there is a limitation on total reproductive effort, and selection is encouraging early reproduction, the reproductive potential of individuals will decline after a certain age. As it diminishes, however, various forms of selection for somatic survival will be diminished. There will be decreasing need, in an evolutionary sense, to keep the body in the pristine shape it enjoyed when reproductive potential was high, and defenses against the ravages of tumors may relax. These changes may proceed slowly, as in humans, where an individual's contributions to inclusive fitness may continue through behavioral means long after direct capacity for reproduction has ended. Alternatively, changes leading to death may be cataclysmic, such as with salmon, where reproductive effort is compressed into a single act with no subsequent parental investment.
This argument is based on the observation that most genes have multiple phenotypic effects and the inference that genes may be beneficial at one stage of the life cycle and neutral or deleterious at another.28 The process of development in fact involves the differential expression of genes, so in general genes can and do have different effects at different times. It is more speculative whether some genes and gene products that play a positive role in developing or in reproductively competent individuals may have deleterious effects later in the life cycle. Such genes would be selected for, but would contribute to the process of senescence. The uncertainty about the effects of estrogen supplements for postmenopausal women, however, provides an example that may make the concept seem more than plausible.
In summary, the notion that selection for continued integrity of the body will be relaxed in post-reproductive individuals is sound. On the other hand, the details of the cellular and subcellular compromises that are made during the life cycle, and their genetic substrate that is sifted by selection, will only become clear as we come to understand the molecular events that take place during development, maturation, and aging.
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