Pleiotropy, Natural Selection, and the Evolution of Senescence

Abstract

A new individual entering a population may be said to have a reproductive probability distribution. The reproductive probability is zero from zygote to reproductive maturity. Later, perhaps shortly after maturity, it reaches a peak value. Then it declines because of the cumulative probability of death. There is a cumulative probability of death with or without senescence. The selective value of a gene depends on how it affects the total reproductive probability. Selection of a gene that confers an advantage at one age and a disadvantage at another will depend not only on the magnitudes of the effects themselves but also on the times of the effects. An advantage during the period of maximum reproductive probability would increase the total reproductive probability more than a proportionately similar disadvantage later on would decrease it. So natural selection will frequently maximize vigor in youth at the expense of vigor later on and thereby produce a declining vigor (senescence) during adult life. Selection, of course, will act to minimize the rate of this decline whenever possible. The rate of senescence shown by any species will reflect the balance between this direct adverse selection of senescence as an unfavorable character and the indirect favorable selection through the age-related bias in the selection of pleiotropic genes. Variations in the amount of fecundity increase after maturity, in the adult mortality rate, and in other life history features would affect the shape of the reproductive probability distribution and thereby influence the evolution of senescence. Any factor that decreases the rate of decline in reproductive probability intensifies selection against senescence. Any factor that increases the rate of this decline causes a relaxed selection against senescence and a greater advantage in increasing youthful vigor at the price of vigor later on. These considerations explain much of what is known of phylogenetic variation in rates of senescence. Other deductions from the theory are also supported by limited available evidence. These include the expectation that rapid morphogenesis should be associated with rapid senescence, that senescence should always be a generalized deterioration of many organs and systems, and that postreproductive periods be short and infrequent in any wild population. Reproduced by permission. G. C. Williams, Pleiotropy, Natural Selection, and the Evolution of Senescence. Evolution 11 , 398-411 (1957).