Genetica 91: 99-109, 1993. 9 1993 Kluwer Academic Publishers. Printed in the Netherlands. Population density effects on longevity Joseph L. Graves Jr. & Laurence D. Mueller Department of Ecology and Evolutionary Biology, University of California, Irvine, CA 92717, USA Received and accepted 22 June 1993 Abstract Population density, or the number of adults in an environment relative to the limiting resources, may have important long and short term consequences for the longevity of organisms. In this paper we summarize the way in which crowding may have an immediate impact on longevity, either through the phenomenon known as dietary restriction or through alterations in the quality of the environment brought on by the presence of large numbers of individuals. We also consider the possible long term consequences of population density on longevity by the process of natural selection. There has been much theoretical speculation about the possible impact of population density on the evolution of longevity but little experimental evidence has been gathered to test these ideas. We discuss some of the theory and empirical evidence that exists and show that population density is an important factor in determining both the immediate chances of survival and the course of natural selection. Theory of population density and senescence The study of the consequences of population den- sity on the longevity of organisms was in fact first explored by scientists interested in problems in evolutionary ecology. In this field there had for some time been interest in understanding those eco- logical conditions which would favor two alterna- tive life history patterns (Cole, 1954). Semelparity is one of these patterns which is characterized by a burst of reproduction shortly after sexual maturity, followed by rapid senescence and death. Semelpa- rous life histories are observed in annual plants, salmon and black widows, in addition to many other diverse groups of organisms. Iteroparity, which is the pattern displayed by humans, fruit flies and many other organisms, is characterized by re- peated episodes of reproduction after sexual matur- ity and thus a prolonged adult life stage. One of the early attempts to understand the eco- logical pressures which may be important to deter- mining which life history pattern might be most advantageous was made by MacArthur and Wilson (1967). In this work, MacArthur and Wilson sug- gest that natural selection will act in qualitatively different ways for populations kept at very high densities as opposed to those kept at very low den- sities. Much of the intuition and theory presented by MacArthur and Wilson was later expanded by "several people including Pianka (1972). Predictions from these verbal theories were that under low den- sity conditions rapid reproduction and thus early maturity and semelparity would be favored, while at high population density repeated reproduction and thus iteroparity should be advantageous. Thus, these ecological theories suggest that low population density would generally accelerate se- nescence while high density would favor delayed reproduction and increased longevity. Many of the logical underpinnings of these verbal theories have been found to be faulty (see Mueller, 1991, for a review). Following the work of MacArthur and Wilson, Pianka, and others, formal theories which specifically took into account population age- structure and density were examined (Charles- worth, 1980). These theories (Charlesworth, 1980) showed that if density simply resulted in a constant increase in the rate of mortality, there would be no change in the form of selection relative to popula- tions living at low density. However, if density- dependent natural selection acted only on pre-adult survival or fecundity, then it is possible that selec-