much significance, especially for biological processes that involve many genes in a redundant way that reduces the harmful effects of any particular allele.
"Antagonistic-pleiotropy" alleles (i.e., genes that have multiple effects that pull in different directions) may also be rare. In particular, such alleles are supposed to come in two varieties: some are helpful at younger ages and harmful at middle and older ages, whereas others are harmful at younger ages and helpful at middle and older ages. The older ages in this model do not count for much, if at all, because few individuals survive to older ages, and those that do have low fertility. A tradeoff of good and bad effects between younger and middle ages is, however, theoretically possible. Mutations may arise that lead to the existence of two alleles at the same locus, with opposite age-specific impacts. All species have to make tradeoffs between fertility and survival, between early and late fertility, between early and late survival, etc. Mutations that produce pairs of antagonistic-pleiotropy alleles may be the mechanism that enables alternative choices about such tradeoffs. Both of the two competing alleles may, however, not survive many generations: one may be driven to extinction as the other goes to fixation (Curtsinger et al., 1994). If this were the case, one would observe that species make tradeoffs, but one would only occasionally observe a genetic locus with two or more antagonistic-pleiotropy alleles with different age-specific effects.
The research of Carey, Partridge, Tuljapurkar and Rose (all of whom have chapters in this volume), as well as Curtsinger (1995a,b), Abrams (1993), and others, is helping clarify the mysterious fact that many individuals survive well past the normal end of reproduction. In addition to the ideas adumbrated above, other possible explanations may prove important. At present, this question appears to be a wide-open, exciting issue on the frontier of knowledge.
This chapter was designed to serve two purposes. My charge was to address the question of what demographers, ecologists, and evolutionary biologists can learn from each other. Actually, demographers, ecologists, and evolutionary biologists have learned a lot from each other about the trajectories of mortality at advanced ages. In explaining what they have learned, the potential for this kind of biodemographic collaboration can be conveyed in a concrete, persuasive way. What has been learned—and what could be learned through further collaboration—is fascinating: outlining this was the dual, complementary purpose of this chapter, the other side of the same coin.
Demographers, ecologists, and evolutionary biologists focus on different topics, have memorized different facts and jargon, and have been indoctrinated with different concepts. They share, however, an interest in populations. Although many demographers rigidly restrict demography to the study of human populations, the methods and concepts of demography are useful in studying