John P. Phelan

BREAKDOWN IN CORRELATIONS DURING LABORATORY EVOLUTION. I. COMPARATIVE ANALYSES OF DROSOPHILA POPULATIONS

Abstract We provide evidence from comparisons of populations of Drosophila that evolutionary correlations between longevity and stress resistance break down over the course of laboratory evolution. Using 15 distinct evolutionary regimes, we created 75 populations that were differentiated for early fecundity, longevity, starvation resistance, desiccation resistance, and developmental time. In earlier experiments, selection for postponed aging produced increases in stress resistance, whereas selection for increased stress resistance produced increases in longevity. Direct estimates of correlations also indicated an antagonistic relationship between early fecundity on one hand and longevity or stress resistance on the other. Laboratory evolution of extreme values of stress resistance, however, led to a breakdown in these evolutionary relationships. There was no evidence that these significant changes in correlation resulted from genotype‐by‐environment interactions or inbreeding. These findings suggest that correlations between functional characters are not necessarily durable features of a species, and that short‐term evolutionary responses cannot be extrapolated reliably to longer‐term evolutionary patterns.

BREAKDOWN IN CORRELATIONS DURING LABORATORY EVOLUTION. II. SELECTION ON STRESS RESISTANCE IN DROSOPHILA POPULATIONS

Abstract We trace the evolutionary correlation between stress resistance and longevity in populations of Drosophila melanogaster selected for stress resistance over many generations. Females selected for desiccation resistance and both females and males selected for increasing starvation resistance initially show concurrent increases in longevity, but then begin to decrease in longevity, even as stress resistance continues to increase. We demonstrate that the correlation between two fitness traits can change and that this change is due to sustained selection rather than a genotype‐by‐environment interaction or inbreeding depression. The breakdown in evolutionary correlation we report underscores the difficulty of extrapolating the results from short‐term selection experiments to predictions of long‐term evolution.

OPTIMAL FORAGING IN PEROMYSCUS POLIONOTUS: THE INFLUENCE OF ITEM-SIZE AND PREDATION RISK

Predictions of optimal foraging theory were tested for Peromyscus polionotus in the laboratory and in two field experiments in which seed item-size was manipulated. In the laboratory, handling times for millet, sunflower seeds, and peanuts were determined. Additionally, energy/handling (E/h) time values were calculated and seed preference experiments were conducted. Mice showed a clear preference for the seeds more profitable in terms of E/h. Seed preference experiments were also conducted in the field to determine the relative influence of optimal foraging and predation risk in diet choice. In the first of the two field experiments, behavioral strategies to reduce predation vulnerability could not influence differential seed selection. As predicted, the mice again showed a preference for the more profitable seed type. In the second experiment, animals were presented with conflicting demands because foraging on a more abundant, but less preferred, seed type afforded the animal an increased ability to avoid predators. Presented with this opportunity to reduce the risk of predation, the animals shifted their preference to the safer, but less profitable food. This shift in seed preference was accompanied by predation-risk-reducing changes in spatial foraging patterns.

Genetic variability and rodent models of human aging

Inbred strains, outbred strains, and natural populations of rodents differ greatly in the amount and nature of the genetic variability they possess. Consequently, as models of human aging they vary with respect to the areas of research to which they are best suited. Inbred strains, in which all individuals are genetically identical, are best suited as models of specific disease processes and for manipulations involving tissue transplantation. Their lack of genetic variability, however, and the disruption of genetic linkage groups that occurs during inbreeding limit their value as models of more general aging processes. Outbred strains exhibit large interindividual genetic variation--a result of ongoing random accumulation of deleterious alleles with late ages of action. This makes them ideal models for studying the diversity of pathologic lesions, connections between pathologies, and susceptibility to pathologic lesions that collectively produce the reductions in reproductive capacity, physiological efficiency, and viability that are characteristic of aging. Natural populations also may exhibit relatively large amounts of interindividual genetic variability. However, difficulties with husbandry, variable parasite loads, and complex population genetics can compromise their suitability as models of human aging. Ultimately, a consideration of the range of animal models available and a more careful matching of the goals of a study with the genetic system of the model will prove fruitful to gerontology.

Differences in the effects of parental age on offspring life history between tropical and temperate populations of milkweed bugs (Oncopeltus spp.)

SummaryWe compare the effects of parental age on several offspring life history traits in two milkweed bug populations: a typically univoltine population ofOncopeltus fasciatus from the Sacramento Valley of California and a typically multivoltine population ofO. cingulifer from Monteverde, Costa Rica. Reared under identical conditions (27°C, 12 h light: 12 h dark photoperiod), these bugs exhibit significant differences in the effects of parental age on offspring life history. As they age,O. fasciatus females from Sacramento lay clutches of eggs of decreasing average weight and a decreasing proportion of their eggs produce offspring that survive to adulthood. Those offspring which do survive have a significantly faster developmental rate and females have a larger body size at adult eclosion. AsO. cingulifer from Monteverde age, they also produce lighter eggs, but there is no significant change in the offspring developmental rate, survival or female adult body size. We suggest that these results are largely explicable as the consequence of different parental investment strategies associated with the predictable relationship between parental age and time of season inO. fasciatus but not inO. cingulifer. AsO. fasciatus from Sacramento age, they may be increasing their investment per developing offspring so as to increase the probability that nymphs hatching late in the season will reach a prereproductive adult diapause before the first killing frost.