Laurence D. Mueller

A genetic polymorphism maintained by natural selection in a temporally varying environment

Environments that are crowded with larvae of the fruit fly, Drosophila melanogaster, exhibit a temporal deterioration in quality as waste products accumulate and food is depleted. We show that natural selection in these environments can maintain a genetic polymorphism with one group of genotypes specializing on the early part of the environment and a second group specializing on the late part. These specializations involve trade‐offs in fitness components. The early types emerge first from crowded cultures and have high larval feeding rates, which are positively correlated with competitive ability but exhibit lower absolute viability than the late phenotype, especially in food contaminated with the nitrogenous waste product, ammonia. The late emerging types have reduced feeding rates but higher absolute survival under conditions of severe crowding and high levels of ammonia. Organisms that experience temporal variation within a single generation are not uncommon, and this model system provides some of the first insights into the evolutionary forces at work in these environments.

Density-dependent natural selection inDrosophila: Trade-offs between larval food acquisition and utilization

SummaryNatural selection at high densities has often been postulated to favour the evolution of greater efficiency of food use. Contrary to this expectation, a previous study suggested the existence of a trade-off between larval feeding rate and efficiency at using food to complete larval development in populations ofDrosophila melanogaster subjected to crowding for many generations. In this paper, we confirm the generality of such a density-dependent trade-off between food acquisition and utilization by demonstrating its occurrence in a new set ofDrosophila populations subjected to extreme larval crowding. We suggest that such trade-offs between food acquisition and food use may represent a general phenomenon in organisms exhibiting scramble competition. We test and reject the possible mechanistic explanation that decreased efficiency of food use in faster-feeding larvae may merely be a consequence of a faster passage of food through the gut, leading to incomplete assimilation of nutrients and energy.

Estimation and interpretation of genetic distance in empirical studies

Linear functions of Neis genetic-distance statistic are calculated frequently in the literature of population genetics. Variance estimates for these linear functions are either not presented or incorrectly calculated. Part of the problem stems from the common assumption that distance statistics are independent random variables. This assumption is not generally correct. We describe methods for estimating the variance of linear combinations of genetic-distance statistics. We also suggest a method for constructing confidence intervals on genetic-distance statistics when these values are small (

The Gompertz equation as a predictive tool in demography

The Gompertz demographic model describes rates of aging and age-independent mortality with the parameters alpha and A, respectively. Estimates of these parameters have traditionally been based on the assumption that mortality rates are constant over short to moderate time periods. This assumption is questionable even for very large samples assayed over short time intervals. In this article, we compare several methods for estimating the Gompertz parameters, including some that do not assume constant mortality rates. A maximum likelihood method that does not assume constant mortality rates is shown to be best, based on the bias and variance of the Gompertz parameter estimates. Moreover, we show how the Gompertz equation can then be used to predict mean longevity and the time of the nth percentile of mortality. Methods are also developed that assign confidence intervals to such estimates. In some cases, these statistics may be estimated accurately from only the early deaths of a large cohort, thus providing an opportunity to estimate longevity on long-lived organisms quickly.

EVOLUTION OF LATE-LIFE MORTALITY IN DROSOPHILA MELANOGASTER

Abstract.— Aging appears to cease at late ages, when mortality rates roughly plateau in large‐scale demographic studies. This anomalous plateau in late‐life mortality has been explained theoretically in two ways: (1) as a strictly demographic result of heterogeneity in life‐long robustness between individuals within cohorts, and (2) as an evolutionary result of the plateau in the force of natural selection after the end of reproduction. Here we test the latter theory using cohorts of Drosophila melanogaster cultured with different ages of reproduction for many generations. We show in two independent comparisons that populations that evolve with early truncation of reproduction exhibit earlier onset of mortality‐rate plateaus, in conformity with evolutionary theory. In addition, we test two population genetic mechanisms that may be involved in the evolution of late‐life mortality: mutation accumulation and antagonistic pleiotropy. We test mutation accumulation by crossing genetically divergent, yet demographically identical, populations, testing for hybrid vigor between the hybrid and nonhybrid parental populations. We found no difference between the hybrid and nonhybrid populations in late‐life mortality rates, a result that does not support mutation accumulation as a genetic mechanism for late‐life mortality, assuming mutations act recessively. Finally, we test antagonistic pleiotropy by returning replicate populations to a much earlier age of last reproduction for a short evolutionary time, testing for a rapid indirect response of late‐life mortality rates. The positive results from this test support antagonistic pleiotropy as a genetic mechanism for the evolution of late‐life mortality. Together these experiments comprise the first corroborations of the evolutionary theory of late‐life mortality.

HOW REPEATABLE IS ADAPTIVE EVOLUTION? THE ROLE OF GEOGRAPHICAL ORIGIN AND FOUNDER EFFECTS IN LABORATORY ADAPTATION

Abstract The importance of contingency versus predictability in evolution has been a long-standing issue, particularly the interaction between genetic background, founder effects, and selection. Here we address experimentally the effects of genetic background and founder events on the repeatability of laboratory adaptation in Drosophila subobscura populations for several functional traits. We found disparate starting points for adaptation among laboratory populations derived from independently sampled wild populations for all traits. With respect to the subsequent evolutionary rate during laboratory adaptation, starvation resistance varied considerably among foundations such that the outcome of laboratory evolution is rather unpredictable for this particular trait, even in direction. In contrast, the laboratory evolution of traits closely related to fitness was less contingent on the circumstances of foundation. These findings suggest that the initial laboratory evolution of weakly selected characters may be unpredictable, even when the key adaptations under evolutionary domestication are predictable with respect to their trajectories.

DENSITY-DEPENDENT POPULATION GROWTH AND NATURAL SELECTION IN FOOD-LIMITED ENVIRONMENTS: THE DROSOPHILA MODEL

The action of density-dependent population growth is modeled through the effects of limited food. Scramble competition for food affects viability and adult size, which are correlated with the fecundity of females. Adult effects on fecundity are also explicitly modeled. In the two submodels considered, changes in the minimum amount of food necessary for successful pupation lead to (1) changes in the minimum size of an adult with no change in overall efficiency or (2) constant minimum size but changes in the efficiency of food use. The resulting population dynamics of the two submodels are qualitatively different. For both submodels, population stability requires some degree of adult effects on female fecundity for parameter values typical of Drosophila. When genetic variation is present for competitive ability and minimum food required, natural selection at equilibrium population size favors increasing competitive ability and decreasing the minimum food requirement. Evolutionary changes in the competitive ability of a population do not affect equilibrium population size. Decreases in the minimum food requirements typically increase the equilibrium adult population size but have variable effects on equilibrium egg numbers, depending on the submodel examined. Biological evidence suggests that competitive ability and minimum food requirements may be positively correlated. Genetic models with this antagonistic pleiotropy can maintain allelic variation without overdominance in either character. Furthermore, contrary to established verbal theory, there is no consistent prediction concerning the evolution of average body size. An advantage of this theory is that parameters of interest may be easily estimated in laboratory populations of Drosophila.

DENSITY-DEPENDENT NATURAL SELECTION IN DROSOPHILA: EVOLUTION OF GROWTH RATE AND BODY SIZE

Drosophila melanogaster populations subjected to extreme larval crowding (CU lines) in our laboratory have evolved higher larval feeding rates than their corresponding controls (UU lines). It has been suggested that this genetically based behavior may involve an energetic cost, which precludes natural selection in a density‐regulated population to simultaneously maximize food acquisition and food conversion into biomass. If true, this stands against some basic predictions of the general theory of density‐dependent natural selection. Here we investigate the evolutionary consequences of density‐dependent natural selection on growth rate and body size in D. melanogaster. The CU populations showed a higher growth rate during the postcritical period of larval life than UU populations, but the sustained differences in weight did not translate into the adult stage. The simplest explanation for these findings (that natural selection in a crowded larval environment favors a faster food acquisition for the individual to attain the same final body size in a shorter period of time) was tested and rejected by looking at the larva‐to‐adult development times. Larvae of CU populations starved for different periods of time develop into comparatively smaller adults, suggesting that food seeking behavior in a food depleted environment carries a higher cost to these larvae than to their UU counterparts. The results have important implications for understanding the evolution of body size in natural populations of Drosophila, and stand against some widespread beliefs that body size may represent a compromise between the conflicting effects of genetic variation in larval and adult performance.

Statistical Inference on Measures of Niche Overlap

Estimates of measures of niche overlap are often reported without any indication of sampling variance or an accompanying confidence interval. We have investigated the delta, jackknife, and bootstrap methods for making statistical inferences on four measures of niche overlap: the coefficient of community, Morisitas index, Horns index, and the Euclidian distance. Our qualitative conclusions are: (1) The bias of these estimators was usually @<10% of the mean unless the sample size was small and the number of resource categories large. The jackknife and bootstrap can significantly reduce this bias. (2) The variance of the bootstrap and jackknife estimators was usually greater than that of the “standard” estimator. (3) Under a variety of circumstances, the population sampled may actually represent several unrecognized subpopulations. In such cases confidence intervals generated by the jackknife and delta techniques can be quite inaccurate, while the nonparametric confidence intervals derived from the bootstrap are highly accurate.

Testing the heterogeneity theory of late-life mortality plateaus by using cohorts of Drosophila melanogaster

Variation among individuals in robustness has been posed as a general explanation for the lack of increase in late-life mortality rates. Here, we test corollaries of this heterogeneity theory. One is that populations that have undergone strong laboratory selection for differentiated stress resistance should show significant differences in their late-life mortality schedules. To test this corollary, we employed 40 410 flies from three groups of Drosophila melanogaster populations that differ substantially in their resistance to starvation. No significant differences between these groups were found for late-life mortality. Another corollary of the heterogeneity theory is that there should be late-life plateaus in stress resistance that coincide with the plateau stage of the mortality curve. In 20 994 flies from six replicate outbred laboratory populations, we measured mortality rates every other day and starvation and desiccation resistance every 7 days. Both male and female starvation and desiccation resistance clearly decreased with time overall. There was no late-life plateau in male desiccation resistance. A late-life plateau in male starvation resistance may exist, however. Together, these two experiments generally constitute evidence against heterogeneity as a major contributor to the phenomenon of late-life mortality plateaus.