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Biomedical subjects

L D Mueller

Publications and source records attributed to L D Mueller.

17 recordsLinked to original sources
PubMed1996

Evolutionary theory predicts late-life mortality plateaus.

Most demographic data indicate a roughly exponential increase in adult mortality with age, a phenomenon that has been explained in terms of a decline in the force of natural selection acting on age-specific mortality. Scattered demographic findings suggest the existence of a late-life mortality plateau in both humans and dipteran insects, seemingly at odds with both prior data and evolutionary theory. Extensions to the evolutionary theory of aging are developed which indicate that such late-life mortality plateaus are to be expected when enough late-life data are collected. This expanded theory predicts late-life mortality plateaus, with both antagonistic pleiotropy and mutation accumulation as driving population genetic mechanisms.

Adult↗
PubMed1996

Genetics of larval urea tolerance in Drosophila melanogaster.

The genetic control of larval tolerance to urea, a nitrogenous waste-product occurring naturally in crowded Drosophila cultures, was investigated in a set of five laboratory populations of D. melanogaster that had been successfully subjected to selection for increased larval urea tolerance. Larva to adult survivorship and development time at three different levels of urea were assayed on the five selected populations, their five matched controls and a set of 10 F1 hybrid populations derived from reciprocal crosses between pairs of selected and control populations. As expected from the results of previous studies, the selected populations exhibited greater larval tolerance to the toxic effects of urea, relative to their controls. Comparison of the hybrid and parental populations with respect to both survivorship and development time indicated that the genetic control of urea tolerance in the selected populations is largely dominant, and has a significant X-linked component. The data also suggested that females from the selected populations exercise a nongenetic maternal effect on the development time of their progeny, regardless of urea level.

Adaptation, Physiological↗
PubMed1993

Population density effects on longevity.

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.

Animals↗
PubMed1993

The effect of superoxide dismutase alleles on aging in Drosophila.

The effects of superoxide dismutase on aging were tested using two different experimental approaches. In the first, replicated populations with postponed aging were compared with their controls for frequencies of electrophoretic alleles at the SOD locus. Populations with postponed aging had consistently greater frequencies of the allele coding for more active SOD protein. This allele was not part of a segregating inversion polymorphism. The second experimental approach was the extraction of SOD alleles from different natural populations followed by the construction of different SOD genotypes on hybrid genetic backgrounds. This procedure did not uncover any statistical effect of SOD genotype on longevity or fecundity. There were large effects on longevity and fecundity due to the family from which a particular SOD genotype was derived. To detect the effects of SOD genotypes on longevity with high probability would require a ten-fold increase in the number of families used.

Aging↗
PubMed1993

Genetic variation and random drift in autotetraploid populations.

The rate of decay of genetic variation is determined for randomly mating autotetraploid populations of finite size, and the equilibrium homozygosity under mutation and random drift is calculated. It is shown that heterozygosity is lost at a slower rate than in diploid populations, and that the equilibrium heterozygosity with mutation and random drift is higher than for diploids. Outcrossing populations as well as populations that randomly self are analyzed. A method of comparing genetic variation between autotetraploid and diploid populations is proposed. Our treatment suggests that the "gametic homozygosity" provides a unified approach for comparing genotypes within a population as well as comparing genetic variation between populations with different levels of ploidy.

Alleles↗
PubMed1991

Evolution of behavior by density-dependent natural selection.

Theories of density-dependent natural selection predict that evolution should favor those genotypes with the highest per capita rates of population growth under the current density conditions. These theories are silent about the mechanisms that may give rise to these increases in density-dependent growth rates. We have observed the evolution of six populations of Drosophila melanogaster recently placed in crowded environments after nearly 200 generations at low-population density in the laboratory. After 25 generations in these crowded cultures all six populations showed the predicted increase in population growth rates at high-population density with the concomitant decrease in their growth rates at low densities. These changes in rates of population growth are accompanied by changes in the feeding and pupation behavior of the larvae: those populations that have evolved at high-population densities have higher feeding rates and are less likely to pupate on or near the food surface than populations maintained at low densities. These changes in behavior serve to increase the competitive ability of larvae for limited food and reduce mortality under crowded conditions during the pupal stage of development. A detailed understanding of the mechanisms by which populations evolve under density-dependent natural selection will provide a framework for understanding the nature of trade-offs in life history evolution.

Animals↗
PubMed1991

Density-dependent natural selection and trade-offs in life history traits.

Theories of density-dependent natural selection state that at extreme population densities evolution produces alternative life histories due to trade-offs. The trade-offs are presumed to arise because those genotypes with highest fitness at high population densities will not also have high fitness at low density and vice-versa. These predictions were tested by taking samples from six populations of Drosophila melanogaster kept at low population densities (r-populations) for nearly 200 generations and placing them in crowded cultures (K-populations). After 25 generations in the crowded cultures, the derived K-populations showed growth rate and productivity that at high densities were elevated relative to the controls, but at low density were depressed.

Animals↗
PubMed1991

Ecological determinants of life-history evolution.

Density-dependent natural selection has been studied, empirically with laboratory populations of Drosophila melanogaster. Populations kept at very high and low population density have become differentiated with respect to important fitness-related traits. There is now some understanding of the behavioural and physiological basis of these differences. These studies have identified larval competitive ability and efficiency of food utilization as traits that are negatively correlated with respect to effects on fitness. Theory that illuminates and motivates additional research with this experimental system has been lacking. Current research has focused on models that incorporate many details of Drosophila ecology in laboratory environments.

Animals↗
PubMed1988

Evolution of competitive ability in Drosophila by density-dependent natural selection.

The theory of density-dependent natural selection predicts that populations kept at extreme densities should evolve different competitive abilities for limited resources. These predictions have been tested with laboratory populations of Drosophila melanogaster. Six independent populations were maintained in two environments, called r and K, for 128 generations. In the r environment, population sizes were small and resources for larvae and adults were abundant. In contrast the populations in the K environment were large and crowded, and resources, such as food and space, were in short supply. The relative competitive ability for food has been estimated for each population. Populations from the K environment consume food at a rate that is 58% greater than the average rate for the r population. The differentiation of competitive abilities in these populations is due to natural selection and is consistent with predictions from the theory of evolutionary ecology.

Animals↗
PubMed1987

Evolution of accelerated senescence in laboratory populations of Drosophila.

Ecological theories of life history evolution predict that natural selection should favor semelparous life histories in environments where juvenile survival is high relative to adult survival and rates of population growth are high. That is, organisms should complete their entire reproductive effort in a short period of time following maturation. Direct empirical verification of this idea has been lacking. Six independent populations of Drosophila melanogaster were maintained in two different environments, called r and K, for more than 120 generations. In the r environment population size was small, larval survival and rates of population growth were high, and reproduction was limited to a few days after eclosion. In the K environment population size was large and larval survival low, but adults were allowed to reproduce indefinitely. The fecundity of females of different sizes from each environment was measured daily for 4 weeks. No differences in fecundity were seen during the first week of adult life for females from the two environments. By the fourth week, however, the fecundity of large females from the r environment was 47-83% less than that of females from the K environment. The accelerated senescence exhibited by females from the r environment appears to be due to the accumulation of deleterious alleles whose effects are expressed late in life, which is consistent with the mutation accumulation hypothesis for the evolution of senescence.

Aging↗
PubMed1985

A direct assessment of the role of genetic drift in determining allele frequency variation in populations of Euphydryas editha.

Estimates of allele frequencies at six polymorphic loci were collected over eight generations in two populations of Euphydryas editha. We have estimated, in addition, the effective population size for each generation for both populations with results from mark-recapture and other field data. The variation in allele frequencies generated by random genetic drift was then studied using computer simulations and our direct estimates of effective population size. Substantial differences between observed values and computer-generated expected values assuming drift alone were found for three loci (Got, Hk, Pgi) in one population. These observations are consistent with natural selection in a variable environment.

Alleles↗
PubMed1985

Natural selection vs. random drift: evidence from temporal variation in allele frequencies in nature.

We have obtained monthly samples of two species, Drosophila pseudoobscura and Drosophila persimilis, in a natural population from Napa County, California. In each species, about 300 genes have been assayed by electrophoresis for each of seven enzyme loci in each monthly sample from March 1972 to June 1975. Using statistical methods developed for the purpose, we have examined whether the allele frequencies at different loci vary in a correlated fashion. The methods used do not detect natural selection when it is deterministic (e.g., overdominance or directional selection), but only when alleles at different loci vary simultaneously in response to the same environmental variations. Moreover, only relatively large fitness differences (of the order of 15%) are detectable. We have found strong evidence of correlated allele frequency variation in 13-20% of the cases examined. We interpret this as evidence that natural selection plays a major role in the evolution of protein polymorphisms in nature.

Alleles↗
PubMed1979

A comparison of two methods for making statistical inferences on Nei's measure of genetic distance.

The delta and jackknife methods can be used to estimate Nei's measure of genetic distance and calculate confidence intervals for this estimate. Computer stimulations were used to study the bias and variance of each estimator and the accuracy of the corresponding approximate 95% confidence intervals. The simulations were conducted using 3 sets of data and several sample sizes. The results showed: (1) the jackknife reduced bias; (2) in 8 out of 9 cases the variance and mean square error of the jackknife estimator were less; (3) a second order jackknife reduced the bias the most but suffered a corresponding increase in variance; (4) both the first order jackknife and delta methods yielded intervals whose confidence levels were approximately equal but less than 95%.

Alleles↗
PubMedDate not supplied

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.

Computer Simulation↗
PubMedDate not supplied

Fitness and density-dependent population growth in Drosophila melanogaster.

The density-dependent rates of population growth were determined for 26 populations of Drosophila melanogaster maintained in the serial transfer system. Twenty-five populations were homozygous for an entire chromosome 2 sampled from nature; the other was a random heterozygous population. Rates of population growth around the carrying capacity cannot explain the large fitness depression of these lines. However, the homozygous lines show large differences in rates of population growth at low densities relative to the random heterozygous standard. The average relative fitness of the homozygous lines, as determined from the growth rates at the lowest density, is 0.51.

Animals↗