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.

Populations of Drosophila melanogaster kept at high population density (K-selected) for 125 generations have higher larval viability than populations kept at low densities (r-selected) when both are raised under crowded conditions. In addition K-selected adults that emerge from crowded cultures are larger than their r-selected counterparts. These differences cannot be explained by differences in efficiency of food use. The minimum food required for successful pupation is actually greater in the K-selected populations. I conjecture that there may be a trade-off between minimum food requirements and competitive ability, which has changed substantially in the K-selected populations. The possibility that K-selected larvae can dig more more deeply and gain access to unused food is examined and rejected as a possible explanation of the viability differences. Evidence is provided supporting the hypothesis that the differences in viability may be due to an increased tendency of the K-selected larvae to pupate off the surface of the medium. © 1990 Chapman and Hall Ltd.

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 2 submodels considered, changes in the minimum amount of food necessary for successful pupation lead to 1) changes in minimum size of an adult with no change in overall efficiency or 2) constant minimum size but changes in efficiency of food use. Resulting population dynamics of the 2 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 competiitve ability and decreasing the minimum food requirement. Decreases in minimum food requirements typically increase the equilibrium adult population size but have variable effects on equilibrium egg numbers. 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. -from Author

A recent study of partial matches in the Arizona offender database of DNA profiles has revealed a large number of nine and ten locus matches. I use simple models that incorporate the product rule, population substructure, and relatedness to predict the expected number of matches in large databases. I find that there is a relatively narrow window of parameter values that can plausibly describe the Arizona results. Further research could help determine if the Arizona samples are congruent with some of the models presented here or whether fundamental assumptions for predicting these match frequencies requires adjustments.

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%.

Estimates of measures of niche overlap are often reported without any indication of sampling variance or an accompanying confidence interval. The delta, jackknife and bootstrap methods for making statistical inferences on 4 measures of niche overlap were investigated: the coefficient of community, Morisita's index, Horn's index, and the Euclidian distance. 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.-Authors

Environments that vary within a generation of an organism provide opportunities for adaptation if the level of variation is severe and predictable. We describe a model of evolution in such environments with genotypes that show trade-offs in viability and fecundity. One genotype develops rapidly and has superior viability but reduced fertility relative to the alternative genotype. Conditions that allow the evolution of the rapidly developing genotypes are explored. We show how the evolution of ovoviviparity and resource specialization in Drosophila sechellia shares many important features of this model. We suggest that our model may capture many of the evolutionary forces responsible for the evolution of niche specialization and ovoviviparity seen in D. sechellia.

Natural 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 of Drosophila 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 of Drosophila 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.

The manipulation of longevity through environmental treatments has provided many insights into the physiological processes affecting life-span. Here, we report the reduction of longevity in adult Drosophila melanogaster after three days of moderate adult crowding. Crowding is shown to reduce life-span through increased age-independent mortality rather than through altering the rate of ageing. Preliminary evidence also suggests that increased age-independent mortality after crowding may result from a reduction in stored energy reserves. The results also suggest that populations of D. melanogaster routinely maintained at high-adult densities may be relatively less susceptible to the detrimental effects of crowding on longevity.

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.