Evolution of ageing since Darwin.

In the late 19th century, the evolutionary approach to the problem of ageing was initiated by August Weismann, who argued that natural selection was more important for ageing than any physiological mechanism. In the mid-twentieth century, J. B. S. Haldane, P. B. Medawar and G. C. Williams informally argued that the force of natural selection falls with adult age. In 1966, W. D. Hamilton published formal equations that showed mathematically that two 'forces of natural selection' do indeed decline with age, though his analysis was not genetically explicit. Brian Charlesworth then developed the required mathematical population genetics for the evolution of ageing in the 1970's. In the 1980's, experiments using Drosophila showed that the rate of ageing evolves as predicted by Hamilton's 'forces of natural selection'. The discovery of the cessation of ageing late in life in the 1990's was followed by its explanation in terms of evolutionary theory based on Hamilton's forces. Recently, it has been shown that the cessation of ageing can also be manipulated experimentally using Hamilton's 'forces of natural selection'. Despite the success of evolutionary research on ageing, mainstream gerontological research has largely ignored both this work and the opportunity that it provides for effective intervention in ageing.

Can simple population genetic models reconcile partial match frequencies observed in large forensic databases?

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 contribution of ancestry, chance, and past and ongoing selection to adaptive evolution.

The relative contributions of ancestry, chance, and past and ongoing selection to variation in one adaptive (larval feeding rate) and one seemingly nonadaptive (pupation height) trait were determined in populations of Drosophila melanogaster adapting to either low or high larval densities in the laboratory. Larval feeding rates increased rapidly in response to high density, and the effects of ancestry, past selection and chance were ameliorated by ongoing selection within 15-20 generations. Similarly, in populations previously kept at high larval density, and then switched to low larval density, the decline of larval feeding rate to ancestral levels was rapid (15-20 generations) and complete, providing support for a previously stated hypothesis regarding the costs of faster feeding in Drosophila larvae. Variation among individuals was the major contributor to variation in pupation height, a trait that would superficially appear to be nonadaptive in the environmental context of the populations used in this study because it did not diverge between sets of populations kept at low versus high larval density for many generations. However, the degree of divergence among populations (F(ST)) for pupation height was significantly less than expected for a selectively neutral trait, and we integrate results from previous studies to suggest that the variation for pupation height among populations is constrained by stabilizing selection, with a flat, plateau-like fitness function that, consequently, allows for substantial phenotypic variation within populations. Our results support the view that the genetic imprints of history (ancestry and past selection) in outbreeding sexual populations are typically likely to be transient in the face of ongoing selection and recombination. The results also illustrate the heuristic point that different forms of selection-for example directional versus stabilizing selection-acting on a trait in different populations may often not be due to differently shaped fitness functions, but rather due to differences in how the fitness function maps onto the actual distribution of phenotypes in a given population. We discuss these results in the light of previous work on reverse evolution, and the role of ancestry, chance, and past and ongoing selection in adaptive evolution.

Short- and long-term effects of environmental urea on fecundity in Drosophila melanogaster.

Previous studies have shown that exposure to urea-supplemented food inhibited fecundity in Drosophila females, and that this inhibition was not expressed when females were given a choice between regular and urea-supplemented food as an oviposition substrate. We assayed fecundity, on both regular food and urea-supplemented food, at 5, 15 and 25 days post eclosion on females from ten laboratory populations of Drosophila melanogaster. The females assayed came from one of two treatments; they were maintained as adults on either regular or urea-supplemented food. We found that exposure to urea-supplemented food inhibited fecundity, relative to the levels exhibited on regular food, regardless of whether the urea was present in the assay medium, or in the medium on which the flies were maintained over the course of the experiment, thereby suggesting that urea has both a long-term (possibly physiological) as well as a short-term (possibly behavioural) inhibitory effect on fecundity of Drosophila females. We also tested and ruled out the hypothesis that prior yeasting could ameliorate the inhibitory effect of urea in the assay medium on fecundity, as this was a possible explanation of why flies given a choice between regular and urea-supplemented food did not exhibit a preference for regular food in a previous study.

Within- and among-population variation in oviposition preference of urea-supplemented food in Drosophila melanogaster.

Oviposition preference for urea-supplemented food was assayed by simultaneous choice trials on five pairs of closely related laboratory populations of Drosophila melanogaster. Each pair of populations had been derived from a separate ancestral population about 85 generations prior to this study. One population in each pair had been subjected to selection for larval tolerance to the toxic effects of urea; the other population served as a control. Considerable variation in oviposition preference was seen both within and among populations, with four of the ten populations showing a significant mean preference for urea-supplemented food. The degree of specificity shown by individual females was surprisingly high, leading to a bi-modal distribution of oviposition preference in some populations. Overall, selection for larval tolerance to urea did not significantly affect oviposition preference. However, the data indicated that pair-wise comparisons between randomly selected populations from the two larval selection regimes would lead to a range of possible outcomes, suggesting, in several cases, that selection for larval urea tolerance had led to significant differentiation of adult oviposition preference for urea in one or the other direction. The results, therefore, highlight the importance of population level replication and caution against the practice, common in ecological studies, of assaying oviposition preference in two populations that utilize different hosts in nature, and then drawing broad evolutionary inferences from the results.