The production of extreme or`transgressive' phenotypes in segregating hybrid populations has been speculated to contribute to niche divergence of hybrid lineages. Here, we assess the frequency of transgressive segregation in hybrid populations, describe its genetic basis and discuss the factors that best predict its occurrence. From a survey of 171 studies that report phenotypic variation in segregating hybrid populations, we show that transgression is the rule rather than the exception. In fact, 155 of the 171 studies (91%) report at least one transgressive trait, and 44% of 1229 traits examined were transgressive. Transgression occurred most frequently in intraspeci®c crosses involving inbred, domesticated plant populations, and least frequently in interspeci®c crosses between outbred, wild animal species. Quantitative genetic studies of plant hybrids consistently point to the action of complementary genes as the primary cause of transgression, although overdominance and epistasis also contribute.Complementary genes appear to be common for most traits, with the possible exception of those with a history of disruptive selection. These results lend credence to the view that hybridization may provide the raw material for rapid adaptation and provide a simple explanation for niche divergence and phenotypic novelty often associated with hybrid lineages. ; Rieseberg & Ellstrand, 1993; Cosse et al., 1995). The generation of these extreme phenotypes is referred to as transgressive segregation, and this is a major mechanism by which extreme or novel adaptations observed in new hybrid ecotypes or species are thought to arise. If transgressive segregation is frequent, then an important evolutionary role for hybridization is more easily explained. Note that transgressive segregation is a phenomenon speci®c to segregating hybrid generations and refers to the fraction of individuals that exceed parental phenotypic values in either a negative or positive direction. This is caused in part by heterosis, which is most pronounced in ®rst-generation hybrids, and is implicated when the mean trait value of the hybrids exceeds (in a positive direction only) the phenotypic values of both parental lines. As will be shown below, the genetic basis of transgressive segregation appears to be largely distinct from that underlying heterosis.Evidence that transgressive segregation facilitates the successful establishment of hybrid lineages is indirect and comes principally from research on plants. Theoretical and empirical studies identify niche separation between hybrid and parental genotypes as the single most important factor favouring hybrid establishment (Lewontin & Birch, 1966; Grant, 1981; Templeton, 1981; Buerkle et al. in review 1 2 ).Without niche di erentiation, new hybrid genotypes are likely to be overcome by competition and/or gene¯ow from parental populations. These predictions are supported by reports that most stabilized introgressants or hybrid species are ecologically divergent with respect to their parental species (Abbo...

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.

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.

Data from populations undergoing experimental evolution can be used to make comparisons between physiologically differentiated populations and to determine evolutionary trajectories. Comparisons of long-established laboratory populations of Drosophila melanogaster that are strongly differentiated with respect to desiccation resistance are used to test alternative hypotheses concerning the mechanisms that fruit flies use to survive bouts of extreme desiccation. This comparative study supports the hypothesis that, in at least one case, D. melanogaster can evolve increased resistance to desiccation by decreasing water loss rates and by increasing bulk water content but not by increasing metabolic water content or dehydration tolerance. While glycogen was involved in water storage, its primary role was in water binding, not the production of metabolic water. Measurement of the trajectories of these component mechanisms during selection for desiccation resistance is used to demonstrate that water loss rate quickly plateaus in response to selection, while water content continues to improve. This disparity reveals the value of studying evolutionary trajectories and the need for longer-term selection studies in evolutionary physiology.

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