Quantitative genetics of growth and cryptic evolution of body size in an island population

Evolutionary Ecology (Impact Factor: 2.41). 01/2007; 21(3):337-356. DOI: 10.1007/s10682-006-9106-z

ABSTRACT While evolution occurs when selection acts on a heritable trait, empirical studies of natural systems have frequently reported
phenotypic stasis under these conditions. We performed quantitative genetic analyses of weight and hindleg length in a free-living
population of Soay sheep (Ovis aries) to test whether genetic constraints can explain previously reported stasis in body size despite evidence for strong positive
directional selection. Genetic, maternal and environmental covariance structures were estimated across ontogeny using random
regression animal models. Heritability increased with age for weight and hindleg length, though both measures of size were
highly heritable across ontogeny. Genetic correlations among ages were generally strong and uniformly positive, and the covariance
structures were also highly integrated across ontogeny. Consequently, we found no constraint to the evolution of larger size
itself. Rather we expect size at all ages to increase in response to positive selection acting at any age. Consistent with
expectation, predicted breeding values for age-specific size traits have increased over a twenty-year period, while maternal
performance for offspring size has declined. Re-examination of the phenotypic data confirmed that sheep are not getting larger,
but also showed that there are significant negative trends in size at all ages. The genetic evolution is therefore cryptic,
with the response to selection presumably being masked at the phenotypic level by a plastic response to changing environmental
conditions. Density-dependence, coupled with systematically increasing population size, may contribute to declining body size
but is insufficient to completely explain it. Our results demonstrate that an increased understanding of the genetic basis
of quantitative traits, and of how plasticity and microevolution can occur simultaneously, is necessary for developing predictive
models of phenotypic change in nature.

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    ABSTRACT: Climate change is expected to induce many ecological and evolutionary changes. Among these is the hypothesis that climate warming will cause a reduction in body size. This hypothesis stems from Bergmann's rule, a trend whereby species exhibit a smaller body size in warmer climates, and larger body size under colder conditions in endotherms. The mechanisms behind this rule are still debated, and it is not clear whether Bergmann's rule can be extended to predict the effects of climate change through time. We reviewed the primary literature for evidence (i) of a decrease in body size in response to climate warming, (ii) that changing body size is an adaptive response and (iii) that these responses are evolutionary or plastic. We found weak evidence for changes in body size through time as predicted by Bergmann's rule. Only three studies investigated the adaptive nature of these size decreases. Of these, none reported evidence of selection for smaller size or of a genetic basis for the size change, suggesting that size decreases could be due to nonadaptive plasticity in response to changing environmental conditions. More studies are needed before firm conclusions can be drawn about the underlying causes of these changes in body size in response to a warming climate.
    Evolutionary Applications 01/2014; 7(1):156-68. · 4.15 Impact Factor
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    ABSTRACT: Hosts may mitigate the impact of parasites by two broad strategies: resistance, which limits parasite burden, and tolerance, which limits the fitness or health cost of increasing parasite burden. The degree and causes of variation in both resistance and tolerance are expected to influence host-parasite evolutionary and epidemiological dynamics and inform disease management, yet very little empirical work has addressed tolerance in wild vertebrates. Here, we applied random regression models to longitudinal data from an unmanaged population of Soay sheep to estimate individual tolerance, defined as the rate of decline in body weight with increasing burden of highly prevalent gastrointestinal nematode parasites. On average, individuals lost weight as parasite burden increased, but whereas some lost weight slowly as burden increased (exhibiting high tolerance), other individuals lost weight significantly more rapidly (exhibiting low tolerance). We then investigated associations between tolerance and fitness using selection gradients that accounted for selection on correlated traits, including body weight. We found evidence for positive phenotypic selection on tolerance: on average, individuals who lost weight more slowly with increasing parasite burden had higher lifetime breeding success. This variation did not have an additive genetic basis. These results reveal that selection on tolerance operates under natural conditions. They also support theoretical predictions for the erosion of additive genetic variance of traits under strong directional selection and fixation of genes conferring tolerance. Our findings provide the first evidence of selection on individual tolerance of infection in animals and suggest practical applications in animal and human disease management in the face of highly prevalent parasites.
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    ABSTRACT: To predict the response of plant pathogens to climate warming, data are needed on current thermal adaptation, the pathogen's evolutionary potential, and the link between them. We conducted a common garden experiment using isolates of the fungal pathogen Rhynchosporium commune from nine barley populations representing climatically diverse locations. Clonal replicates of 126 genetically distinct isolates were assessed for their growth rate at 12°C, 18°C, and 22°C. Pop-ulations originating from climates with higher monthly temperature variation had higher growth rate at all three temperatures compared with populations from climates with less temperature fluctuation. Population differentiation in growth rate (Q ST) was significantly higher at 22°C than population differentiation for neutral microsatellite loci (G ST), consistent with local adaptation for growth at higher temperatures. At 18°C, we found evidence for stabilizing selection for growth rate as Q ST was significantly lower than G ST . Heritability of growth rate under the three temperatures was substantial in all populations (0.58–0.76). Genetic variation was lower in populations with higher growth rate at the three temperatures and evolvability increased under heat stress in seven of nine popula-tions. Our findings imply that the distribution of this pathogen is unlikely to be genetically limited under climate warming, due to its high genetic variation and plasticity for thermal tolerance.
    Evolutionary Applications 01/2013; 6(3):524-534. · 4.15 Impact Factor

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