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

Evolutionary Ecology (Impact Factor: 2.37). 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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