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: Exaggerated sexual displays are often supposed to indicate the indirect benefits females may receive from sexual reproduction with displaying males, but empirical evidence for positive relationships between the genetic quality and sexual trait quality is scant. The explanation for this might lie in the fact that mixing of reproductive individuals whose development has been influenced by genotype-by-environment interactions (GEIs) can blur the relationship between the individual male genetic quality and phenotype as perceived by females. Strong GEIs can generate an ecological crossover, where different genotypes are superior in environments that are separated either in space or time. Here, we use a stochastic simulation model to show that even a weak GEI, which does not generate an obvious ecological crossover, can neutralize or even reverse the relationship between genetic quality and sexual trait size in the presence of environmental heterogeneity during development. Our model highlights the importance of developmental selection in evolution of traits and allows us to predict the situations in which sexual displays might not be reliable indicators of genetic quality.
    Proceedings of the Royal Society B: Biological Sciences 01/2009; 276(1659):1153-9. · 5.68 Impact Factor
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    ABSTRACT: 1. Efforts to understand the links between evolutionary and ecological dynamics hinge on our ability to measure and understand how genes influence phenotypes, fitness and population dynamics. Quantitative genetics provides a range of theoretical and empirical tools with which to achieve this when the relatedness between individuals within a population is known. 2. A number of recent studies have used a type of mixed-effects model, known as the animal model, to estimate the genetic component of phenotypic variation using data collected in the field. Here, we provide a practical guide for ecologists interested in exploring the potential to apply this quantitative genetic method in their research. 3. We begin by outlining, in simple terms, key concepts in quantitative genetics and how an animal model estimates relevant quantitative genetic parameters, such as heritabilities or genetic correlations. 4. We then provide three detailed example tutorials, for implementation in a variety of software packages, for some basic applications of the animal model. We discuss several important statistical issues relating to best practice when fitting different kinds of mixed models. 5. We conclude by briefly summarizing more complex applications of the animal model, and by highlighting key pitfalls and dangers for the researcher wanting to begin using quantitative genetic tools to address ecological and evolutionary questions.
    Journal of Animal Ecology 01/2010; 79(1):13-26. · 4.84 Impact Factor
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    ABSTRACT: Recent years have seen a rapid expansion in the scope of quantitative genetic analyses undertaken in wild populations. We illustrate here the potential for such studies to address fundamental evolutionary questions about the maintenance of genetic diversity and to reveal hidden genetic conflicts or constraints not apparent at the phenotypic level. Trade-offs between differ-ent components of fitness, sexually-antagonistic genetic effects, maternal ef-fects, genotype-by-environment interactions, genotype-by-age interactions, and variation between different regions of the genome in localized genetic correlations may all prevent the erosion of genetic variance. We consider ways in which complex interactions between ecological conditions and the expression of genetic variation can be elucidated, and emphasize the ben-efits of conducting selection analyses within a quantitative genetic frame-work. We also review potential developments associated with rapid advances in genomic technology, in particular the increased availability of extensive marker information. Our conclusions highlight the complexity of processes contributing to the maintenance of genetic diversity in wild populations, and underline the value of a quantitative genetic approach in parameterizing models of life-history evolution.
    Annu. Rev. Ecol. Evol. Syst. 01/2008; 39:525-48.

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