Genetic Variation for Clutch Size in Natural Populations of Birds From a Reaction Norm Perspective

Abstract

Abstract. Genetic variation for ecologically important traits determines the potential for evolutionary changes and should be measured directly. Such measurements of genetic variation based on quantitative genetic theory rely on assumptions of environmental constancy. These assumptions are not likely to hold in nature. Instead, natural environments are structured, and systematic variation in environmental conditions is an important determinant of phenotypic variation. Here we provide an introduction to quantitative genetics using a reaction norms approach, because we believe that this provides us with a good framework for combining ecology and genetics. We subsequently review the literature on genetic variation for clutch size of birds, and we show that, in spite of the inherent limitations of the methods employed, there is strong evidence that clutch size has a heritable component in natural populations of several species. However, the number of studies on the amount of genetic variation for clutch size in different species and across a range of environmental conditions is still far too small to study patterns in the relationship between heritable variation and properties of species and/or their environments. Furthermore, the role of both correlations and interactions with the environment in these estimates requires much more attention. Above all else, however, we need more information on the structure and magnitude of the environmental variation present in these studies. Future work should focus on how to obtain such data, and how to subsequently incorporate them into the proposed reaction norm framework. This requires the search for, and measurement of, relevant ecological variables. Also a more detailed investigation of the within-individual variation and the use of animal model methodology may prove to be valuable. Such additional data are essential for interpreting the amounts of genetic variation present for clutch size as a model system in the general problem of better understanding the maintenance of genetic variation in heterogeneous environments and the estimation of evolutionary potential.

Parasitism Reduces the Potential for Evolution in a Wild Bird Population

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Extended Laying Interval of Ultimate Eggs of the Eastern Bluebird

“…The response of birds to removal/addition experiments has been described as some species laying to achieve a particular clutch size (indeterminate layer) and others laying a particular number of eggs (determinate layer). Although phylogenetic constraints play a role in the determination of clutch size (Yom-Tov 1987, Postma andvan Noordwijk 2005), the termination of laying has been attributed to (1) extrinsic cues that trigger the cessation of laying (e.g., the tactile stimulation of the brood patch; Haywood 1993a, b), (2) nutrient limitations on egg production (Houston et al 2007), and (3) hormonal and circadian regulation of clutch size (Haywood 1993a, b, Zivkovic et al 2000. If any of these mechanisms play proximate roles in…”

A framework for the study of genetic variation in migratory behaviour

“…Having reduced our bird to one trait value, we can use the concept of reaction norms to study phenotypic variation of a series of genotypes over a range of environmental conditions (e.g. van Noordwijk 1989;Postma and van Noordwijk 2005;Rappole and Helm 2006). Each genotype is represented as a line that specifies the phenotype over a range of values on an environmental axis.…”