Natural selection on body size is mediated by multiple interacting factors: A comparison of beetle populations varying naturally and experimentally in body size

Ecology and Evolution (Impact Factor: 2.32). 09/2011; 1(1):1-14. DOI: 10.1002/ece3.1
Source: PubMed


Body size varies considerably among species and among populations within species, exhibiting many repeatable patterns. However, which sources of selection generate geographic patterns, and which components of fitness mediate evolution of body size, are not well understood. For many animals, resource quality and intraspecific competition may mediate selection on body size producing large-scale geographic patterns. In two sequential experiments, we examine how variation in larval competition and resource quality (seed size) affects the fitness consequences of variation in body size in a scramble-competing seed-feeding beetle, Stator limbatus. Specifically, we compared fitness components among three natural populations of S. limbatus that vary in body size, and then among three lineages of beetles derived from a single base population artificially selected to vary in size, all reared on three sizes of seeds at variable larval density. The effects of larval competition and seed size on larval survival and development time were similar for larger versus smaller beetles. However, larger-bodied beetles suffered a greater reduction in adult body mass with decreasing seed size and increasing larval density; the relative advantage of being large decreased with decreasing seed size and increasing larval density. There were highly significant interactions between the effects of seed size and larval density on body size, and a significant three-way interaction (population-by-density-by-seed size), indicating that environmental effects on the fitness consequences of being large are nonadditive. Our study demonstrates how multiple ecological variables (resource availability and resource competition) interact to affect organismal fitness components, and that such interactions can mediate natural selection on body size. Studying individual factors influencing selection on body size may lead to misleading results given the potential for nonlinear interactions among selective agents.

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    • "), high temporal variation in resource availability (McLain, 1993; Badyaev & Ghalambor , 1998; Lehman et al., 2005) and/or high levels of resource competition (Amarillo-Su arez et al., 2011). One of the clearest examples of this is with the marine iguanas of the Galapagos (Wikelski & Trillmich, 1997; Wikelski et al., 1997; Wikelski, 2005). "
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    • "Many laboratory rearing experiments are performed under what researchers hope are optimal conditions, but discovery of trade-offs associated with increased growth rates are most likely when conditions are suboptimal (Fisher et al. 2007; Dmitriew 2011). Beetles selected for large size suffered a greater reduction in body mass when reared under stressful conditions such as being fed on small seeds or reared at high larval density (Amarillo-Suarez et al. 2011). Yellow dung flies selected for large size grew fast on optimal diets, but showed more strongly reduced growth and greater mortality than control lines or lines selected for small sizes when reared under food-restricted conditions, supporting the idea that stressful conditions reveal costs of large size/high growth rates (Teuschl et al. 2006; Blanckenhorn et al. 2011). "
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