Evol Ecol Res 13: 439-459 (2011)     Full PDF if your library subscribes.

Partitioning the relative fitness effects of diet and trophic morphology in the threespine stickleback

Daniel I. Bolnick1 and Márcio S. Araújo2

1Howard Hughes Medical Institute; Section of Integrative Biology, University of Texas at Austin, Austin, Texas, USA and  2Departamento de Ecologia, Instituto de Biociências, Universidade Estadual Paulista, Rio Claro, SP, Brazil

Correspondence: D.I. Bolnick, Section of Integrative Biology, University of Texas at Austin, One University Station C0930, Austin, TX 78712, USA.
e-mail: danbolnick@mail.utexas.edu


Background: Numerous models show that if morphology and diet are correlated, frequency-dependent competition will lead to fitness differences among phenotypically dissimilar individuals within a species.

Hypothesis: Selection acts primarily on diet, and only indirectly on morphology via its correlation with diet.

Field sites and organism: British Columbia, Canada; 340 individual threespine stickleback (Gasterosteus aculeatus) from McNair Lake and 430 individuals from First Lake.

Measurements: Stable isotopes (δ13C and δ15N; a proxy for diet); trophic morphology (quantitative traits and geometric shape variables); and growth rates (RNA/DNA ratios; a proxy for the component of fitness arising from competitive or foraging ability).

Analysis: Linear and quadratic regression of growth rate on stable isotopes and morphological variables to calculate the relationship between growth (a fitness proxy) and diet and/or morphology. When both morphology and isotopes affected growth rates, we used a path analysis to separate their effects.

Conclusions: In the McNair Lake population, growth was dependent primarily on diet type and only indirectly on trophic morphology. In a second population, path analysis found that isotopes and body shape separately explain variation in growth rates. We infer that, in stickleback, selection on trophic morphology is often a correlated side-effect of selection on diet composition, rather than direct fitness effects of morphology per se.

Keywords: directional selection, frequency-dependent selection, fitness landscape, function-valued trait, Gasterosteus aculeatus, stabilizing selection, stable isotopes, trophic morphology.

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