Establishing the relationship between elemental composition, phenotypes, and functional performance is a key step toward understanding the role of stoichiometry in adaptive evolution. The clearest example of such a relationship is the positive association among P content, RNA content, and growth rate described by the GRH (see Organismal Ecophysiology). The GRH has obvious evolutionary implications because growth rate influences an organism's age and size at maturity, traits that often affect survivorship and fecundity.
A test of the GRH in an evolutionary context comes from a comparison between Daphnia species from different geographic regions. This comparison found that Daphnia from the arctic have higher P content and faster growth rates than Daphnia species from temperate lakes. These results suggest that the short arctic growing season selects for rapid growth, which is reflected in whole-body stoichiometry. Indirect support for the GRH comes from studies on zooplankton that show positive associations between body P concentration and growth rate across ontogenetic stages.
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