Most empirical studies of specialists and generalists have focused on comparisons among different species. Interspecific comparisons have recently been criticized because phenotypic trade—offs can evolve after ecological specialization. As a result, it has been suggested that intraspecific variation is the appropriate level to demonstrate functional trade—offs. We demonstrate functional trade—offs for two components of fitness in pumpkinseed sunfish (Lepomis gibbosus) from a single trophically polymorphic population: condition factor and growth rate. Condition factor was estimated using somatic lipid content, while growth rate was determined from scale measurements. Fish with extreme benthic and limnetic morphologies are on average superior to fish with intermediate morphologies, especially in the pelagic habitat. Our study is a particularly stringent test of functional trade—offs at the intraspecific level because the morphological variation is very subtle in this population.
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Robinson et al. (1996) studied this question.