Adaptive Radiation in Sticklebacks: Size, Shape, and Habitat Use Efficiency
I examined habitat use efficiencies of two morphologically distinct threespine sticklebacks (Gasterosteus spp.). The species make up one of several coexisting pairs that apparently formed in the past 13 000 yr in coastal British Columbia lakes. Previous comparative work has shown that morphology and habitat use are correlated within and among populations. Sticklebacks occurring alone in lakes are intermediate in morphology and habitat use, suggesting that differences between the sympatric species result from ecological character displacement. I tested two hypotheses to explain these patterns, and which are implicit in general views of the causes of adaptive radiation: (1) Each resource (habitat) subjects its species to unique selection pressures, owing to the advantages of certain combinations of traits for foraging; (2) Phenotypes intermediate between habitat specialists suffer a competitive disadvantage. These hypotheses were evaluated by measuring the foraging rates of the two species and their interspecific hybrids in the two main habitats (benthos and open water) provided in the laboratory. Efficiencies of habitat exploitation matched obeserved morphological differences. In the benthic habitat the larger, deeper bodies species (having also a wide mouth and few short gill rakers) was superior to the smaller, more slender species (having a narrow mouth and many long gill rakers). Success of hybrids was intermediate. This rank order of species efficiencies was reversed in the open water habitat. These results support the hypothesis that different traits are favored in different habitats, and that adaptation to one habitat has occurred at the expense of feeding rate in the other. Average foraging success of hybrids in the two habitats fell near to below the average of the two parental species, suggesting natural selection against intermediate phenotypes. Foraging efficiency in open water was greatest in the smaller size classes of fish, hinting that small size of the open water species is the result of selection for high foraging efficiency. However, size could not explain most differences in feeding efficiency between species, and it is likely that many traits contribute.
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