| Summary: | This thesis investigates competition's effects on the evolution of sympatric threespine
sticklebacks in small, post-glacial, coastal lakes of southwestern British Columbia. A few
lakes contain two morphologically and ecologically distinct stickleback forms ("limnetics" and
"benthics"), whereas most lakes contain a single "intermediate" form. These forms are
believed to be derived from marine threespine sticklebacks that invaded lakes following the
retreat of glaciers. I examine body shape differences of marine and freshwater stickleback
forms and I test whether competition experienced by planktivorous marine sticklebacks in
experimental ponds is consistent with the hypothesis that sympatric lake forms evolved, at
least in part, in response to interspecific competition.
A thin-plate spline geomorphometric analysis quantified differences in body shape
among four stickleback forms: the marine [ancestral] and three [descendent] lake forms
(limnetics, intermediates, and benthics). I found significant shape differences among all forms:
marine and limnetic sticklebacks were most similar in body shape, while intermediates fell
between them and benthics. This pattern matches the divergence observed in other characters
among these forms. The main differences were in head size, distance between fin insertion
points, and first dorsal spine location. Convergence in body shape among the freshwater
sticklebacks may represent anti-predator adaptation, whereas divergence may represent
adaptations for foraging.
To test whether marines compete with intermediates, I placed juvenile marine
sticklebacks either alone or with intermediate sticklebacks in experimental ponds. I also tested
whether there is an adaptive shift in marine morphology between treatments due to natural selection, phenotypic plasticity, or both. Marine sticklebacks competed with intermediates;
marines experienced both significant reduction in growth and increased consumption of
zooplankton when intermediates were present. Marine morphology also showed an adaptive
shift; however, this shift could not be unambiguously attributed to the presence of
intermediates.
I tested whether competition experienced by marines declines as the competitor
becomes more divergent by pairing marines with either intermediates or benthics.
Competition experienced by marine sticklebacks decreased when the competitor was more
ecologically and morphologically differentiated; marines with intermediates grew significantly
slower than marines with benthics.
This research provides both a more complete description of morphological
differentiation among sticklebacks and crucial evidence that interspecific competition is an
important factor in evolution.
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