Contemporary Morphological Evolution in a Lacustrine Population of Threespine Stickleback

Date of Award


Degree Type


Degree Name

Master of Science in Biology



Chief Instructor

Susan A. Foster

Second Reader

John A. Baker

Third Reader

Philip Bergman


In the face of rapidly changing environments, some populations go through contemporary evolution in response to altered selective pressures. In this situation, changes in morphology and behavior can be observed to monitor phenotypic transitions. In adaptive radiations such as the threespine stickleback, Gasterosteus aculeatus, species complex, both re-expression of ancestral phenotypes and the expression of novel phenotypes have been detected. I monitored a population of threespine stickleback in Lynne Lake, Alaska, which is undergoing rapid environmental changes due in part to an increase in primary productivity in the lake. Changes have been documented in the body size and foraging behavior of this population in a previous study, causing me to ask whether body morphology had evolved in a direction paralleling these behavioral changes. I predicted a shift from the historically limnetic phenotype to a more benthic body shape and jaw morphology, following the recent expression of benthic foraging behavior and an increase in productivity in the lake. I measured aspects of trophic structure and overall body morphology and interpreted the results by looking at shape components, two indices (jaw opening displacement advantage and suction index), and linear measurements of the jaw structure. I found certain aspects of body form to match the expected trends (including head shape, buccal cavity size and gape width), but many did not (such as overall boy shape, suction index and jaw protrusion). Suites of traits evolve in complex ways, both interacting with each other and following separate patterns. There are often observable differences in evolutionary rates in various traits as well. The morphology of the threespine stickleback population in Lynne Lake may be in the middle of a rapid morphological change, but the future pattern of change and where the phenotype will stabilize along the benthic-limnetic axis is still unclear.

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