UNDERSTANDING THE EVOLUTION OF BEHAVIOR IN THREESPINE STICKLEBACK: THE VALUE OF GEOGRAPHIC VARIATION

The endemic radiation of the threespine stickleback in North America possesses features that will permit us to gain unique insights into evolutionary pattern and process. Already, comparison of populations has demonstrated a remarkable level of adaptive differentiation in reproductive behavior across populations in both southern British Columbia, Canada, and the Cook Inlet Region of Alaska, U.S.A. In small, shallow lakes, threespine stickleback feed predominantly on benthic invertebrates in the shallow littoral zone. They form large groups that attack nests defended by males, and consume all of the young within. In contrast, in larger, more oligotrophic lakes, adult stickleback feed primarily on plankton in large open-water foraging groups. They do not form groups that cannibalize young in nests. In these non-cannibalistic populations, males perform more conspicuous courtship behavior than do those in cannibalistic populations, apparently because risk of attack by visually hunting conspecifics is low. Courtship in these lakes more often includes the conspicuous zig-zag dance, and less often, meandering and dorsal pricking. Anadromous populations tend to be cannibalistic, but also are often behaviorally intermediate. They incorporate the zig-zag dance in courtship more often than do males in cannibalistic freshwater populations, but also usually incorporate the less conspicuous dorsal pricking behavior. These results suggest that the ancestral condition was intermediate and that both cannibalistic and non-cannibalistic freshwater populations display some derived elements in their reproductive repertoires. This kind of comparison among populations thought to have been independently derived from the marine form can provide insights into the patterns and causes of adaptive diversification. For populations such as these, carefully chosen from disparate geographic locations so as to preclude common evolutionary history after colonization of freshwater,