Susan A. Foster
Trends in Ecology & Evolution, 1999, 14:5:190-195
Box 1. Postglacial population differentiation in the threespine stickleback
As glaciers began to recede following the last glacial maximum (22 000 years ago or less), marine threespine stickleback (Gasterosteus) colonized the new freshwater habitats, giving rise to populations that subsequently experienced rapid adaptive differentiation452650. Because marine threespine stickleback are remarkably uniform morphologically and behaviourally, and because they have changed little morphologically in the past eleven million years, phenotypes common to all marine populations studied to date can reasonably be interpreted as ancestral relative to the recent fresh-water radiation. Thus, the direction of evolutionary change can be inferred with unusual certainty. A notable element of this radiation, with respect to behaviour, is that a survey of more than 25 freshwater populations in southern British Columbia (Canada) and the Cook Inlet Region of Alaska (USA) has failed to reveal any novel motor patterns, although adaptive differentiation has occurred involving behavioural traits as diverse as sneaking behaviour by males, courtship behaviour, antipredator behaviour and cannibalistic proclivities. In all cases, differentiation has been achieved by altering the frequency of expression of the motor patttern or by loss of all or part of a behavioural repertoire.
The most pervasive pattern of ecotypic differentiation among lacustrine populations has been driven by differences in trophic conditions between (a) shallow, comparatively eutrophic lakes and (b) deep, oligotrophic lakes. In shallow lakes, the populations display morphological specialization for foraging on abundant benthic invertebrates, and large foraging groups routinely cannibalize young in the nests guarded by parental males. In these 'benthic' populations (sensu McPhail4), behaviour most closely resembles that observed on marine breeding grounds. Large cannibalistic foraging groups are common, males respond to these groups with all or part of a complex ancestral diversionary display repertoire, and courtship behaviour is relatively inconspicuous because the vigorous zig-zag dance is rare or absent and slow circling and dorsal pricking are major components of the interaction. In contrast, populations occupying oligotrophic lakes (limnetic ecotype) are specialized for feeding on plankton, have lost the ancestral tendency of group cannibalism of young and the associated diversionary response to groups, and exhibit extremely conspicuous courtship behaviour that predominantly incorporates the zig-zag dance. Ecotypic differentiation along this trophic axis has been repeated in at least six benthic and four limnetic populations occupying geographically disparate lakes, and all adaptive behavioural differentiation appears to have occurred as a consequence of loss of ancestral behaviour patterns or shifts in the frequency of their expression.