Meeting Abstract
During the last glacial maximum, the Cordilleran Ice Sheet covered mainland British Columbia including much of Vancouver Island. Sea level was 125-145m below the present level, creating ice-free habitats on the low-slope continental shelf where estuaries develop. Thus, there likely existed a zone of refugia for marine, anadromous, and possibly freshwater species along the coast. The conventional model of post-glacial evolution in freshwater threespine stickleback (Gasterosteus aculeatus) assumes independent colonization of many lakes from a relatively uniform oceanic ancestor. However, extant oceanic populations around Vancouver Island vary in frequency of morphological traits, and trait distributions map well on sea surface salinity variation. Here we use a GIS approach to map glacial retreat and relative sea level change to model estuary development from 25,000 yr ago to the present using an adjusted sea level curve. We infer relative surface salinity changes from broad glacial melt patterns. The resulting time series of habitat development, migration, and loss suggests two general colonization routes for refugial Pacific Ocean populations into recently-deglaciated watersheds; the southern group via the low-salinity Salish Sea and the northern group via the high-salinity LaBouchere passage. If adequate estuarine habitat existed through the last glacial maximum, ancestral stickleback populations from two geographically, and possibly genetically, distinct sources were likely colonizers of freshwater habitats, rather than a single genetically uniform ancestral population as is often assumed. This could account for some apparent variation among both oceanic and freshwater populations on Vancouver Island.
