Caddisflies of the Yukon - Department of Biological Sciences ...

840 G.B. Wiggins and C.R. Parker
FIG. 18. Global distribution of variant characters in Agrypnia colorata Hagen (129) (Phryganeidae). Dark-coloured
adults with tibial spurs constant at 2,4,4 occur throughout northern Eurasia and Beringia; light-coloured adults with
tibial spurs variably reduced occur through most of North America outside Beringia; intermediate dark specimens
with variable tibial spurs occur in northwestern North America between the two areas outlined.
Failure of these East Beringian species to broaden their postglacial Nearctic range
perhaps could be attributed to competition for resources from species advancing from the
south which were better adapted to the freshwater habitats of the deglaciated terrain, or in
any case from species that reached the new habitats first. As discussed previously, closure
of Beringia by conjunction of the Laurentide and Cordilleran ice sheets probably would have
caused some extinction in Nearctic Beringia; consequently a wider range of ecologically
coordinate species from the much larger, and biologically more diverse, West Beringia
would have encountered reduced competition when they dispersed to East Beringia. But this
advantage of the Palaearctic species might well have been less effective in postglacial time
when Nearctic species were reassembled in more tightly packed communities. For these or