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

Caddisflies of the Yukon 825
recognize that if correct for some species, this morphological guideline may be illusory for
others. One indication that the species did not diverge at uniform rates is provided by
members of category V; several of these are widespread Holarctic species that must have
passed between Asia and North America at some time before the onset of harsh Pleistocene
climates because there is little evidence that they are adapted now to far northern latitudes.
Secondly, fossil evidence of known age is rarely available for species of Trichoptera, but in
Coleoptera where it is available, extant species of the north were found to be unexpectedly
old and clearly in existence well before Pleistocene time, with some identifiable in late
Miocene 5.7 ma B.P. (e.g. Downes and Kavanaugh 1988). A very slow rate of evolutionary
change in some Trichoptera is demonstrated by the small structural differences between a
Baltic amber species of Lype (Psychomyiidae) and a Recent European species (Ross 1958,
fig. 1); the age of the amber species is placed at upper Eocene—some 50 ma B.P. Perhaps
this is an isolated and atypical example, but for insect species generally an age of 25 million
years has been estimated (e.g. Briggs 1966). Comprehensive analysis of the fossil record
reveals that insects show very low rates of extinction; among vertebrates, by contrast, few
living species are more than a million years old (Labandeira and Sepkoski 1993). The age
of species and the time required for their origin are different concepts; but inferences
reflecting the rate of evolution for insect species must be tempered by the evidence that at
least 760 species of Drosophilidae have evolved on the Hawaiian Islands since their origins
6 million to 500 000 years ago (Kaneshiro 1993).
Finally, there is the larger question of the age of the cold, far northern biomes
themselves. Tundra develops in regions that are too cold for the growth of trees, and arctic
tundra is believed to have arisen in the north as disjunct areas of ecological regression in late
Miocene forests (Matthews 1979b; Danks 1981). The Seward Peninsula of Alaska, just
below the Arctic Circle (67°N lat.), was forested up to about 5.7 million years ago in late
Miocene time (Hopkins et al. 1971). Palaeobotanical evidence indicates that the forest-tundra
ecotone probably lay just to the north of the Seward Peninsula at that time, suggesting that
tundra probably covered most of the North American Arctic Archipelago by 6 ma B.P.
(Matthews 1979b). Thus, tundra may be a relatively young biome in the north, and adaptation
of tundra species a rather recent phenomenon. Some arctic tundra Trichoptera may have been
derived from alpine tundra habitats, which in Beringia would have dated from the time that
the high mountains of the western Cordillera and of northeastern Asia reached elevations
which precluded the growth of trees. Evidence from fossil insects suggests, however, that
species may have adapted to arctic tundra conditions from lineages occurring in the boreal
forest (Matthews 1979a; Danks 1981). Because of their diversity and broad ecological
penetration, Trichoptera could be an appropriate group for testing these hypotheses. Phylogenetic
analysis in the highly diverse boreomontane genus Limnephilus would be a promising
avenue of investigation to reveal the habitats of the phyletic ancestors of the present
inhabitants of arctic and alpine tundra. However, a good deal more basic information about
the biology, distribution, taxonomy, and phylogenetic relationships of the species of Limnephilus
will be required. Trichoptera established in pre-Pleistocene arctic tundra might have
found glacial refuge in alpine tundra of accessible mountain ranges, or in disjunct lowland
tundra refugia along the southern margin of the continental glacier.
I. Nearctic Species Widespread in North America
This is the largest category of the Yukon Trichoptera—98 species comprising 68 per
cent of the total fauna. None of these species is known from Europe or Asia, and their
generally widespread occurrence now at more southerly latitudes in North America suggests