California insects surveyed for their phylogeographic structure display a variety of patterns, depending on their vagility, habitat requirements, and reproduction. Although insects have been present since the Paleozoic, they experienced significant extinction during at the end of the Permian. Modern insect lineages expanded and migrated in the mid-Mesozoic with the expansion of freshwater ecosystems and with associations with angiosperms in the Cretaceous. Later colonizations of western North America by insects were likely the result of repeated migrations across Beringia and from eastern North America. Molecular evidence from taxa in the Klamath-Siskiyou region, southern Cascades, and northern Sierra Nevada support migration from Asia. Endemism in the Sierra Nevada is surprisingly low (at 0.9 percent) and occurs primarily at higher elevations; however, mountainous species—particularly alpine taxa—reflect the effects of Pleistocene climatic change on their genetic structure. Butterflies, in particular, have the capacity for long-range dispersal, but because they are vulnerable to environmental change, they provide a good model for the study of phylogeographic change. Low-dispersing species have emerged as particularly useful in providing genetic signatures of past vicariant events.
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