Biogeography of North American Andmexican Insects, and a Critique of Vicariance Biogeography

Abstract

Insects comprise approximately 79% of all described animals, probably form an even greater percentage of the animal component of terrestrial biotas and are excellent subjects for biogeographic studies. This paper applies vicariance biogeography methods to samples of insects from Canada, the United States and Mexico. Analysis of the reduced area cladograms of 22 genera representing 11 families and 6 orders shows that North America (Canada and continental United States) and Mexico have ten zones of disjunction which separate allopatric sister taxa. The predominant pattern among the cladograms is for the various pairs of sister areas to each have a different outgroup area. The general lack of compatibility among reduced area cladograms is apparently due to four factors. (1) Some groups of insects have probably dispersed across barriers. (2) Barriers within at least 6 of the zones of disjunction have been cyclic in occurrence, alternating between being present and then temporarily subsiding; these cycles have offered opportunities for cyclic dispersal and cyclic vicariance. (3) The geographical ranges of many insect groups have apparently changed greatly in the past, thereby obscuring any orderly vi- cariance patterns. (4) Extinctions probably have played a major role in producing incompatible reduced area cladograms. Selective pressures at times have probably favored the most vagile insect groups and thereby selected for those least likely to have compatible reduced area clado- grams while selecting against more sedentary groups, those most likely to have compatible cladograms. The methods of vicariance biogeography may be unable to fully elucidate the historical bio- geography of many major components of continental biotas due to factors such as those affecting insects. Such methods may be most appropriate for relatively sedentary organisms found in areas outside the ranges of drastic environmental changes such as Pleistocene ice sheets. The methods of vicariance biogeography need to be further tested with data from real organisms rather than from simplified or hypothetical taxa; such testing may lead to the devising of more robust procedures. For many groups of organisms the methods of vicariance biogeography will be an important subset of a more multidisciplinary approach. (Biogeography; vicariance; dis- persal; extinction; Nearctic; Insecta; Coleoptera; Hemiptera; Homoptera; Hymenoptera; Lepi- doptera; Plecoptera.)