Historical Biogeography

as physical and biotic factors. All tropical deserts, for example, will not have the same species or genera of darkling beetles or all high mountains the same butterfly groups. Past dispersals and evolution of organisms will result in different assemblages of taxa in geographic areas regardless of similarities they may have in their environment (Mooney 1977). These assemblages, particularly the existence of endemics, give taxonomic character to any region from which it may be recognized as a distinct biogeographic unit with biotic uniformity.

The Neotropical Region itself is one of the earth's broadest and most distinctive of such categories. The reason for this seems to be due primarily to the fact that South America and Africa, before their disjunction, constituted the largest block of land (western Gondwanaland) in the tropics and was therefore a separate evolutionary center (Raven and Axelrod 1975). Subsequent long isolation permitted undisturbed and independent development of an already rich biota (Fittkau 1969).

The composition and spatial arrangement of the world's biogeographic regions have changed through time. Past evolutionary patterns have been postulated for some other organisms (Hallam 1973) but are not known for insects in Latin America, mainly because of their poor fossil record there. The origins of these regions and contemporary insect geography are determinable, nevertheless, by combining knowledge of a group's morphology, mobility, geographic and ecological dispersal opportunities in the past, and historical disruptions in its range. There are relatively few such studies on Neotropical insects (Halffter 1975, 1976, 1987), but it is common for taxono-mists to treat the zoogeography of their groups routinely in systematic papers (e.g., Nielsen and Robinson 1983).

In the Neotropics, different systems of classification of today's biogeographic sub-regions have been applied by authors choos ing varied organisms as indicators and more or less inclusive amounts of land area. For all or most of Latin America, few are based on the entire flora (Gentry 1982, Cabrera and Willink 1973) or fauna (Gil-more 1950, Goldman and Moore 1946, Fittkau 1969), fewer on both (Darlington

1965, Udvardy 1975). Some are derived from an analysis of diverse entomofaunal elements (Halffter 1964, 1974, 1975, 1987; O'Brien 1971) or specific taxa (Porter 1980, Ichneumonidae; Kuschel 1969, beetles; Halffter 1975, dung beetles). Most, however, are limited to much narrower taxonomic groups and restrictive areas (Peña

1966, Chilean darkling beetles; Scott 1972, Antillean butterflies; de Armas 1982, Antil-lean scorpions; Lane 1943, sabethine mosquitoes). Some of the latter have produced complex results, such as Lamas's (1982) recognition of forty-eight biotic regions for Peru, based on the country's butterfly fauna alone.

The aquatic medium has been treated to a limited degree. Stream insects fall into two main groups, the cool-adapted (oligosteno-thermic) types found in certain mountainous areas (Andes, especially southern Andes, Guianas, southeastern Brazil), and warm-adapted (polystenothermic) types spread almost everywhere else in the lowlands (lilies 1965, 1969).

For entomological purposes, in Latin America, the simple classification of biogeographic regions shown in figure 2.2 is useful (although a much more detailed system is proposed by Udvardy [1975]).

References

Cabrera, A. L., and A. Willink. 1973. Biogeografía de América Latina. Org. Est. Americanos, Ser. Biol., Monogr. 13: 1 — 122. Darlington, Jr., P. J. 1965. Biogeography of the southern end of the world. McGraw-Hill, New York.

de Armas, L. F. 1982. Algunos aspectos zoogeo-gráficos de la escorpionfauna antillana. Poeyana 238: 1-17. Fittkau, E. J. 1969. The fauna of South Amer-

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