Global patterns of distribution reflect latitudinal gradients in temperature and moisture and natural barriers to dispersal. A. Wallace (1876) identified six relatively distinct faunal assemblages that largely coincide with major continental boundaries but also reflect the history of continental movement, as discussed later in this section.Wallace's biogeographic realms (Fig. 7.1) remain a useful template for describing species distributions on a global scale. Many taxa occupy large areas within a particular biogeographic realm (e.g., the unique Australian flora and fauna). Others, because of the narrow gap between the Palearctic and Nearctic realms, were able to cross this barrier and exhibit a Holarctic distribution pattern. Of course, many species occupy much smaller geographic ranges, limited by topographic barriers or other factors.
Some distribution patterns, especially of fossil species, are noticeably disjunct. Hooker (1847, 1853, 1860) was among the first to note the similarity of floras found among lands bordering the southern oceans, including Antarctica, Australia, Tasmania, New Zealand, Tierra del Fuego, and the Falklands. Many genera,
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