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• â-* The North American creosote bush (Larrea tridentata) has undergone a simple cytogeographic differentiation, with the ancestral diploid population in the Chihuahuan Desert, and with tetraploid and hexaploid derivatives in the Sonoran and Mohave Deserts, respectively. The chromosomal races have annectant but largely allopatric distributions, which coincide remarkably well with the boundaries of their respective desert provinces. However, during the glacials of the Pleistocene, the lowlands of these provinces were extensively invaded by evergreen woodlands dominated by various species of Juniperus, Pinus, and Qaercus; and the generality of the phenomenon during the Wisconsin glacial has been abundantly documented by some of the most detailed macrofossil evidence of relatively xerophytic plants ever obtained. The u C-dated macrofossil assemblages, preserved in rock-sheltered Neotoma deposits, demonstrate a persistence of woodland conifers and consistent absence of Larrea throughout most of its present range in the Southwest during the last major glacial episode of the Pleistocene from > 40,000 BP to about 11,000 BP. The oldest reliable record of Larrea in North America is dated on macrofossils of Larrea itself at 10,580 BP, and the site, near Wellton, Yuma County, Arizona, is one of the lowest ( 162 m ) and most arid in the present Sonoran Desert. Hence, the major features of the modern geographic pattern of Larrea tridentata in North America must have originated during the climatic transition from the late Wisconsin to the Holocene, when a desiccating climate gripped the immense lowlands of the Southwest, opening a vast and varied desert niche into which a burgeoning population of Larrea could have expanded and differentiated explosively.The biogeography of Larrea during the Pleistocene is a particularly interesting enigma because there is an intriguing possibility of a late, intercontinental dispersal from South America, where the genus exhibits markedly greater ecomorphological and chemical diversity than it does in North America. Also, the genera most closely allied to Larrea are endemic to South America. While several "r"-selected, herbaceous genera of Zygophyllaceae have much wider world-distributions, Larrea is unique among woody Zygophyllaceae in having an amphitropical, intercontinental disjunction in its range. The specialization of Larrea on intensely arid deserts with relatively open plant communities would facilitate establishment after dispersal to a new desert area. However, the migration of Larrea to North America must have lacked continuity. If the extremely xerophytic species, Larrea divaricata, had been able to follow a hypothetically continuous route across the wet tropics during a period of drier climate, then a much greater amphitropical disjunction of American zygophyllaceous genera should be expected. Long distance transport of seeds across the wet tropics is the only feasible alternative for Larrea.The creosote bush [Larrea tridentata (Sesse & Moc. ex DC.) Cov.] is the most abundant a...