On the Geographical Origin of Larrea Divaricata (Zygophyllaceae)

After assessing all available evidence concerning the present distribution of Larrea divaricata Cav., we have suggested that reached North America from South America (Ilunziker et al., 1972a, 1972b). This point of view was sustained previously by Johnston (1940). Axelrod (1950) has pointed out that the present occurrence of these relict austral types in the north can be explained by transtropic migration in the late Cenozoic. Long-distance dispersal across the tropics may have occurred through an intermediate series of appropriate semi-arid habitats that served way-stations (Raven, 1963). The latter might have arisen a result of the expansion of the world's arid climates in the late Tertiary and early Quaternary (Axelrod, 1970; Porter, 1974). An alternative hypothesis is that the present disjunct range of L. clivaricata originated by the evolution of the Chihuahuan and Argentinian types from a common trans-tropic prototype, which later became extinct (Barbour, 1969). A third hypothesis has been suggested by Turner (1972) who claims that it is equally likely that L. divaricata developed a diploid population in North America millions of years ago, and that subsequently was established in South America through long-range Porter (1974) believes that the genus might just well have arisen in North America and later have been dispersed to South America. Turner (1972: 161-162) writes that at least one species, L. divaricata, is so similar to its southern hemisphere counterpart, L. tridentata, that some authors have considered them to be the same species. On p. 163 he states that as yet we do not know the origin of the apparently autojunct Larrea divaricata. Hunziker (personal communication, this symposium) interprets his preliminary protein-band data suggestive of a South American origin for the group a whole. I am more cautious, for while I recognize the better development of Larrea (as to number of species) in South America, I am also aware that the North American element is made of diploid, tetraploid, and hexaploid populations, all of which show considerable variation, indicative of a species with some considerable tenure in the North American deserts. I suggest, therefore, that is equally likely that L. divaricata developed a diploid population in North America millions of years ago, and that subsequently was established in South America through longrange dispersal. The fact that there are two other species of Larrea in South America implies little, for L. divaricata may be only remotely related to these. Unfortunately, Dr. Turner has apparently overlooked the cytogenetic, taxo-