<P> Understanding evolution within the core cacti clade is difficult as of February 2012, since phylogenetic relationships are still uncertain and not well related to current classifications . Thus, a 2011 study found "an extraordinarily high proportion of genera" were not monophyletic, so were not all descendants of a single common ancestor . For example, of the 36 genera in the subfamily Cactoideae sampled in the research, 22 (61%) were found not monophyletic . Nine tribes are recognized within Cactoideae in the International Cactaceae Systematics Group (ICSG) classification; one, Calymmantheae, comprises a single genus, Calymmanthium . Only two of the remaining eight, Cacteae and Rhipsalideae, were shown to be monophyletic in a 2011 study by Hernández - Hernández et al . For a more detailed discussion of the phylogeny of the cacti, see Classification of the Cactaceae . </P> <P> No known fossils of cacti exist to throw light on their evolutionary history . However, the geographical distribution of cacti offers some evidence . Except for a relatively recent spread of Rhipsalis baccifera to parts of the Old World, cacti are plants of South America and mainly southern regions of North America . This suggests the family must have evolved after the ancient continent of Gondwana split into South America and Africa, which occurred during the Early Cretaceous, around 145 to 101 million years ago . Precisely when after this split cacti evolved is less clear . Older sources suggest an early origin around 90--66 million years ago, during the Late Cretaceous . More recent molecular studies suggest a much younger origin, perhaps in very Late Eocene to early Oligocene periods, around 35--30 million years ago . Based on the phylogeny of the cacti, the earliest diverging group (Pereskia clade A) may have originated in Central America and northern South America, whereas the caulocacti, those with more - or-less succulent stems, evolved later in the southern part of South America, and then moved northwards . Core cacti, those with strongly succulent stems, are estimated to have evolved around 25 million years ago . A possible stimulus to their evolution may have been uplifting in the central Andes, some 25--20 million years ago, which was associated with increasing and varying aridity . However, the current species diversity of cacti is thought to have arisen only in the last 10--5 million years (from the late Miocene into the Pliocene). Other succulent plants, such as the Aizoaceae in South Africa, the Didiereaceae in Madagascar and the genus Agave in the Americas, appear to have diversified at the same time, which coincided with a global expansion of arid environments . </P> <P> Cacti inhabit diverse regions, from coastal plains to high mountain areas . With one exception, they are native to the Americas, where their range extends from Patagonia to British Columbia and Alberta in western Canada . A number of centers of diversity exist . For cacti adapted to drought, the three main centers are Mexico and the southwestern United States; the southwestern Andes, where they are found in Peru, Bolivia, Chile and Argentina; and eastern Brazil, away from the Amazon Basin . Tree - living epiphytic and climbing cacti necessarily have different centers of diversity, as they require moister environments . They are mainly found in the coastal mountains and Atlantic forests of southeastern Brazil; in Bolivia, which is the center of diversity for the subfamily Rhipsalideae; and in forested regions of Central America, where the climbing Hylocereeae are most diverse . </P> <P> Rhipsalis baccifera is the exception; it is native to both the Americas and the Old World, where it is found in tropical Africa, Madagascar, and Sri Lanka . One theory is it was spread by being carried as seeds in the digestive tracts of migratory birds; the seeds of Rhipsalis are adapted for bird distribution . Old World populations are polyploid, and regarded as distinct subspecies, supporting the idea that the spread was not recent . The alternative theory is the species initially crossed the Atlantic on European ships trading between South America and Africa, after which birds may have spread it more widely . </P>

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