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Evolution and Phylogeny of Cacti Cactus Evolution Cactus Phylogeny Cactus Evolution Perhaps about 50 million years ago, the Cenozoic cooling resulted in regionally arid conditions within which various xerophytes evolved. Among such xerophytes was a particular jungle thorn bush, which bore the seven characteristics by which all cacti are defined. This was defined as the ancestor of all cacti. Its descendants diversified, bit by bit, as each evolved successful survival responses to changing conditions in its own habitat. The popular Pereskia aculeata var. godsefiana, (Lemon Vine), a hardy woody shrub with succulent leaves which produces the delicious "Barbados Gooseberry", is thought to resemble the ancestor closely. This ancestor was the first to evolve the Areole, the specialized Axillary Bud, unique to and a very important definer of the Cactaceae. Each axillary bud carries the spines and a bud which may (or may not) differentiate to become a flower o r a side shoot, depending upon many local factors. Top of Page Cactus Phylogeny Finding a series of characteristics common to a plant of interest, and following the line of development, reveals the phylogeny or evolutionary history of a plant family. In the Cactaceae, the process of developing a phylogenetic scheme rarely identifies clear definable groups, as the descriptions of many genera and most species of cactus are incomplete. The extent of cactus variation and geographical distribution is insufficiently known due to the poor fossil record. No fossil cacti are known to exist and most cases of natural hybrids have only been observed as isolated examples. For this reason, groups within the cactus family are continually split up, re-united and definitions are modified. To make some general conclusions about the evolutionary history of cacti, scientists today must rely on the external form of present day cacti and more commonly, on biochemical, chromosomal and DNA data. Genealogical Tree of the Family Cactaceae (Adapted from Barthlott, 1979) There are over 2000 species of cacti, and these can be arranged in a system based on their presumed degrees of relationship. Within the cactus family it is generally accepted that plants fall into three natural groups, called sub-families, which evolved perhaps by 20 million years ago as a result of a gradual adaptive evolutionary process. By this time, Plate Tectonics had spread the Atlantic sea floor to such a width that these adaptations were absolutely confined to the Americas. Consequently, Cactaceae are native only to the Americas. Sub-Family Pereskioideae Of the two genera, Pereskia and Maihuena, forming this most primitive sub-family, the Pereskiae are a genus of about 20 species of jungle trees, vines and shrubs native to Central America and the West Indies. Plants are scarcely succulent, woody, usually shrubby or tree-like. All species have persistent leaves and shiny black seeds. Seedlings bear fleshy cotyledons. Maihuena, the second genus, is a mound forming, low plant. One species of pereskia has been naturalized on the west coast of Africa, Although no proof has been found, there is no doubt that it was introduced by ocean curr ents, birds, or even, perhaps, by ancient mariners. Pereskia godsefiana Sub-family Opuntioideae Members of this sub-family range in form from low shrubs to large tree-like plants. There are 9 genera and approximately 300 species within this sub-family. Plant members are unique for having deciduous leaves that soon dry up, and for possessing a second type of very small spines called glochids, which are barbed and very easily detached. Seedlings possess fleshy cotyledons similar to those of the Pereskiae. There are three sub-tribes of Opuntiae; the 'Cylindropuntias', Chollas with cylindrical, sausage-like, linked stems which are the most closely related to the Pereskiae through their early-deciduous, large and terete leaves; the 'Tephrocacti'with many globe-shaped linked stem segments; and the 'Platyopuntias', Prickly Pears with flat pads. These latter cacti comprise the Flat-stemmed Opuntiaand the Nopalea,and are the most widely distributed, of all cacti, having evolved selected traits allowing them to inhabit many otherwise inhospitable environments. They are the least primitive plants of the Opuntioideae. The Opuntioideae sub-family is particularly interesting because its members occupy the greatest geographic range as compared to any other major group of cacti. The Opuntiae range in both North and South America, from mid Saskatchewan to Patagonia and have become naturalized in southern Africa and Australia where they are now considered noxious weeds. In some Mediterranian lands, they are cultivated for their fruit. Opuntia engelmannii Sub-family Cactoideae This is the most highly differentiated sub-family of the Cactaceae, containing about 90% of all species diversity within the family. There are over a thousand species showing a wide range of morphological diversity. All plants are more or less fleshy and can be either terrestrial or epiphytic. They have no leaves and no glochids and the flowers are usually funnel shaped. Spines are usually present and vary in color, structure, desnity and arrangement. The cotyledons of the Cactoideae members are microscopic in the seed and unlike the Pereskiae and Opuntiae, they do not enlarge in proportion to the growing embryo. Cactologists today recognize about 8 evolutionary independent lineages called tribes. The first six contain stem-succulent plants that have ribs, tubercles, areoles bearing spines and flowers having a long tube. The last two are epiphytic, tree or cliff-dwelling jungle cacti. Within the sub-family Cactoideae can also be inferred a system of four lineage groups based on perceived relationships of morphology and adaptation which very roughly conform to four American regions, the Carribean, the Central-, the North- and the South- American regions. Group I   contains certain Carribean genera descended from Pereskia which first developed a columnar morphology as thin stemmed, erect and creeping types. They are thought to be the ancestors of all Cactoideae, evolving differently through several sub-groups and dispersing as the next three groups. Group II  are the Epiphytes, three recognizable sub-groups of so-called "Jungle cacti" (a) the tree-like or climbing Carribean So. American genus Harrisia, (b) the thin stemmed or flat stemmed climbers and trailers of Central America and (c) the So. American epiphytes familiar as the "Holiday-, Orchid-, and Mistletoe cacti" among others. Group III consists of the Mexican-No. American spherical cacti descended from Group I through a sub-group of Echinocereus, the "Hedgehog cacti" via Central America, the primary dispersal centre for Group III as as for the following, Group IV. Group IV comprises the spherical and columnar cacti of South America which are more or less distinct from their No. American counterparts because of the different evolutionary forces which acted on them. Cacti in groups III and IV have evolved deeply notched ribs and often tubercles, an advantage to coping with sporadic water supplies. When water is scarce the folds sink in between the ribs and when water is plentiful the folds swell out. In this way, the overall form remains more or less similar at either extreme. A ribbed body can change its volume while the area of the skin remains unchanged. This contrasts with the flat pads of Opuntiawhich wrinkle up in an uneven manner upon water loss. Cacti which bear notched ribs and large tubercles are believed to be those which have developed farthest from the basic cactus form. Columnar cactus Epiphytic cactus Notched ribs Top of Page