& OTHER SUCCULENTS
Evolution and Phylogeny
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.
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
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.
There are three sub-tribes of Opuntiae;
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.
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 II are the Epiphytes, three recognizable sub-groups of so-called "Jungle cacti" (a) the tree-like
Group III consists of the Mexican-No. American spherical cacti descended from Group I through a
Group IV comprises the spherical and columnar cacti of South America which are more or less distinct
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.
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