Evolution of Cactus ... 4/4

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The other possibility is that cacti had appeared/evolved up to rhipsalis stage BEFORE the splitting up of Gondwanaland and the Rhipsalis that were left stranded in Africa did not evolve further, while it cousins carried away on the South American landmass evolved further in response to changing climates, so the Rhipsalis genus is the ancestor of all cacti.

This scenario put forward by Phil Maxwell is that Pereskia evolves in West Gondwanaland (area of future South America) from the Portulacaceae(?) family, Opuntia form a evolutionary branch (or may even be a separate independent evolution of 'cactus') at an early stage, the Pereskia evolves into a epiphyte - a Rhipsalis - with CAM and no leaves which spreads rapidly throughout West Gondwanaland, then the American and the African populations become genetically isolated due to plate movements/oceans and begin to diverge. The African Rhipsalis reach an evolutionary 'cul-de-sac' while the American ones go on to form the cacti genus we know today. Also another evolutionary branch that diverged from Pereskia early on is the Hylocereeae e.g.. Hylocereus, Epiphyllum and Selenicereus according to Phil.

Cacti evolve into 3 subfamilies

Three subfamilies, which had one common ancestor but evolved about 20 MYA are now recognised by (most) botanists as below:(see above "Rhipsalis" above for an alternative evolutionary path).

• PERESKIOIDEAE
  

  The smallest (18 species) and most primitive subfamily ( Maihuena may be the ancestor of all cacti) of the Cactaceae are found predominantly in South America/Mexico. Pereskia plants are usually shrubby, tree-like with leaves.
  Pereskioideae may be on an independent evolutionary lineage from the other cacti, and have not been as "successful" as the other two subfamilies in terms of specialisation and distribution.

• OPUNTIOIDEAE
  

  This subfamily (300 species) has small bristles called glochids which are unique to opuntioids. It is the most widely cactus distributed from coast to coast and Canada to southern south America, plus with the help of humans now the Mediterranean, Africa and even Australia (where it is considered a pest).
  Economically it is important as the "prickly pear" or "tuna" for human consumption, or as a host for the red dye cochineal insect in places from Sicily to Egypt to Mexico to South America.

• CACTOIDEAE
  

  In most amateur collections more than 95% of the cactus cultivated are from this subfamily, with 1,000 to 2,000 species of all shapes and forms (probably 1,600 depending on whether you are a lumper or a splitter).
  There are eight to ten, possibly independent, tribes within this subfamily.
  Four tribes which evolved in North America (Cactacae, Pachycereeae, Echinocereeae, and Hylocereeae), and their parallels in South America (Notocacteae, Trichocereeae, Browningieae, Cereeae, and Rhipsalideae).
  Barrel cacti, columnar cacti, and epiphytic cacti have parallel evolved in both North and South America after spreading from point of origin then being separated when the central American land bridge disappeared for a while.

Further Reading

How cacti survive drought from LiveScience.