From the Digest

Cactus Evolution – Part II

By Joe Merkelbach (April 2009)

Cacti are confined to the New World – North and South America – with the exception of a species of Rhipsalis that is widely distributed from Africa to Sri Lanka. Cacti are believed to have developed after South America separated from Africa.

The center of origin is thought to be either in the Caribbean basin or South America. Plants radiated out from this center in all directions. Development and speciation occurred along similar paths thoughout the North and South American continents as they joined and then separated and rejoined.

The most primitive forms are Pereskia and Maihuenia, the Pereskioideae that retain fleshy leaves, although they also have areoles with glochids and spines. Most of these are shrubby or mound forms. The Opuntiodeae, with their fiendish glochids, are considered intermediates between pereskias and the largest, most evolved group, the Cactoideae.

The cactus groups in North and South America are different in details, indicating their separate development, but they have very similar forms: barrel, columnar and epiphytic or clambering. The opuntias also have similar forms: chollas, balls and pads, represented from South and North America.

These stuctural forms are very good examples of parallel evolution, the development of living forms into similar shapes due to exposure to similar conditions. There are conditions that favor treelike columnar cacti, spherical barrel forms and epiphytes in similar habitats on both continents.

The specific forms of both continents that live in restricted habitats are the ones most threatened with extinction in the wild. Examples are the Florida species that we discussed last month. Restriction to small islands is decidedly a mark of rarity and a potential threat for eradication in the wild.

Protection of the habitat, with enough space for the plants to thrive and reproduce is thus essential for survival in nature. This is the direction that plant and wildlife conservation should be pursuing in the future to maintain diversity and the continuity of evolution on the planet.