Cactus Evolution – Part III

By Joe Merkelbach (May 2009)

The article on cactus photosynthesis and respiration that Martin Schwieg contributed last month ( “C3, C4, CAM Photosynthesis”) is a good launching point for a discussion on one of the methods that systematists and taxonomists use for evaluating the direction and sense of timing for biological evolution.

The rule called Occam’s Razor says that the simplest, least complicated explanation of an event is usually the correct one. A corollary in biology is that organisms generally evolve from simple and common, or widespread, toward more complex and specialized.

The succulent plants, with cacti as one of the largest and best known of the group, have moved from simple C3 metabolism to more complicated C4 and CAM as an evolving adaptation to difficult environments. Deciduous leaves, the physical location for typical simple C3 photosynthesis, are the older, more common form. C4 and particularly CAM have developed as plant forms evolved to persist in harsher, drier environments.

Pereskias, with their full-size, persistent, slightly succulent leaves and ability to use C3 metabolism, are considered the most primitive form of cacti. Their shrub, vine and tree forms are common plant shapes as opposed to the pads, cylinders and spheres of more evolved cacti. However, the presence of fuzzy aeroles and spines on the stem, as well as the flower form, indicate that pereskias are cacti and probably best represent the primitive form from which more highly evolved types differentiated over long expanses of time.

The intermediate species, such as maihuenias and opuntias, have much smaller persistent or semi-persistent succulent leaves which evolved as an effective water conservation measure. The majority of cacti, as well as other many other succulents, have evolved body forms that conserve water very efficiently in order to persist in their generally harsh environments.

The highly modified leaves of mesembs and crassulas are other superb examples of this tendency to co-evolve form and function, such as C4 and CAM photosynthesis, in response to habitat changes over time.

Succulence, as a means of conserving water, is thus an evolutionary format that has given us the many interesting forms that we enjoy growing and showing.

Find additional information and photos at http://en.wikipedia.org/wiki/Pereskia.