The Cactus’s Doppelgänger: Convergence in the Desert

At least on this side of the pond—Britannica’s offices are in Chicago—mention of the desert is sure to evoke a landscape punctuated by towering, columnar plants armed with rows of spines: cacti. Indeed, the deserts of the Western Hemisphere bristle with members of the family Cactaceae, ranging from the emblematic saguaros and organ pipes—the species whose vertical stems most people probably think of—down to the 1 cm buttons of Blossfeldia liliputana, the smallest cactus in the world.
Echinocactus grusonii. Credit: Mike Keeling

The Cactus’s Doppelgänger: Convergence in the Desert

Echinocactus grusonii, a species of cactus. Credit: Mike Keeling Attribution-NoDerivs 2.0 Generic (CC BY-ND 2.0)

But what of the flora eking out an existence in the arid regions of the other side of the world?

Upon superficial examination, the botanical character of the deserts of Africa and Asia doesn’t seem terribly dissimilar from that of, say, the Atacama Desert of Chile or the Mojave Desert of the U.S. Speaking broadly, these regions are home to often patchy vegetation, the majority of which sports easily discernible evolutionary adaptations to the harsh climate: succulent stems to hold water, thick, waxy leaves or no leaves at all, thorns to defend against parched herbivores and protect living tissues from harsh sun and wind.

It is worth noting that cacti are not the only plants to have evolved means of coping with the challenges of desert life. Aloes and agaves, Crassulas and echeverias, among others, join cacti in cloaking the world’s sands and rocks in fleshy greenery. Even knowing this, the casual observer might reasonably conclude that at least some of the spiny plants erupting from the floor of the Namib Desert or towering over the Kalahari were indeed cacti. After all, the signatory qualities are there in many cases: columnar, fleshy stems adapted to storing water for long periods, few if any discernible leaves, and, of course, the requisite armature of wickedly pointed spines.
Euphorbia columnaris. Credit: Mike Keeling

The Cactus’s Doppelgänger: Convergence in the Desert

Euphorbia columnaris, a species of Euphorbia. Credit: Mike Keeling Attribution-NoDerivs 2.0 Generic (CC BY-ND 2.0)

Despite appearances, though, such a conclusion would be erroneous. The fact is, there are no cacti native to the Eastern Hemisphere. (One genus, Rhipsalis, does grow in eastern Africa but many consider it either a human introduction or one made in the relatively recent past by birds depositing seeds in their droppings.)

Like as not, a “cactus” spotted in Africa or Asia is actually a member of the globally distributed family Euphorbiaceae, usually of the genus Euphorbia. Its most recognizable member, native to comparatively wet areas of Mexico and Central America, is that ubiquitous holiday decoration, the poinsettia. The Euphorbias are a remarkably plastic group, having evolved into forms—ranging from trees to lawn weeds—adapted to nearly every conceivable type of habitat and climate, deserts included.

Subjected throughout their evolution to natural forces similar to those experienced by the ancestors of cacti—searing heat, blistering winds, long periods without rain—the desert forms bear striking similarities to their western analogues. Such evolution of similar forms from disparate ancestors is known as evolutionary convergence. More noticeable occurrences of this phenomenon appear in the animal kingdom: think of the independently evolved wings of insects and birds.

Fascinatingly, in instances of convergence, the end results are often not derived from the same part of the ancestral organisms. In insects, wings evolved from extensions of the exoskeleton, while in birds they evolved from modified forelimbs supported by an internal skeleton. While both cacti and succulent Euphorbias both responded to dry conditions by evolving progressively thicker stems as a means of storing water, their spines derive from totally different features. In cacti, spines evolved from either modified leaves or bud scales and in Euphorbias they evolved from stipules, small, leaflike structures that protrude from the bases of leaves in leafy plants. (In many cases, the spines of Euphorbias are paired, a relic of the stipule structure. Some species form spines that are sterile peduncles, or flower stalks.)

The next time you’re poring over the houseplant options at Home Depot and get jabbed by an errant spine, have a look at the offending protuberance once you’ve stanched the blood. It may provide clues as to the provenance of your desktop ornament.

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