Hoodia pilifera: like a liliputian cactus, but from Africa

You can't wade very far into the study of evolution, plant taxonomy -- or the pursuit of growing exotic succulents at home -- without smacking right into convergent evolution. That's the notion that certain distantly related plants have evolved features which make them seem much more alike than their DNA (or evolutionary pedigree) would predict.

Hoodia pilifera, 3.5" tall, like a tiny saguaro
South Africa (Western Cape, Little and Great Karoo)
photo by Sentient Meat

As I've learned more about it, I've come to the conclusion that there are two kinds of convergent evolution: the superficial, obvious kind and the subtle, fool-a-scientist kind. The more subtle kind of convergent evolution -- the kind that can fool a scientist -- shows up in the study of plant taxonomy (the classification of plants into species, genus, and family based on how closely they're related). Just as DNA science has revolutionized how crimes are solved (and revealed that some imprisoned felons are actually innocent), the study of DNA in plants has revealed that plants once believed by scientists to be closely related (due to similarities in stem, leaves, flowers, or seeds) are actually quite distant relatives. They developed their similarities independently -- and well after their ancestors had split off from each other. As a result, the family tree (for example, of Euphorbia¹) has been in a state of ongoing upheaval as the contemporary field of molecular phylogenetics revises our knowledge of evolutionary relationships and reveals many cases of subtle, scientist-fooling, convergent evolution.

Actual saguaro, approx 100x taller than our H pilifera
photo by Leonard G. / Wikipedia - Creative Commons ShareAlike 1.0 license

In grade school we learn the more obvious kind of convergent evolution -- a sort of gee-whiz, ain't-Nature-grand, intercontinental shape-matching game, in which we are amazed that plants from Africa (like spiny euphorbs and today's feature, Hoodia pilifera) bear such a striking, if superficial, resemblance to desert cactus (like the majestic saguaro, pictured), which hail exclusively from the Americas.
--

¹Credit goes to Brian Dorsey from University of Michigan for turning me on to the turmoil and excitement in genus Euphorbia (or wails of anguish, depending how attached you are to old classifications) in his talk, "Systematics of the xerophytic and succulent Euphorbia" at 28th Succulent Plants Symposium. Much of this turmoil comes from correcting errors in classification (and our accompanying model of the evolutionary family tree) which can be blamed on subtle cases of convergent evolution within Euphorbia.