15 January 2019
As I mentioned yesterday, Galapagos land iguanas eat mainly the flesh and fruits of the Opuntia cacti that grow all over the dry lowlands of these islands.
As with most other cacti, the Galapagos Opuntia have spines to protect themselves from being munched by herbivores…
… Except where they don’t!
That’s right – In this video, you see my own defenseless hand, petting the cactus as if it were a cat!
Here’s a photo showing similar “cactus petting:”
Rather than being stiff and sharp and painful, these cacti have “spines” that behave more like hairs.
But doesn’t this leave the poor cactus defenseless against attack by vicious iguanas?
Yep! It sure does. …Good thing that doesn’t matter.
This is what happens when the cactus have a long time to grow and reproduce without being molested by vegetarian lizards.
On islands where land iguanas roam, cactus individuals are under strong selective pressure to have some sort of defense. Stiff spines are therefore essential for survival. Hungry, hungry lizards keep the pressure on the cacti to maintain strong defenses. On islands where the cactus grows and is never attacked by iguanas, however, that pressure is released. It’s no longer a “deal breaker” for survival if a mutation makes a cactus grow non-stiff spines. It doesn’t need spines any more, so it doesn’t matter. On iguanafied islands, spinelessness is a recipe for death. Dead iguanas don’t have babies. On noniguanified islands, spinelessness is either (a) fine and neither a good thing nor a bad thing or (b) maybe even a good thing, if making stiff spines is in some way energy or resource intensive — those resources are then freer to be reallocated elsewhere. All else being equal, the spineless, hairy cacti can survive, no problem! And that means they can reproduce, which means they can pass this trait on to their offspring.
Evolutionary prediction: if you were to release land iguanas on these “hairy cactus” islands, the iguanas would go to town on the defenseless cacti, strongly selecting against the “hairs-not-spines” trait. If there were a few individuals that still made stiff, sharp spines, they would be at a huge selective advantage. They would be selected as the survivors, and then would be eligible to go on and be the breeding stock that would be the foundation of the future cactus population on those newly-iguanified islands. The proportion of spines/hairs in the population would be predicted to increase.
It’s a neat trick to pet a cactus and not come away bleeding, but it’s also a nice prompt for a little lesson in how natural selection works.