Around the world, in a range of different environments, pythons and boas resemble each other. A new study reveals that these commonalities come not from common ancestry, but because the two families of snakes found similar solutions to the same problems, a process known as convergent evolution.
In wetlands and forests, pythons prey on small mammals, birds, and frogs. To the untrained observer, an aquatic python looks more like an aquatic boa than a tree python. Nevertheless, the reason we classify all the pythons as one family and boas as another, is because they each descended from a common ancestor not shared with anyone else, rather than just lumping species together because they resemble each other.
In many different environments pythons and boas adapt in such similar ways they can be hard to tell apart. Damien Esquerre.
Australian National University PhD student Damien Esquerre compared the head shape of 1,073 specimens from 34 species of pythons and 45 boas, to show how, when confronted with matching environments, each went for remarkably similar solutions. He measured head shape, because in a limbless predator like a snake, it is an extremely important feature.
“People in the past thought pythons and boas were part of the same family,” Esquerre told IFLScience. “But DNA shows their last common ancestor was 70 million years ago, during the age of the dinosaurs. Their closest living relatives are small and burrowing snakes that don't look anything like either.”
Examples of convergent evolution are common, but in Ecology Letters Esquerre points out: “Such cases often are based on general similarities in appearance (e.g. numbats and anteaters or wolves and thylacines in the comparison between the radiations of placental and marsupial mammals), rather than rigorous quantitative similarities.”
The two snake families allowed Esquerre to address this. Boas have adapted to many ecosystems on all continents other than Australia and Antarctica, while pythons are widespread through Africa, Asia, and Australia. Their diversity has ranged from modest burrowers to the enormous Anacondas and Reticulated Pythons more than 8 meters (26 feet) long.
As an example of their commonalities Esquerre said: “Aquatic boas like anacondas, and Australian water pythons, have evolved the same head shape, with eyes and nostrils on the back not the sides, which is ideal for moving through water and observing their environment from below.” On the other hand, he said, the tree dwellers of each family “have eyes on the sides and very wide heads, ideal for grabbing prey from hanging branches.” Rather than simply noting the commonalities, Esquerre used specialized techniques to measure and quantify skull shape, to demonstrate mathematically how tight the convergence has been.
Pythons and boas also display convergent evolution in other aspects. The incentive for camouflage, for example, has led to similar coloring across the inhabitants of the same sorts of habitats, to the extent that green tree pythons and their emerald tree boa equivalents look almost indistinct to the untrained eye.
Esquerre told IFLScience the observations have implications beyond our legless friends. “Convergent evolution is an interesting topic,” he said. “Evolution is thought to be generally unpredictable, but this shows it has some predictability. It shows us something about what we might expect about life on another planet if we know the conditions. There is an old question: if we rewound the tape of life, would the same sort of animals and plants have evolved? Is it all determined by random events? Strong cases of convergent favor the idea that there is some predictability and we might expect to see something similar emerge.”
Reticulated Python (Malayopython reticulatus) in all its glory. Damien Esquerre