Why pythons and boas look alike

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An Emerald tree boa native to the Amazon Basin.
Credit: Joe McDonald

The evolution of two of the largest snake families in the world – boa constrictors and pythons – tells a fascinating story about natural selection, and how adaptation impacts an animal’s traits.

Pythons and boas are two very distinct families in the snake world, each encompassing dozens of diverse species that may reside in trees, in water or on land.

To understand more about these distinct families, a pair of researchers at the Australian National University (ANU) in Canberra put their heads together – or rather, the heads of their subjects – to study the ecology of the species.

The team, comprising of biologists Damien Esquerre and J. Scott Keogh, studied the size and shape of the heads of 2000 pythons and boas, ranging from America to Australia and representing more than 80% of known species.

They found species from the different families that lived in similar habitats had developed strikingly similar features – and this occurred across at least five different habitats.

The heads of aquatic boas, for example, were shaped like the heads of aquatic pythons, while land-dwelling boas closely resembled land-dwelling pythons. Burrowing snakes, for example, showed a broader, shorter head with smaller eyes, while semi-aquatic species had a more streamlined, funnel-shaped head.

This, the researchers say, points directly to environmental impacts. Traits associated with a particular environment or ecological factor that have evolved repeatedly in different species across different lineages, are likely to be adaptations to that environment, the researchers say.

Until now, most research into reptile genetics has focused on related species from similar regions, so Esquerre and Keogh were keen to conduct a broad, habitat-based comparison.

This phenomenon is known as convergence – that is, when two distinct species develop similar traits that weren’t present in their last common ancestor. The last common ancestor of pythons and boas lived around 70 million years ago, in the mid-late Cretaceous period.

The evolutionary paths taken since that distant cousin is, according to the researchers, an exemplary demonstration of how adaptation can be related to habitat.

Other animals show convergence. Dolphins and sharks, for example, are two distinct species with strikingly similar physical characteristics, probably developed in response to their environment.

“The finding of such a strong case of convergent evolution demonstrates the power of natural selection and adaptation in living organisms,” says Esquerre, a biologist at ANU and lead author on the study.

“If we see that different groups evolve the same things independently when they face the same challenges, we can find predictability in evolution.”

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