Trans-Continental Convergences

Possibly the most compelling evidence for natural selection comes from instances of convergence—not a surprising thing for me to say, being one of Jonathan Losos’ students! But as phenomenal as the repeated evolution of Anolis ecology and morphology on the Greater Antilles may be, the convergences I find the coolest are those with one or more members each in the Neotropics and the Palaeotropics (i.e. tropical regions in the Americas vs. everywhere else). What makes these convergences interesting is that so many other organisms are completely different between the Americas and Africa, Asia, or Australia. When I first moved from India to the U.S., identifying even the simplest bird was a challenge—this had many reasons, including a shift from tropical to temperate systems and my mediocre birding skills. But the differences persisted even on trips to the tropical parts of the Americas, and the floral and faunal similarities to home stood out from the unfamiliarity.

A bit about convergence: in biological terms, convergence is the evolution of similar behaviours, morphologies, or physiologies from different starting points, likely due to similar selective pressures exerted by an organism’s environment. A simple, if crude, example is that of bat, bird, and insect wings—all are solutions to the problem of flight, but each has evolved independently from very different wingless ancestors. To know if two similar organisms have converged, we need to know how they are related to each other, and whether the two similar organisms have in fact descended from different ancestors, each of which lacks the traits that makes their descendants similar.

The first trans-continental convergence that really made me stop to think was that of plumage coloration in diverse fish-eating birds such as Osprey, Brahminy Kites, and Bald Eagles. I wrote a popular science piece about this convergence for the Amherst Element, and have posted it here. But the more I travelled, the more I came across convergences that I found equally compelling. Here are some of my favourite examples of Neotropical-Paleotropical convergence:

Sunbirds (Nectariniidae) and hummingbirds (Trochilidae): evolved to pollinate some wonderful flowers, these tiny birds have both evolved dramatic bill morphologies, iridescent plumages, and a proclivity for nectar.

Sunbirds and Hummingbirds (photos from Wikipedia and by Ethan Temeles, respectively)
Sunbirds and Hummingbirds (photos from Wikipedia and by Ethan Temeles, respectively)

Vine snakes (e.g. Ahaetulla and Oxybelis): as impressive as their convergence is the ability of these snakes to remain completely camouflaged in their forest environments.

Vine snakes, Ahaetulla and Oxybelis (photos from Wikipedia)
Vine snakes, Ahaetulla and Oxybelis (photos from Wikipedia)

Hump-nosed pit viper (Hypnale hypnale) and Hog-nosed pit viper (Porthidium nasutum): why on earth might turned-up noses be a selective advantage? Does it allow a snake to breathe from under the leaf-litter?

Hump-nosed and Hog-nosed pit vipers
Hump-nosed and Hog-nosed pit vipers (photos from Wikipedia)

The convergence of individual species in the Old and New World is exciting, no doubt, but it’s even more remarkable that whole communities of animals have been shown to converge across continents. This level of convergence suggests a big role for natural selection, exerted not only by the an organism’s habitat but also by other species in the community competing for the same habitat resources. One nice example is that of diurnal, insect-eating, desert-dwelling lizards in North America (iguanids) and Australia (agamids). A study by Jane Melville and colleagues examined the correlations between ecological and morphological variation in lizards in these two communities, and found evidence for across-continent community convergence–the degree of morphological similarity between two species, one in each community, could predict the degree of ecological similarity between the species. Here are the three most similar species-pairs from these communities:

Convergent lizards from the deserts of North America and Australia (from Melville et al 2006)
Convergent lizards from the deserts of North America and Australia (from Melville et al 2006)

Not surprisingly, trans-continental community convergences aren’t common–there are many ways to be different, and chance events ensure that even similar species will diverge over time. But I’m going to keep my eyes open and continue to build my list of convergences, in the hope that someday I’ll chance upon a convergent food chain…Let me know if you have other examples of neotropical-palaeotropical convergence!

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