Backpedalling on backwards evolution

In a recent post I wrote about irreversible colour changes in morning glory flowers, and how this was promoted as evidence that evolution does not work in reverse1. This is called Dollo’s Law, after the 19th century Belgian paleontologist Louis Dollo. He spent most of his life digging up and reconstructing Iguanodons, but his name lives on in our concept of evolutionary trees. In linguistics, for example, the “stochastic Dollo” model refers to the scenario where words can only arise once in a family of languages2.

Louis Dollo supervising the reconstruction of an Iguanodon

Louis Dollo supervising the reconstruction of an Iguanodon. From Wikimedia Commons.

And the reason Dollo’s name is forever tied to the idea of history not repeating? He wrote that “an organism is unable to return, even partially, to a previous stage already realized in the ranks of its ancestors.”3

Frogs might prove Dollo wrong. A new study by John Wiens, a herpetologist at Stony Brook University, proves that a South American frog re-evolved the long lost bottom teeth of its ancestors, after going more than 200 million years without them4.

Guenther’s marsupial frog lives in the tropical forests of Colombia and Ecuador. It is the only frog we know of among thousands of species with true teeth in its lower jaw. Nearly all frogs have teeth, but only on the upper mandible. They use their single set for grasping prey items that they swallow whole, rather than chewing. The closest amphibian relatives to frogs, salamanders and worm-like caecilians, have maintained upper and lower teeth. This means that somewhere along the line, early frogs lost their bottom set.

As the odd one out, Guenther’s marsupial frog was originally placed in its own family within the taxonomic order Anura. But early studies of tadpole development and immunological proteins suggested that something was off5. In some ways, the marsupial frog was similar to typical tree frogs in the family Hylidae.

John Wiens’ latest results provide a more comprehensive picture of the early amphibian family tree than ever before. He compiled genetic data from 170 species of frogs, salamanders and caecilians. The new phylogeny firmly places Guenther’s marsupial frog in the family Hemiphractidae, and, by calibrating the new tree with evidence from the fossil record, Wiens can pin down the timing of events in amphibian history. His results show that frogs evolved from a salamander-like ancestor and lost their bottom teeth over 230 million years ago – and Guenther’s marsupial frog regained them quite recently, within the last 17 million years.

Hoatzin chick

The marsupial frog is just the latest in a long line of putative exceptions to Dollo’s Law. Others include fossil ammonites that have lost and regained the coiled form of their shells several times3, and modern slipper limpets that regained the coiled shells of their snail-like ancestors6. Some lizards have re-evolved long lost fingers and toes7. The strange bird known as the hoatzin, which has claws on its wings that are used for climbing until its wings are fully developed for flight, might have re-evolved this feature from its dinosaur ancestry. These examples have led some authors to argue that Dollo’s Law has outlived its usefulness8. Should it go the way of chewing teeth in hens and (most) frogs?

Continue reading →