In 1893 a Belgian palaeontologist by the name of Louis Dollo formulated his law of irreversibility which stated that evolution is not reversible. According to Dollo, structures that have been lost over evolutionary time (such as gills in mammals) cannot re-evolve. To put it in his own words: –
“an organism is unable to return, even partially, to a previous stage already realized in the ranks of its ancestors”
However, if there’s one thing you should know about biology it’s that rules almost always have exceptions and often it is these exceptions that are the most interesting areas of research. A paper published last year in the journal Evolution and written by John J. Wiens documents one such case, an apparent violation of Dollo’s law, the loss and re-evolution of teeth in the lower jaw of frogs.
The class amphibia is divided into three major groups; the little known caecilians, the salamanders and the frogs and toads. Of these groups both the caecilians and salamanders have teeth in their lower jaws, or mandibles. This is the ancestral condition that has been retained ever since the first amphibians pulled themselves from the water during the Devonian period. However, in the lineage leading to frogs and toads, the order Anura, mandibular teeth were lost at some point after it split from its sister group, the salamanders, between 225 and 338 million years ago. This is shown in the phylogenetic tree below.
This is where it gets interesting. If, as we know they were, mandibular teeth were lost in the frog lineage hundreds of millions of years ago then, according to Dollo’s law, they should be found in no species of frog alive today. But this is not the case. Gastrotheca guentheri, or Guenther’s marsupial frog, does possess teeth in its lower jaw, in fact it is the only living species to have this trait.
If you look closely at the phylogenetic tree at the top of this post you can see G. guentheri is buried deep within the Anuran clade indicated by an arrow (re-evolution of mandibular teeth). No other species within the genus Gastrotheca, or indeed any other species of frog, possesses these teeth which means that they must have evolved independently in G. guentheri relatively recently. What we have here is an anomaly, G. guentheri is a species that violates Dollo’s law.
Before we consider why teeth have re-evolved after such a long period of absence it is worth considering why they were lost in the first place. Although teeth play a crucial role in prey capture for most carnivorous animals, many frogs instead catch their prey with their tongues which they flip out and downwards over the lower jaw. Mandibular teeth are therefore not required for most frogs and may actually be a hinderance when it comes to catching their prey, this may explain why they were lost so long ago.
The tongue method of hunting is however, only effective for those species that feed on relatively small prey items such as insects. Some species of frogs tackle much larger prey (such as other frogs) and have evolved stiffer mandibles and large fang-like teeth in the upper jaw to facilitate this. A few of these species have even evolved tooth-like structures on the lower jaw known as odontoids or serrations, these are not actual teeth but serve a similar function. As Wiens points out, it is clear that for some species of frogs mandibular teeth would be a significant advntage when hunting. Why they have re-evolved only in G. guentheri and not in other species is not known but may be due to developmental constraints that limit the re-evolution of mandibular teeth in most species of frogs.
Wiens study used genetic data to estimate the divergence times of the major groups of frogs and to pinpoint the times at which mandibular teeth were lost and when they later re-evolved in the lineage leading to G. guentheri. His results show that teeth were lost between 338 and 225 million years ago didn’t reappear again until only 20 million years ago, thus they were absent for 200 million years at least and most probably much longer. Further research is required to reveal why it is that mandibular teeth have only re-evolved once and what constraints may be preventing their re-evolution in other species.
If there’s one thing we can take away from this study it is that Dollo’s law is not really a law at all but a rule. This means that while it is true in most situations there are exceptions as we have seen. Dollo’s rule then is really just a statement about the statistical improbability of complex traits evolving in exactly the same way in the same lineage twice. It’s still important but it is not a law.
Wiens J.J. (2011). Re-evolution of lost mandibular teeth in frogs after more than 200 million years, and re-evaluating Dollo’s law, Evolution, 65 (5) 1283-1296. DOI: 10.1111/j.1558-5646.2011.01221.x