Evolving Thoughts

Evolution, culture, philosophy and chocolate! John Wilkins' continuing struggle to come to terms with impermanence... "Humanus sum, nihil humanum a me alienum puto" - Terence

Tuesday, February 22, 2005

The reciprocal illumination of paleontology and molecular systematics

I recently attended my last Systematics Forum at the Melbourne Museum. This lovely little series covers issues at a technical and theoretical level to do with classification in biology, which is my obsession du jour.

The forum was a special one, a talk by Michael S. Y. Lee (aka Mike) of the South Australia Museum, and the University of Adelaide, entitled Whale teeth, bird hands and snake legs: why palaeontologists need to talk to molecular biologists (and vice versa).

Mike is one of those way too handsome and way too young types who seems to have a handle on everything (despite his advocacy of the Phylocode, for which I forgive him, although Gary Nelson may not). This was an especially interesting talk.

Mike used three examples of a correction of paleontology by molecular systematics and vice versa to show that both disciplines need each other. The first example is the "whippo" hypothesis - that the closest living relatives of whales are hippos and not some other Artiodactyl, such as the Mesonychids:

Some of the other examples were the origins of snakes and their relationship to "aquatic goannas", and the evolution of avian wings.

These cases depend traditionally on the use of morphological data. In the case of the whales, the phylogeny relied upon teeth, which were extremely similar to mesonychid teeth. This led researcher to think that whales evolved from these carnivorous animals. But with Gingerich's discovery of whale ancestral astralugus bones (see the diagram) in the ankle, it became clear that they were more closely related to the herbivorous animals of which hippos are a member taxon.

Molecular data removed all doubt. Whales are in fact more closely related to hippos than to mesonychids, and the similarity in teeth is a convergently evolved feature.

It is often the case that teeth are used as the criteria for classification, as they are persistent features of organisms, and indicate a lot of information about the ecology of the animals. A famous case is, of course, the evolution of horses (pics here), where the shift from browsers of tree leaves to grazers of grass-eaters not only marked the shift of an ecological feature (that is, the evolution of silaceous grasses) but also the evolution - once - of a feature which thereafter was common to all horses that persisted.

The trouble is that convergence can hide or confuse the phylogenetic signal. Teeth are only informative if they happen to match the evolutionary tree - in the case of cetaceans and mesonychids, they didn't. Once that character was removed, said Mike, the rest of the morphological data made sense and matched the molecular data.

The founder of what he called "phylogenetic systematics", which has become known as "cladism", Willi Hennig, wrote at length in his epoch-making book Phylogenetic systematics of the need for the various disciplines in biology to "reciprocally illuminate" each other when the evolutionary past was being uncovered. This is a beautiful case of exactly that.

We don't approach our classifications of the world nakedly, so to speak. We are always clothed in prior knowledge - well, we have to be, really, as we wouldn't even know what to look at otherwise. Thus do we evolve better knowledge of things...

Hennig, Willi. 1966. Phylogenetic systematics. Translated by D. D. Davis and R. Zangerl. Urbana: University of Illinois Press.