What isn’t homologous converges

I took part in a discussion over at Peaceful Science a week or two ago, in which the slam dunk evidence of nested hierarchies for evolution was (not for the first time) being disputed – by me, amongst others.

Not that I disagree with evolution, but with the claim (a) that nested hierarchies are overwhelmingly neat and tidy and (b) that clades prove evolutionary nested hierarchies rather than, as the pattern cladists have always insisted, just the best tree-like pattern the software can produce from any particular set of data, however related or not.

Thr subject of homology versus convergence came up, in that the only way to distinguish them is by ultimately subjective assumptions about evolutionary relationships. That led, if I remember, to what might possbly constitute as evidence against homologous relationships, such as the famed Precambrian rabbit.

Fishing for examples, I thought of suggesting that feathered pterosaurs would put the cat among the pigeons, but decided to be less contentious, and contented myself with talking about the three disputed phylogenies of pterosaurs, the two of turtles, and the two possibly independent origins for bats (amongst the insectivores and the primates).

But lo and behold, in Today’s BBC science news, this! They have found my Precambrian-rabbit pterosaur with feathers anyway! Not true bird-like feathers, to be sure, but feathers very like those discovered in the last decade or two in fossils of the later theropod dinosaurs believed (possibly – see this post) to have given rise to the birds.

The link between theropods and birds is based on the homology of the feathers and a number of other key features – though it has to ignore some other significant non-homologous features and assume they will be explained one day, the pelvis being one that has actually led to a reclassification of the whole range of dinosaurs after a century of certainty.

But what about these pterosaurs? As I said above, pterosaur phylogeny is disputed, but in all models they diverge from the reptile lineage long before the dinosaurs . That’s one reason their warmbloodededness, combined with hair-like skin covering is not thought to be homologous with the warmbloodedness and feathers of the dinosaurs, or the mammals.

For that reason, it is absolutely certain that, if confirmed, the newly discovered feathers of pterosaurs will not raise any concerns about where they fit in the nested hierachy of feathers amongst theropods and birds. The’re too obviously not related – despite the fact that they have evolved identical feathers entirely independently

The explanation is that, like the pterosaurs themselves, the complex organs we call feathers just happened to evolve twice, by convergence. Which I suppose is no problem if, as Ernst Mayr said, complex eyes evolved independently between 45 and 60 times.

But it does push the pterosaurs a little further outside the nested hierarchy. There’s a cuckoo about somewhere.