Thursday, March 19, 2009

What does homoplasy mean?

Nature's got another feathered dinosaur report, but this time it's an ornithischian. I guess I shouldn't say it's "feathered," since it's really not. The authors describe it as a "dinosaur with filamentous integumentary structures" that they suspect are like protofeathers.

Zheng X-T, You H-L, Xu X, and Dong Z-M. 2009. An Early Cretaceous heterodontosaurid dinosaur with filammentous integumentary structures. Nature458:333-336.

What makes this special is that it's an ornithischian. The famous protofeathered and feathered dinosaurs are theropods, which makes evolutionary sense, if the theropods are the closest relatives to birds. But theropods are saurischians. This new dinosaur Tianyulong, is from a completely different group of dinosaurs that is not supposed to be closely related to birds. If Tianyulong has protofeathers, then it makes the evolution of feathers complicated. I can think of three possible evolutionary interpretations.
  • Tianyulong has protofeathers, and they are homologous to theropod protofeathers. This would imply that the common ancestor of the ornithischians and saurischians (basically all dinosaurs) had protofeathers and that protofeathers should be common in dinosaurs. This possibility is unparsimonious since we know only a few dinosaurs with protofeathers, which implies that the dinosaur ancestor had no protofeathers.

  • Tianyulong has protofeathers but evolved them independently of the theropod protofeathers. Basically, it's an astonishing homoplasy. If this were the case, we'd have to conclude that feathers are easy to evolve, or perhaps that the ancestor of the dinosaurs had some kind of genetic predisposition to evolve protofeathers. That's not unprecedented in evolution (see C4 photosynthesis).

  • Tianyulong does not have protofeathers but some other kind of filamentous integumentary structures. The resemblance to protofeathers is merely coincidental. This is the least exciting option.

As a creationist, I'm particularly fascinated by the nature of homoplasy and the meaning of intermediates. Homoplasy is similarity that is not attributable to a common ancestor. Homoplasy is everywhere, but it's typically treated as noise or nondata, except for the really spectacular examples. For example, the eye of the squid and the vertebrate eye are remarkably similar (but still different in important ways). This is more than just evolutionary happenstance. It's usually given as an example of convergent evolution, evolution of similar solutions to similar problems. Same for the body shape of the dolphin, ichthyosaur, and shark.

What if homoplasy has nothing to do with evolution, though? It's a such a large part of the pattern of life, what if it's telling us something? If it is, what does it mean? Is it nothing more than something to resist the construction of evolutionary trees? Is it just an antievolutionary foil? That seems lame. It must be more important than that. It's a large part of God's design, and that must count for something.

Exactly what it counts for, I don't know.