bauplan convergence

[Kendall Clements <k.clements@auckland.ac.nz>]

The evo-devo approach advocated in the McNamara and Long book is in my 
opinion one of the most exciting advances in evolutionary biology. Rudi 
Raff's book "The shape of life" is a great read, although 4 years after 
its publication it is already somewhat outdated (especially the 
invertebrate phylogeny, which predates the tripartite  bilaterian tree).
I have two comments with respect to the quoted passage from "The 
evolution revolution:"

1. Many of the molecular biologists in this area have been very naughty 
with their use of the word homology. Here's an example from a 
recent paper by Jason Hodin (J. Exp. Zool. (Mol. Dev. Evol.) 288: 1-20 
2000) which discusses the role of the gene Pax6 in regulating eye 
development in both the mouse and Drosophila:

"This example really brings into focus the problems encountered with 
the use of the word "homology" to describe both molecules and 
morphology. Yes, the "homologous gene" (properly, the "orthologous 
gene") is used to build both the fly and mouse eye. But are they used 
in the same way? The appropriate way to address this question is not to 
see if the mouse gene works in fly eye development. The mouse gene was 
found to regulate eye development in Drosophila (Halder et al. 1995), 
yet there is a functional Pax6 in C. elegans [a nematode], an organism 
that lacks eyes altogether. Based on its sequence similarity, I wager 
that C. elegans Pax6 would also work in fly eyes. A positive result 
tells you only that the biochemical properties of the protein have been 
conserved, not necessarily that its function within a certain 
morphological structure has also been conserved."

Returning to the insect wings example, one could say that the 
regulatory genes are orthologous (homologous), but whether the 
resulting morphological structures are homologous is another matter. 
Incidentally, the link between crustaceans and insects may be closer 
than Averof and Cohen imagined. Wilson et al. (Mol. Biol. Evol. 17: 
863-874 2000) suggest that malacostracan crustaceans (e.g. prawns) may 
be closer to insects than they are to branchiopods (brine shrimp). Thus 
crustaceans may be paraphyletic, and insects are actually derived 
crustaceans!

2. Evo-devo approaches can be used to test hypotheses on dinosaurs. 
Examples include Steve Gatesy's work on avian vs non-avian theropod 
locomotion, and Richard Prum's work on feather evolution. I think this 
sort of reasoning has the potential to be far more productive and 
meaningful than what often passes for scientific discourse on this list.

Kendall

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Kendall Clements k.clements@auckland.ac.nz