Thursday, January 28, 2010

Phylogenetically structured variance in felid bite force: the role of phylogeny in the evolution of biting performance

I have been too busy lately.

Firstly because I started a postdoc and settling down etc takes up some time. But also, I've been revising a manuscript on dinosaur jaw biomechanics that came back from review; I am currently struggling with a method new to me that one of the reviewers suggested I use.

But before all this happened, I was pretty much occupied by this single paper that finally came out online:


I checked the dates on this submission and it was received by the Journal of Evolutionary Biology on 22 January 2009, and accepted 30 November 2009, but didn't become available on EarlyView until 14 January 2010. The actual print date is March 2010. So a year since submission. And three rounds of review in between.

Phew.

This was based on a chapter from my thesis that I thought was fairly robust and well-written that it would be the easiest chapter to convert to a manuscript for publication. Boy, was I wrong. Through the course of the review process, I had introduced so many new analyses that except for the basic premise, the final paper is nothing like my original thesis chapter.

Thanks to that, it is a much better paper; or so I think.

Anyway, I suppose I shall provide a brief overview of the paper here.

The main idea is to investigate just how much, if any, of the variability inherent in cat biting performance measures can be explained by phylogeny. In other words, do cat biting performance measures exhibit a strong phylogenetic signal. A strong phylogenetic signal would imply that the evolution of biting performance (and thus its underlying morphofunctional character complex) exhibits some phylogenetic conservatism. I discussed phylogenetic conservatism in an earlier post so I shan't repeat myself here. But determining whether a morphofunctional character complex is showing phylogenetic conservatism/signal or not is quite important to our understanding of the evolutionary processes in functional morphology.

For instance, if a functional character shows phylogenetic conservatism it could very well be that the associated morphological changes could be: a, somehow constrained (genetically, developmentally, structurally, geometrically, etc) and character change is limited; or b, conforming to a Brownian motion model of evolution and character change accumulation is proportional to branch length (or in other words, the differences between two taxa would be proportional to their phylogenetic distances). If in the case of the former, your functional character of choice may actually be something that is very difficult to modify because of constraints put upon it (perhaps because it requires modifications to vital organs or whatever). On the other hand, if in the latter case, your functional character may not be exhibiting much directional evolution or responding to external selective pressures and thus the function may not be something that is under selection (like its relationship with ecology/environment is very stable).

However, on the other hand, functional characters could also exhibit a strong phylogenetic signal and still show directional evolution (i.e.neither constrained nor with inertia [in the sense used here]); for instance, if there were trends across phylogeny. So perhaps there would be a functional change that is consistent with phylogeny but with directionality. For instance, maybe panthera lineage cats persistently evolve higher bite forces throughout their lineage.

This is all very interesting to speculate about, but we'd need to know if our functional characters of interest exert phylogenetic signal or are independent of phylogeny, in order to actually discuss anything in detail. It would be pretty difficult to determine exactly what the underlying mechanism would be (i.e. constraint, inertia, or perhaps directional) but its effect can be observed as phylogenetic signal.

There are several methods to do that, but I can't be bothered to provide a review here (maybe that's a future post, if I can ever get around to it...). Anyway, I tested for phylogenetic signal in biting performance measures (absolute and relative bite force) in 36 extant and 3 extinct felids but also the proportion of variance in bite force that can be attributed to phylogeny. In other words, I quantified how much of the disparity in bite force values is due to phylogenetic distances. I chose cats, initially because I supervised a Masters student who worked on the biting performance of the sabre-tooth cat Smilodon fatalis (we have a nice skull specimen in our Department), but also because I can get a large enough taxonomic sample from the very plentiful British collections of extant cat skulls, thanks to all the fine Victorian gents (Sirs, Colonels, Lt. Generals, etc.) who just shot the heck out of these poor animals, and to their widowed wives who after the deaths of their esteemed husbands donated the cat skulls to local museums (like the Bristol City Museum and Arts Gallery).

I kind of ran out of steam here, so I think I'll save my findings or interpretations for a later date.