Putting morpho-phylogenetics in an Evo-Devo context

Posted by: Bryn Gaertner

Today we had a seminar speaker who talked about ways to map morphological data (the technique is a way to quantify shape, called Geometric Morphometrics) onto existing phylogenetic trees, and then inferring when vital morphological transitions happened (and specifically whether big shape transitions happened, or whether it’s gradual).  Brian Sidlauskas gave three specific examples of these strategies. Conceptually, his MO is to synthesize the fields of phylogenetics and morphometrics.  His argument is that each of them on their own are modestly informative, but in the words of G.G. Simpson, “we’re learning more and more about less and less.”  By synthesizing the two fields, we’re able to learn more about both than by studying either of them separately.

What did we learn?  We learned in Africa that the Distichodontidae and Chodontidae clades of fishes show morphological divergence, and that there is also phylogenetic divergence.  Brian had the bright idea to do a PCA on morphological differences, then essentially draw the phylogenetic branch onto the points represented by each individual, and connecting the points with the know phylogenetic relationships (nodes).  What we find is that they segregate together quite nicely.  And, the points segregate right where a new bone joint showed up in the fossil record.  One branch was able to radiate out once it got the new bone joint, and the other had to stay point because their functionality was limited with the “old” jaw.  And we see a similar story with a group of South American fishes.

A wrench was thrown in the works with a meta-analysis of body size and body shape data for a broad sampling of supposed adaptive radiation events.  Here, we expect massive shifts in body size and shape because of some type of founding effect, where there are a limited amount of competing taxa and a lot of niches to fill.  However, this is not what they found.  Instead, they found that most of the changes in body size/shape in an adaptive radiation are due to Brownian motion– random steps from one size to another, or more aptly put, gradual change over time, more change with more phylogenetic differences.  Not to worry though, it’s likely that a lot of the changes seen during an adaptive radiation wouldn’t have been found using their data.  For example, in Darwin’s finches, it’s beak depth that seems to have done all the adapting, but this might not have shown up in the size/shape data.

What got interesting to me was the measures of allometry in the fossil record of tetrapods (from amphibians till… mammals?).  Some parts of the jaw scale with others, but other parts don’t. It got me thinking about the functional systhesis that most of the graduate students in our department strive towards, and that is the topic of this quarter’s journal club.  What genetic changes do we expect to see when we have this phylo-morphometric data?  In the case of the African fishes, perhaps we expect a gene duplication, missing a bit of the regulatory region, that made that extra jaw bone.  In the case of the non-allometric scaling, there are some bones that scale with size and some that don’t.  What genes are in charge of this part of the body plan?  Can we make valid predictions of which changes to expect, and then how do we test them?

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