The chaotic evolution of colony size in ants. (Tree re-analyzed from Brady et al 2006, colony data taken from Hoelldobler & Wilson 1990 and other sources)

This tree depicts how colony size evolves in ants.  The purple/blue colors represent small colonies with only a few to a few dozen ants, while the yellows and oranges represent species with enormous colonies of tens or hundreds of thousands of individuals.  What’s exciting about this rainbow-colored figure?

If you were expecting ant evolution to be an inexorable march towards larger and more complex societies, this tree should come as a surprise.  Ant colony size is all over the place.  Not only is there no general trend towards larger colonies, some lineages seem to be shrinking down from more populous ancestors.

Colony size evolution is not the subject of this post, though.  I’m going to whinge instead about how frustrating I found the process of making this figure. 

You see, there are ants we know a fair bit about.  We know what they eat, how many queens they have, and how large their colonies are.

Then there are the species that the NSF-funded “Assembling the Ant Tree of Life” (AToL) group sampled for the ant phylogeny.  Those two sets of taxa do not show much overlap.  And this lack of overlap means that using the AToL trees as a platform for revealing patterns in ant evolution will be a slower and more complicated slog than it ought to be.

I made the above tree by taking the AToL molecular data set from Brady et al (2006) and re-inferring it in MrBayes using only those species for which I could find reliable colony size data.  Most of the colony data come from table 3-2 in Hoelldobler & Wilson (1990), but I also drew from the literature.  Of the 162 AToL taxa, I arrived at only 30 with available colony size information. Even so, I still fudged on a couple of species by swapping data in from congeners.

Why did AToL choose the species they did?  Well, most of the AToL PIs are taxonomists. From a taxonomic perspective, Lasius californicus is locally available and works just fine as a representative for the genus.

But for downstream users of the phylogeny, for the folks who wish to use the tree to study how social behavior evolves in ants, the AToL design is odd indeed.  The most researched ant species tend to be either trampy or european, but AToL largely sampled ants from California and Madagascar.  The result is a tree connecting a bunch of species we don’t know much about.

For a taste of the AToL taxon sampling, consider the following.  The most studied Lasius is the common european garden ant L. niger (Google Scholar hits: 3,370); AToL sampled Lasius californicus (G.S. hits: 6).  The most studied Formica species are F. rufa (G.S. hits: 3,280) and F. polyctena (G.S. hits: 1,830);  AToL sampled F. moki (G.S. hits: 39).  The most studied Eciton army ant is E. burchellii (G.S. hits: 830); AToL sampled E. vagans (G.S. hits: 51).  Of the ten most studied ants only one, Linepithema humile, is included in AToL.

Absent field studies to fill in data for the AToL species, we have two ways to wed our knowledge of ant biology to the mismatched phylogeny.  First, interested researchers could drop the money to sequence relevant loci from taxa of interest and re-analyze the AToL data to produce a new, more comprehensive tree.  This option is the more correct one, but it will also be expensive.  (Do any of my independently wealthy readers wish to fund the AToL Patch Project? It’d be the best $100,000 you ever spent.)

Alternatively, and more cheaply, researchers could use the AToL tips as proxies for well-studied taxa.  For example, we could assume that the position and branch lengths for Solenopsis xyloni are reasonable phylogenetic surrogates for Solenopsis invicta and plug in the biological data for the better known species.  An easier option, but one that rests on a shakier set of assumptions.

As way of disclaimer, I don’t mean this post as an affront to AToL researchers.  After all, they are among my absolute favorite people.  And, they’ve done a simply fantastic job covering the global diversity of ants from a systematic perspective.   It’s just that, well, they could have anticipated what the larger ant community might wish to use their trees for.  We’re on the cusp of some powerful analyses on how and why ants evolved, and the lack of phylogenetic coverage of well-studied ants is a frustrating speed bump.


Sources:
Brady, S.G., Fisher, B.L., Schultz, T.R. & Ward, P.S. (2006) Evaluating alternative hypotheses for the early evolution and diversification of ants. PNAS, 103, 18172-18177.
Hölldobler, B., Wilson, E.O. 1990. THE ANTS. Harvard University Press, (Cambridge MA, London UK) pp 732.