Monomorium kiliani, an Australian myrmicine. The narrow, two-segmented waist is characteristic of this subfamily.
We’re only halfway through the year, but already 2014 will be remembered as pivotal for studies of ant evolution and classification. Following right on the heels of Schmidt & Shattuck’s massive ponerine revision comes an important new study from the Ant Tree of Life group. Ward, Brady, Fisher, and Schultz (2014) have reconstructed the first thorough genus-level phylogeny of the great ant subfamily Myrmicinae.
How important is this study?
Roughly half of all ants are myrmicines, both in abundance and in species diversity. Their numbers include fire ants, harvester ants, leafcutter ants, big-headed ants, acrobat ants, and so on, to the tune of some 6,000+ species.
So… Boom! Suddenly, we’ve been given a detailed picture of the evolution of half the ants. This is big. It is so big I cannot cover the paper in detail. Instead, I’ll just give a few preliminary thoughts, as follows:
1. This is a well executed study, as we’ve come to expect from the Ant Tree of Life team, applying a thorough analysis to over 250 carefully selected taxa and 11 genes. It’s also a shining example of an older generation of genetic techniques, alas, and while I am confident the stronger results will mostly endure, be aware that an incoming next-gen tide of full genomes, and the 6,000 yet-unsampled myrmicine species, may yet overturn some of the findings.
2. The deep history of Myrmicinae, starting 100 million or so years ago, mostly occurred on those continents that drifted to become the Americas. Echos of these earliest divisions are heard in six clear, genetically distinct groups that Ward et al have formally set the up as a new system of tribes, replacing an earlier, messier scheme. The six groups are listed here in their order of divergence: Myrmicini (Myrmica & Manica), Pogonomyrmecini (Hylomyrma & Pogonomyrmex), Stenammini (Aphaenogaster, Messor, Stenamma, and relatives), and three sprawling groups with thousands of species: Solenopsidini, Attini, and Crematogastrini.
The myrmicine big picture. (Sharpie on office paper, 2014, limited edition print available, unless I recycle it).
3. The news is not all good. The clarity deep in Ward et al‘s tree fades for slightly younger events. Early relationships within some of the the six tribes are discouragingly ambiguous. This study has resolved some problems, myrmicine taxonomists face a difficult road ahead. Many of the world’s greatest genera do not form natural groups and will have to be redone. These include Aphaenogaster, Pheidole, Tetramorium, and especially Monomorium, which splatters almost comically across the Solenopsidines.
What, really, is Monomorium? Modified from Figure 1 of Ward et al (2014).
Distressingly, fuzzy resolution in a data set with this many markers and taxa means achieving proper resolution, if at all, will likely be expensive. Myrmicines may have speciated so explosively that we may never be able to reconstruct what happened with confidence.
4. The authors correct a few of the more obvious instances of paraphyly. Notably, the New World “Messor“, being unrelated to their old world doppelgangers, were moved to a revived Veromessor, and several social parasites like Protomognathus and Anergates have been sunk into the host genera from whence they evolved: Temnothorax and Tetramorium, respectively. There are other changes, too; they are listed in the abstract.
Most of the identified problems- such as what to do with Monomorium and Aphaenogaster- were left for targeted future research.
5. Remember the dispute over Pyramica vs. Strumigenys? The argument was fundamentally over how ant mandibles evolve. Apparently, high energy trap-jaws arise easier than anyone imagined. According to Ward et al, not only is the assemblage of trap-jaw ants formerly included in dacetini a polyphyletic splatter, even within the genus Strumigenys the trap jaw has arisen at least twice.
A phylogram of Strumigenys, modified from Figure 1 in Ward et al 2014, showing strong support for the parallel evolution of trap-jaws in the genus.
6. The rare and bizarre African myrmicine genus Ankylomyrma is not a myrmicine at all! Rather, Ward et al‘s results unambiguously tie it to the equally bizarre Tatuidris of the Neotropics, sitting on a distant branch of the ant tree. Peas in a poneromorph pod…
Ultimately, Ward et al have crafted a sobering view of how little we still know about ant evolution, and how much remains to be done.
Aphaenogaster fulva, photographed in Illinois.
source: Ward PS, Brady SG, Fisher BL, Schultz TR (2014) The evolution of myrmicine ants: phylogeny and biogeography of a hyperdiverse ant clade (Hymenoptera: Formicidae). Systematic Entomology, online early. DOI: 10.1111/syen.12090
disclosure: I received my Ph.D. from Phil Ward’s lab where much of this study was completed, and I contributed a few of the samples, but I was long gone by the time the study was initiated and have had no other involvement with the research.