Amblyopone australis: a primitive ant?

Earlier I chastised Christian Peeters and Mathieu Molet for misinterpreting the term “basal” in a phylogenetic context.  What was that about?

The issue relates to the classic fallacy of viewing evolution as a linear progression from primitive to advanced. Popular conceptions of evolution aside, the process is not linear like a ladder so much as branching like a bush.

I don’t know what quirk of human psychology so strongly predisposes us to frame ideas in linear narratives, but the fact that we do so makes evolution an unfortunately difficult concept to grasp. We imagine a tidy parade of forms from simple to complex, even though evolution in the real world is entirely different. Real evolution runs in duplicating strands in many directions at once, some gathering complexity, other losing it.

Ant science has its own version of the monkey-to-man mythology, whereby brutish prehistoric hunting ants evolved into sophisticated superorganisms. This narrative is apparent in the Peeters & Molet study:

In higher ants, polyphenism can be extreme: high queen–worker size dimorphism and worker polymorphism allow for better success at colony founding by lone queens, better division of tasks, and ultimately access to new ecological niches. However in basal ants, polyphenism is much more limited, restricting them to narrow niches.

While it is undoubtedly true that the largest and most intricate ant societies emerged from less refined ones, it would be wrong to extrapolate that progression as a general rule into “higher” and “lower” ants. Many modern “higher” ants live in small colonies (Strumigenys, for instance) and evolved from relatively complex ancestors. And, as Peeters & Molet document in their Amblyopone data, some “lower” ants have morphological complexity in their worker caste. Ant evolution is not a straight line. Overarching patterns are obscured by numerous reversals and exceptions, so that reducing evolution to a story not only oversimplifies the process but actively misrepresents it.

Here is a common view of the phylogenetic position of Amblyopone australis, the subject of Peeters & Molet’s study:

They justify the placement of Amblyopone as a “lower” ant by pointing to the basal position in this tree and noting that it is close “to solitary vespoid wasps.”

But basal does not mean primitive. Nor does it imply evolutionary proximity to external groups like vespids.   To see why, let’s look at a different tree.  Here I have included a different set of species, retaining only A. australis and S. invicta.  This tree is just as accurate as the preceeding one- I’ve taken it from a study by Corie Moreau- it just shows the relationship of a different set of taxa:

Among these taxa, Solenopsis invicta is older and basal and Amblyopone australis is recent and apical. Exactly the reverse of the first tree. Is Solenopsis invicta closer to wasps than Amblyopone australis?

Well, no. These diagrams are only contradictory if we assume that modern species serve as stand-ins for ancestors.  Solenopsis invicta and Amblyopone australis cannot both be ancestral to other ants.  And indeed they are not.  Both are equally distant from their wasp progenitors, as both are modern species with the same amount of elapsed time since the Ur-ant ancestor.   Here is a combined tree:

It should be clear that a basal phylogenetic position has an awful lot to do with the taxon sample, and not a whole lot to do with the amount of evolution.

Many researchers have taken to using the term “basal” instead of “primitive” as more phylogenetically acceptable. But the word never was the problem. The word was a symptom. The problem was the underlying misconception of evolution as a ladder of progress, and just swapping the terminology won’t cut it.   Many biologists still can’t think clearly about phylogeny.

“Primitive”, “advanced”, “lower” and “higher” are dangerous words. They come laden with baggage, imbuing values on species that bear little resemblance to the species’ actual biology. Worse, these words make us think we know things that, in fact, we don’t.  They allow us to substitute prejudice for knowledge.  If we assume that Amblypone australis is primitive, then we are surprised when it turns out to have some derived features.  We shouldn’t be. It’s standing on just as much evolutionary time as the other ants in the tree.

Finally, here’s our own species added to the mix:

Draw your own conclusion.