I’ve been surprised to learn that my dissertation is a “living document,” meaning that people are actually reading and citing the damn thing as we speak. Most dissertations either get swiftly hot-chopped into MPUs and read/utilized that way, or were written as MPUs in the first place, with the dissertation step hastily shoving them together in pretense of being a meaningful document, and never expected actually to be read as such. (For non-academics, that’s minimal publishable unit, simultaneously both standard procedure and a derogatory term.)
I’m not expecting anyone actually to read it through via this blog, but these Count Bacula posts are intended to bring out some of the neat stuff in there for discussion. Here, I want to talk about the technique of phylogenetic character mapping, which is incredibly simple, but as it happened was part of a big intellectual crisis for evolutionary biology at the time, twenty years ago.
In fact, my work was perfectly timed to be a nexus for several such crises of the moment: what homology actually is, whether selection bakes the cake or frosts it, and how phylogeny should be employed when comparing some feature across different species. The latter becomes even more fun (term used definitely in retrospect) when the creatures in question are currently undergoing intense revision of their phylogeny.
In the case of the megabats, someone reading this probably remembers the fuss about whether bats were “the” bats, or two kinds of bat entirely, mega vs. micro. It’s generally resolved now in favor of “one bat, big and little subsets,” but from the mid-1980s through the late 1990s it was worth a limb to piss off someone committed to one or the other side, and I walked carefully in examining the genital anatomy of Dermoptera, a one-species Order of mammals that’s mixed up in the bats-bats-primates. Fortunately that’s not my topic today.
Today, it’s about the then-called Macroglossinae, a bunch of nectarivorous microbats with scary slurpy long tongues. I mentioned them in Same thing(s), and if there’d been one single solid phylogeny to use then, I would have. As it was, I had to do the thing I wanted three times over … OK, so what was this thing?
Nothing more nor less than marking the character states of the feaure(s) you’re interested in onto the phylogeny, for those creatures that have it. Character states are the version of the feature(s). In my case, it was penis anatomy and cervix anatomy. I wanted to see whether certain changes in one consistently preceded, or perhaps were simultaneous with, certain changes in the other. The point should be apparent: do genitals evolve into different shapes in one sex due to changes in the genitals of the other? (The cervix, incidentally, is a much more anatomically interactive site in many mammals, compared to humans. Given shape features, that was at least plausible to consider in these species, if not rigorously known.)
This is a pic from Craig Hood’s 1986 paper Comparative morphology and evolution of the female reproductive tract in macroglossine bats (Mammalia, Chiroptera):
This isn’t character mapping. In this case, the changes in uterine features (numbers) are used to help make the phylogeny. The point is that, of the bats considered to be macroglossines, one of them, Megaloglossus, didn’t “go” with the others in terms of the uterus and cervix. Since its uterus isn’t derived but rather ancestral (see how Megaloglossus has no numbers), he said, this guy isn’t a macroglossine at all.
My goal was a little different. I took this phylogeny as given (just as I did with the other two that were available at the time), and I mapped the penis and baculum features for the various species onto it. That meant I had to go and prepare the various specimens myself, describing some of them for the first time, especially since I was interested in soft tissue features as well as the bone. Here they are for Megaloglossus – pics on the left are the original organ and the ones on the right are cleared (rendered transparent) and the bone stained for easy viewing:
I ended up with ten features of the soft and bony tissues, and the mapping was easy – I just drew’em onto the phylogeny, or rather, symbols to represent the evolutionary changes.
Check it out. By themselves the penis features are utter crap to compose a phylogeny for these animals (that very boring analysis is in the dissertation), but they tell quite a different saga via the map. The “basic” or plesiomorphic penis for them is apparently built with a ventral crater (on the underside), with or without spines over the surface (did I mention that many mammals’ penis is extremely spiny?). That changes to a distal crater (on the end), and from there, you either lose the crater entirely or it changes to a dorsal crater (on the top). This is a pretty linear and interesting sequence of events along which the species represent various points.
It also tracks the uterine changes to some extent, if not 1:1, at least enough to make any functional anatomist really want to know what, if anything, those craters are doing with the various conformations of the cervix represented by the numbers. If you asked me whether penis and cervix shape were co-evolving based on this analysis, I’d be inclined to say yes. (In the dissertation, I say no. It strikes me now as rather timid.)
However, the baculum doesn’t “behave” as nicely. Here’s the map:
Briefly, baculum evolution (positing that this is the correct phylogeny) is a bit of a mess, with all sorts of things happening at different points in different species. Therefore one is left with a conundrum about whether and how the bone associated quite intimately with the erectile tissues, and presumably with their operation, evolves “out of tandem” with them. Fortunately that kind of conundrum is what really good hypotheses are made from, so despite the lack of a soundbyte answer, I’m calling it a win.
OK – all of that posited that Hood’s phylogeny is correct, and this analysis for example mainly supports it (it differs in splitting Nyctimene very far away from the others, which doesn’t affect my analysis), but that said, it also so happens that every phylogeny proposed for these animals, different as they may be, preserves the relative positions of the genital features. Which is pretty cool and suggests that this relative arrangement is indeed correct. Meaning: that my basic point about the continuity of penis soft-tissue and uterine tissue, and the discontinuity of penis soft-tissue and baculum, is rather strongly supported and could easily be the foundation for a functional anatomy / copulatory physiology study among them, exactly the kind of multi-level comparative approach I’ve always favored to address the questions about why (in the world) different creatures copulate so differently.
So that’s character mapping. The idea is that you build a phylogeny based on some features (actually quite a lot), and then you map some other feature, that you’re interested in, onto it to see how it changed in that context. Basically, what turned into what, and in whom. One can then take into organismal, physiological studies as I said above, as well as, if the information is available, to impose the whole thing onto geography to see how historical genital changes relate to instances of speciation.
Next: Count Bacula, Part 3: The case of the missing baculum