Thursday, April 21, 2016

The joys of single-character taxonomy

Time for a little rant. Two days ago I tried to identify an Australian native Asteraceae. I already knew that it had to belong to one of two genera, and had always wondered why those two genera were recognised as distinct in the first place. If you put a randomly chosen species from the first next to one from the second you will be hard pressed to see any difference beyond hair cover or suchlike.

I assumed there would be some fruit character, for example feathery versus smooth pappus bristles. That would be bad enough because it would probably still mean that one genus is phylogenetically nested within the other, as is usually the case when there is only a difference in one trait. This is because then one genus is defined by having the trait and the other merely by lacking it; in systematics, we call that an 'apomorphic segregate'. But okay, such a fruit character, even if evolutionarily irrelevant and phylogenetically uninformative, is at least user-friendly. You can look at the pappus (or beak, or whatever) and quickly conclude: ah yes, it must be this genus.

What was the difference in the present case? "Florets homogamous" or "florets heterogamous". Before we consider the trait itself, hands up everybody who knows what that means! Yes, that's what I thought. The identification key in question was apparently written for an end user group of about half a dozen fellow taxonomists in Australia or so, but certainly not for conservation managers, community ecologists, or plant-enthusiastic non-scientists.

Now the trait itself. It means whether the flowers in the daisy flower-head are all of the same type or if there are different types present; and here we are not talking about the presence or absence of petal-like ray florets or anything easy to see like that. We are talking about one of the two genera sometimes having a few female flowers at the edge of the flower-head in addition to the normal, bisexual flowers. In other words, get out the anatomy grade tweezers and a dissecting microscope!

And as expected we are dealing with a single character difference. It is extremely unlikely that the two genera are reciprocally monophyletic, so they probably don't make sense in modern systematics. But even from a so-called 'evolutionary' taxonomy perspective this is weird. Again, you place species from the two genera next to each other, you will not see any significant difference; and surely having or not having a few female flowers in the head is not going to put a species into a different 'adaptive zone' or something. So what is the idea?

What is weirder is that this criterion is not even applied consistently. Another closely related genus has got several homogamous and one heterogamous species.

Of course this is not the first time I have seen a situation like that. The genus I did my honours on was Suessenguthia (Acanthaceae), a group of (now) six species with four fertile stamens and little hooks on the anthers. Some of its species are pretty similar to those of the larger genus Sanchezia except that the latter has only two fertile stamens. In addition, there is a monotypic genus Trichosanchezia that looks exactly like certain hairy, northern Peruvian Sanchezias but has four fertile stamens without the little hooks. Even better, there was once a likewise monotypic genus called Steirosanchezia characterised by two fertile stamens without hooks; that one, however, has already been put out of its misery and sunk back into Sanchezia.

So once there were four genera based merely on minutiae of the androecium, for species that are so similar that they constantly get misidentified to each other's genera, and obviously all forming one tight natural group. How is that helpful? How did that ever make sense even before Phylogenetic Systematics, even before the Theory of Evolution?

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