Thursday, January 30, 2014

No arguments from authority please, even if it is Charles Darwin

Continuing with my ruminations on Richard Zander's Framework. Another thing that got me when reading the book was how often proponents of paraphyletic taxa claim that Charles Darwin was on their side.

In contrast to Zander, who very openly cites the Origin's chapter on classification for support, many of them do so through code phrases; they will write "evolution is descent with modification" and then assume the matter to be decided in favour of paraphyly. Personally, I don't quite get how that definition is supposed to be an argument against cladism anyway. But they think it is, and they think it will impress people because it is supposedly Darwin's own definition of evolution, and he is the authority, amirite?

Well, for starters there are legions of people who will vehemently disagree, maybe even call you a naif, if you come to them with that definition. No, they will say, evolution is the change of allele frequencies in a population, did you not pay attention in Biology 101? Example 1, example 2.

Why would they do so? Well, presumably they would, if challenged, go on to say that we have learned quite a bit more since the days of Darwin, genetics, modern synthesis and all that. We are not 'Darwinists' any more, regardless of how often creationists call us that, our understanding of evolution is now quite a bit different from his.

And regardless of the preferred definition of evolution, this is also my first and more important point with regard to what Darwin would have thought about cladism. Willi Hennig came up with his conceptual and methodological suggestions decades after Darwin had died so obviously Darwin never had a chance to consider the arguments.

Citing Darwin to argue against phylogenetic systematics is like citing Isaac Newton to argue against the general theory of relativity, or citing Linnaeus to argue against the theory of evolution. Yes, those were bright people who generated impressive science, but science has marched on since their time. It is an interesting though purely hypothetical question whether these people would have accepted the ideas in question given the run-down and presented with currently available evidence but in reality they never had the chance.

So I hope for what follows it is abundantly clear: Nobody should care if Darwin was in favour of what we would now call paraphyletic taxa unless the arguments he supposedly presented for such a position are strong enough, even after his venerable name is removed, to stand in the face of our conceptual and methodological progress since his time. Conversely, and please keep that in mind, nobody should care if Darwin was in favour of what we would now call phylogenetic systematics either unless, again, his arguments are good enough on their own.

And now for the fun part. What did Darwin actually write in the Origin's chapter on classification?

Well, he spent a long time developing his argumentation (as he generally did) by pointing out that we humans have always classified organisms by descent instead of similarity. Insect larvae and imagoes can look so different as to appear to belong to completely different phyla of the animal kingdom and they can have extremely different ecological adaptations, but we still consider them to be members of the same species because one brings forth the other. The males and females of some animals (and plants) can look so different that we would, if we did not know any better, not even expect them to be of the same genus, but we still consider them to be one species because they can be each others children.

After discussing these precedents for several pages, Darwin then concluded:
In considering this view of classification, it should be borne in mind that the element of descent has been universally used in ranking together the sexes, ages, dimorphic forms, and acknowledged varieties of the same species, however much they may differ from each other in structure. If we extend the use of this element of descent, - the one certainly known cause of similarity in organic beings, - we shall understand what is meant by the Natural System: it is genealogical in its attempted arrangement, with the grades of acquired difference marked by the terms, varieties, species, genera, families, orders, and classes. --- page 403 of the 6th edition of On the Origin of Species.
So there you have it: he argued we should classify by genealogy, by descent, in all cases.

In case the connection isn't immediately clear, proponents of paraphyly like to complain that cladists classify only by descent while supposedly ignoring 'evolutionary divergence' (i.e. superficial similarity).

Not that it matters (see above), but I really do not understand how one can read Darwin and come to the conclusion that he would, provided with contemporary terminology, have supported anything but a phylogenetic classification. Perhaps some selective perception is involved, either on my side or on theirs.


  1. Yes, but Darwin said we should use grade levels in creating higher level groups. This is the so called, 'evolutionary taxonomy' prevalent until the rise of cladistics in the late 1960s. This classification was indeed paraphyletic, at least in classification of fishes. In a way, the grade level taxonomy is easier to teach because the grade units are more compact in characteristics, compared to clade units. Took me a while to convert.

  2. That wasn't what I got out of reading the chapter in question, but I am happy to be corrected.

    Still doesn't mean that Darwin gets to decide about classification in the early 21st century of course...

  3. Someone should do a modern cladistic analysis of Darwin's barnacle taxonomy, to see what he actually did. A matter of historical interest only, of course.