The problem with “derived groups”

Here I am, again, fighting against the nefarious scala naturae. No surprises here because, indeed, this is the subject of most of my book. I know that this is an uphill struggle against something that will hardly disappear, once for all, from the way we view and organize the living world. However, we must try …

My issue now is the (wrong) concept of derived group. There are primitive features and derived features, plesiomorphic characteristics and apomorphic characteristics. But “primitive group” and “derived group”… that makes no sense! However, even the way a cladogram is constructed, i. e., its graphical form, can lead to wrong interpretations such as this one in particular.

(I’m not discussing the concept of basal group, which is a bit more complicated, and that I shall address in a future post)

I remember a good zoology teacher once I had warning us about something that usually students do not realize: in slanted cladograms, the viewer is driven to think that the main line that goes from bottom left corner to top right corner leads to the “normal” group, to the “more evolved” group, and that all other taxa are deviations from this “normal” destination, thereby forming “less evolved” groups. To make the misinterpretation even worse, we must add that most of Indo-European languages are written from left to right (except for the boustrophedon in ancient greek, I can’t remember any right-to-left indo-european writing system), increasing the importance of groups that are at the right side of slanted cladograms…

There are several good examples to illustrate this. I’ll work here with a simplified cladogram of the fungi (kingdom Fungi), about which we so commonly think of Zygomycota as the “least developed” group and of Basidiomycota as the “more evolved” group. Let’s see:

Cladogram #1

Cladogram #1

The Basidiomycota occupy the position of prominence that we mentioned earlier: the top right corner. The impression we have, because the node that separates the Zygomycota from the monophyletic group formed by {Ascomycota + Basidiomycota} is in a lower place in the cladogram (and thereby earlier in the time scale), is that the Zygomycota are more primitive than the other two. Moreover, only three characteristics are given: 1, present in all fungi, 2, exclusive of Ascomycota and Basidiomycota, and 3, exclusive of Basidiomycota.

For start, let’s make a graphic change that does not change the information delivered by the cladogram, because we can rotate the nodes freely:


Cladogram #2

Now, the place occupied by Basidiomycota has much less psychological impact. But there is still a problem: in our cladogram, only Basidiomycota have unique characteristics, i. e., features that other groups do not have… So we can say they are “more evolved”, right? Well, let’s fix this, for it is obvious that the Zygomycota have their unique characteristics as well, and so the Ascomycota:

Cladogram #3

Cladogram #3

Now we have two new features: 4, found only in Zygomycota, and 5, exclusive of Ascomycota. This point is crucial, because here we finally realize that, despite Basidiomycota possessing characteristics that do not occur in Zygomycota, Zygomycota also have characteristics that do not occur in basidiomycota… The only certainty (scientifically speaking) that we have is that they share a common ancestor, that’s all.

But there is still something missing … The group formed by {Ascomycota + Basidiomycota} occupies a larger space in the cladogram, that is, they include more elements, more terminals. Let us do the following: dividing Zygomycota group in two (for all monophyletic group can be divided into two monophyletic groups), called Zygomycota A and Zygomycota B. So, we’ve got:

Cladogram #4

Cladogram #4

The image is now much more balanced, and the impression that Basidiomycota are “more evolved” is thus much weaker.

But we still have a graphic problem … These triangular or triangular cladograms, that we call slanted cladograms, as already said, have a great line that leads to the upper right corner, and all other lines are smaller than that one. We should not underestimate the psychological importance of this. Thus, it is normal that we still see Basidiomycota as “the inexorable fate that the fungi had to take” (or Ascomycota in cladogram #4, or any other group that you put in upper right corner).

Let us then turn the cladogram into a rectangular one, more appropriate to our purposes:

Cladogram #5

Cladogram #5

You can see that the difference is huge: despite the fact that Ascomycota is on the right (but this can be easily changed, as we can rotate the nodes freely), the way the information is delivered does not make us spot or think of a more “derived” or a less “derived” group.

To further reduce the ill effects of slanted cladograms, rectangular cladograms can be made horizontally, a very common procedure when there are multiple branches, facilitating the typing and reading:

Cladogram #6

Cladogram #6

Basidiomicota and Zygomycota have now pretty much the same weight, the same importance. Remember: there is no “derived group” or “evolved group”, there is no “less derived group” or “less evolved group”; what do exists is derived character. There is not anything like an “apomorphic group” or a “plesiomorphic group”: apomorphies and plesiomorphies refer to characters, and not to entire taxa.



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