That got your attention, didn’t it?
I know this sort of statement is up there with heresy and treason but let me try to explain my position. The British televisual institution who is Sir David Attenborough has presented a lot of shows in his time, his most recent offering being the ambitious (both in title and in scope) ‘David Attenborough’s Rise of the Animals: Triumph of the Vertebrates’, of which the second episode is available to UK readers here.
This is the episode that I have the most problems with, although it seems like the entire series is going to consist of shows which try to cover too much material in too short a time, and therefore not going into enough depth for my liking. Hopefully future episodes will prove me wrong, or maybe that’s just me as a scientist wanting to see deeper understanding being communicated rather than only covering the most superficial of details.
My biggest bugbear of the second episode concerns the short amount of time that he dedicates to explaining a series of diagrams which are flashed up on-screen. In this time, we are introduced to the terms ‘monophyletic’, ‘polyphyletic’ and ‘paraphyletic’ all in reference to the term ‘clade’. I am perfectly happy with his explanation of the term clade, which is a grouping of animals which includes a common ancestor and all of the descendents it gave rise to, whether they are extant or extinct. He also explains what each of the terms means while these diagrams are on-screen.
I would have added an additional 60 seconds or so of information explaining why these clades are shown as they are and hence how cladistics can be sometimes be misleading. Indulge me, if you will, to fill this gap left by Attenborough when discussing this particular diagram:
Cladistics is a largely subjective process which relies on a researcher choosing reliable traits with which to identify related species.
The monophyletic clade (yellow) in the above diagram is a true clade. It includes the last common ancestor of all ‘reptiles’ including the birds, which evolved from theropod dinosaurs, similar to Tyrannosaurus rex, as we are indeed told by Sir David Attenborough himself in both episodes of the series.
The paraphyletic clade (blue, most commonly referred to as the reptiles) is so called because it does not includes the last common ancestor, but not all of its descendants; as we can see is does not include Aves (birds). This may be because the creator of this clade diagram based their cladistic analysis on the ability to fly: no other reptiles can fly so, by this logic, birds aren’t reptiles.
A polyphyletic clade (red) is defined as one which does not include the last common ancestor of the organisms contained within it, and can be created as a result of two separate lineages coming up with the same solution to a common problem. In fig. 1, the polyphyletic clade includes mammals and birds who separately developed warm-bloodedness; they did not inherit it from a common ancestor. This same debate showed up in my last post, Chimps and their doctrines, when I mentioned the issue of knuckle walking in chimpanzees and gorillas. Similar examples of convergent evolution are flight in birds and bats, and a fully aquatic lifestyle in fish and whales.
Unfortunately, when dealing with extinct animals all we have are fossils so it is (near–)impossible to carry out genetic analysis. This means we are forced to choose anatomical traits from the fossils with which to recreate their relationships and this process produces a cladogram. Animals are assigned to clades based upon features which they appear to share and are assumed to have been inherited from a common ancestor. Choosing reliable markers, however, can be especially tricky in fossils which may have undergone all sorts of deformation since they were originally formed.
So, cladistics is not the best thing to rely on to represent true evolutionary relationships.
That being said, they are useful once we’ve confidently monophyletic clades, since they can be used to describe grouping that fall in between the constrictive taxa that are traditionally used. (Also see my classification of Apes in A Primate Primer).
And Another Thing.
As a more general rule, the way he refers to the animals ‘putting evolution into reverse’ is irresponsibly misleading. Similar turns of phrase which imply that the organisms have control over the fate of their descendants are littered throughout his narration. He even uses the same tone when talking of plants developing sweet, fleshy fruit. What makes me most angry is that this is Sir David Attenborough. He has had a huge influence on my life, and on many of my friends’ lives, and I would not be surprised if many people are inclined to take his word as gospel, if you’ll forgive the theistic turn of phrase. My opinion is that such a large personality as Attenborough has a duty of honour to strive for accuracy in an increasingly science-hungry world.
Or maybe I’m underestimating the British TV audience, and they do have the capacity to realise that Attenborough’s turns of phrase are just that: metaphors, and that individual animals do not suddenly decide to wind back millions of years of evolution of just their eyes until they reach the stage where they had colour vision. And then decide to stop there and keep it.
Because that’s not what happened.
Rant over. Please tell me if I’ve been unfair in my opinions of either Attenborough or the British viewing public.