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Jul 26

The magnolia misunderstanding

T. Ryan Gregory has a great post at Genomicron on the ‘Platypus Fallacy’. He imagines a platypus professor explaining the wonders of the Human Genome Project to a group of student platypodes:

“The lineage of which humans are a part is a very ancient offshoot of our mammalian family tree, so it was 166 million years ago that we last shared a common ancestor with humans, and that puts them somewhere between mammals and reptiles, because they lack a lot of specialized characters that we have gained but the ancestral amniote also lacked; for instance, they have no electroreception, no bills, no webbed feet, and no venom. So we can use them to trace the changes that have occurred as we went from being a reptile, to having fur to making milk to having our specialized features.”

The casual acceptance of this fallacy causes otherwise clever people to made the most asinine statements. I remember – as an undergraduate in the last millennium – being told by a now long retired entomologist that:

“Mosquitoes are lower Diptera [flies], so it’s surprising they are so successful, given how primitive they are”

I distinctly remember the dismissive lip-curl that accompanied the “lower”, as if mosquitoes had passed the port to the right at some point in their evolutionary history.

Describing groups as “basal” is merely the latest step on the euphemism treadmill that started with “lower”, and which has only recently started to view “primitive” with suspicion. All three words are (mis)used in exactly the same way: to describe a group of organisms that “branched off early” from the rest of a group, with the implication that such “basal” organisms are primitive, relictual, lower forms of life that deserve pity rather than study.

“With suspicion” is exactly how these words ought to be viewed. When they are applied to groups of organisms rather than to character states of those organisms, these words are wholly inappropriate. In the botanical world, it is the magnolias and their relatives which have long been on the receiving end of the “primitive” slur. Magnolias retain a number of features thought to be possessed by the common ancestor of all flowering plants, such as having pollen grains with one hole rather than three, and having spirally arranged petals in their flowers:

Angiosperm phylogeny stressing nonmagnoliids [cc-by-sa-3.0 Steve Cook]: magnolia appears as an outgroup to the ((thistle,rose),(onion,maize)) clade

Magnolia is basal to thistles and roses and onions and maize, oh my…

The magnolias do indeed appear to branch off early from the “main trunk” of the flowering plants, but this entirely depends on what you define as the main trunk. Here is another phylogeny based on the same data, but including a different but no less arbitrary subset of the flowering plants:

Angiosperm phylogeny stressing magnoliids [cc-by-sa-3.0 Steve Cook]: thistle appears as an outgroup to the ((magnolia,bay),(pepper,birthwort)) clade

But the thistle is just as “basal” to the magnolia (and bay trees and black pepper and birthworts) as the magnolia is to the thistle (and rose, etc).

With a little change of perspective, the allegedly “advanced, derived”  thistles appear as the basal group, branching off before the splendid diversification of the magnolia and its close relatives. The thistles presumably suffer from various evolutionary deficiencies such as having non-ephemeral antipodal cells. This is no less ridiculous a thing to choose as an overall marker of the “primitiveness” of an entire group than is the number of holes in a plant’s pollen grains.

There’s simply no objective way to define a “main trunk” in a phylogeny because there isn’t any such thing. There are branches, and every branch-point is a rotatable, symmetrical fork. Some of these branches, like those leading to the platypus (and echidna), or to the lungfishes, or to the magnolias, may not lead to a great plethora of still-living organisms compared to their sister groups (the other mammals, the land vertebrates, and most of the other flowering plants respectively), but this is nothing to do with their being “primitive” or “basal” as a whole.

There are perfectly good alternative ways to describe the relationship between magnolias and other plants:

  • The magnolia is an out-group with respect to (most of) the rest of the flowering plants. But, absoutely equivalently, any random member of the rest of the flowering plants is an out-group with respect to the magnolias and their relatives.
  • Magnolias and their relatives are the sister-group of (most of) the rest of the flowering plants, which stresses the symmetrical nature of the relationship.
  • The pollen of the magnolias retains the primitive character state of having a single pore through which the pollen tube germinates. But, this does not mean that magnolias as a whole are primitive: like the platypus and the cycads, the magnolias and their relatives have experienced genetic drift, natural selection, diversification and extinction like all other living, evolving things.

Isn’t it about time we started using these terms rather than value-laden arbitrary terms like “basal”? Isn’t it about time I got around to fixing the appalling article on Wikipedia rather than writing a rant to an audience of zero here? Ah…

5 comments

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  1. Robert Swift

    My scientist son aimed me at your blog. It is fascinating: intelligent criticism of received wisdom. I particularly like the concept of a platypus’s view of humanity; and I shall paste a copy of your notes on the “primitive” nature of magnolias into one of my botany books.

    You do not have an audience of zero!

    1. polypompholyx

      Cheers! I can’t take credit for the platypus’s viewpoint (that belongs to T. Ryan Gregory), but it’s long bugged me that people who should know better are still trying to shoe-horn a half-baked Great Chain of Being onto the Tree (or Mucky, Webby Bush if you prefer) of Life. It’s like trying to appreciate the three-dimensional structure of a flower but insisting on pressing the thing flat before you look at it.

  2. Ileasn

    Not zero audience!
    Also, I do not quite get the platypus thing either. The features listed by platypus professor are autapomorphies which are not informative in constructing a phylogeny. The platypus can be put anywhere on the phylogeny if based only on these characters. I think, from what I have learnt, platypus professor should list some characters that are shared between platypus and many other mammals (synapomorphies) to prove that platypus branched off late and for the same reason, list some synapomorphies that human do not process but platypus shares with many other mammals. Otherwise, the phylogeny suggested by platypus professor is not parsimonious because platypus would have lost many synapomorphies shared by many other mammals and many convergence would have happened during human evolution. Though using words like ‘primitive’ is misleading, the fact that platypus branched off early from mammal tree is undeniable. The statement thus in my view is novel and inspiring but less rational. Or there is something that I have misunderstood?

    1. polypompholyx

      This is a fair point; however, it’s not really about what the features are (autapomorphies, synapomorphies or plesiomorphies); it’s about how they are typically viewed by humans (or Prof. Platypus). Whatever the current interpretation of the evolutionary history of a feature, its evolution will still be viewed through a lens of “I am the bestest”. Autapomorphies will be viewed as defining of our specialness; synapomorphies with other closely related organisms will be viewed as important stepping stones on the way to our specialness; and plesiomorphies that have been retained but radically changed in other groups will either be ignored, or the apomorphies of other groups will be dismissed as charming but flawed alternatives to our sensibly conservative choice to stick with the traditional solution.

      Humans big-up their autapomorphic bipedality. They see their synapomorphic big ape brains as an important feature perfected in humans, with the other apes’ middle-weight brains as also-rans. And they completely ignore many of the plesiomorphies they share with other mammals, tetrapods – even eukaryotes in general – that have been radically altered in other clades. Humans still piss like losers (urea rather than uric acid, an innovation mostly of the ‘reptiles’), they still breathe like losers (with a diaphragm rather than the air-sacs of a bird), they still have to schlepp everywhere like losers (walking rather than flying like a bee), and they can’t even make their own food (unlike all the marvellous green things). But when have you ever heard a human admit their inability to photosynthesise is a major evolutionary disadvantage?

      So, for the platypus, it’s possible that the venom glands are a synapomorphy of all mammals, later lost in the therians (but not the monotremes); however, if this were the case, then no doubt Prof. Platypus would then either ignore the venom glands and focus her energies on why webbed feet are so damned great instead; or she would insist that loss of venom glands was a stupid move on the part of the therians. After all, who doesn’t want venom glands?

    2. polypompholyx

      Also…

      the fact that platypus branched off early from mammal tree is undeniable

      I deny it! This implicitly assumes there is a ‘main trunk’ off from which the platypus could branch. What defines this ‘main trunk’? There’s no very sensible way of defining a ‘main trunk’ in a phylogeny: every node is attached to a symmetrical bifurcating fork. At best, you could try to define the ‘main trunk’ by the number of species ultimately dangling off some given node, but – within the animals – this means vertebrates dropped the ball half a billion years ago when they failed to evolve lots of jointed appendages, and therefore branched off early from the main trunk of life, which – of course – leads to the insects.

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