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…

Leave a Reply

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.