Sunday, April 12, 2009

Twisted Tree of Life Award #4: Hoxful Monsters Blog on "Primitive" Animals

Nothing gets me more riled up in evolutionary writings than the use of the term primitive to describe organisms on a "deep" branch in an evolutionary tree. Thus for the following tree
many people would refer to Species 1 as a primitive organism solely on the basis of this tree. In fact, I just saw this in the Hoxful Monsters blog (see here) where it says:
In April 2008, Cassey dunn et el published their famous phylogenomics work in Nature, which placed Ctenophores at the base of the tree making them most primitive animal on Earth. Before that work Sponges were considered as first animals to branch off from rest of the animals and hence occupied a most basal position in the tree of animals for long time. However, some months later another phylogenomics study carried out by Bernd Schierwater et al ; changed things dramatically by placing Trichoplax as most primitive animal replacing Ctenophores. Now a new phylogenomic study published in online section of Current Biology is making headlines , where Sponges regain their position as most primitive animal on Earth.
This is simply wrong as the term primitive, which I avoid at all costs because it is so frequently misused, should only be used to refer to features of organisms not to the branching pattern in an evolutionary tree. Forget for a minute that "primitive" implies that something else is "advanced", as if we could determine which is which. And lets pretend instead for a minute that "primitive" could be used in an equivalent manner as "ancestral." Ancestral is a term used to refer to features of organisms present, as the term implies, in an ancestor. Thus is the organism(s) at the node in the tree labelled with the X had some feature (e.g., lets say, they were green) and Species 4 is green while Species 1 is orange and Species 2 and 3 are purple. In this case, in terms of color, Species 4 has ancestral feature and the other species have derived features. And thus, when I am feeling generous, I will grant that one could sort of use the term primitive to say "Species 4 is primitive in terms of color." I don't like this usage, because primitive implies something about quality that ancestral and derived do not. But it is not so horribly wrong that I cringe.

But I would like to make two more points in terms of the way primitive is misused.
  • First, even if one species had a large number of ancestral features (and thus for those features resembled an ancestral organism), that species certainly will have some derived features as well. Thus ancestral, derived, and primitive (ick) should only be used to refer to features, not organisms.
  • Second, the particular position a species occupies in a tree does not tell use whether it has ancestral or derived features. Thus is my color example above, Species 1 does not have the ancestral phenotype -- it has a derived one. Thus calling organisms that branch deeply in a tree "primitive" is wrong not only because it is referring to an organism not a feature but also because deep branching does not imply ancestral features.
In other words, the fact that if you look only at species in the tree, that Species 1 is the deepest branching individual species, does not in any way tell whether Species 1 has primitive (i.e., ancestral) features. Features present in the common ancestor of the entire group (i.e., the organisms at node X) are the ancestral features. These features could change on any of the branches leading to Species 1,2,3, and 4. If they change, then the descendants will have derived features. If they stay the same they will have ancestral features. The branching order of the species does not explicitly tell us which one retains ancestral features.

This inaccurate use of phylogenetic trees to imply primitive features drives me batty. I was disappointed to see this phylogenetic gobbledygook being used in the HoxFul Monsters blog. And thus, Hoxful Monsters is the winner of my fourth "Twisted Tree of Life" award.

8 comments:

  1. I think that you should print up posters about this and put them up in Biology departments throughout the country. I hear people say things like this all of the time!!! YUCK!

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  2. I think that the use or primitive comes from the misapprehension that something further back in the evolutionary past is somehow not as evolved as those individuals at the end of branches, and thus primitive is another way of saying this. It would seem that some forget that changes arise at random and when the current state of the organism is less favorable than a new state that has arisen, we may have a forking of the branches. One could argue that the use of primative is always wrong as organisms are always hovering around the state of 'just good enough'. We should stick with more tangible terms like ancestral, contempory, extinct and extant.to avoid all the confussion.

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  3. Thanks for correcting me Mr. Eisen ....I am student of dev bio ......trying to understand a bit of evolution since last one year....i was thinking a organism will be considered primitive ,if it branches out first from rest of the animals...I know that present day sponges and trichoplax are highly derived ...i didnot know that we shld not use word primitive for organisms ......but use it for characters only......I will try to keep these in mind in future........just a question then....how do u mention sponges when compared to other animals(shall we just say they branched off first) based on phylogenetic tree??? ....by Harvey Phillipe...in CB latest issue.......
    Thanks
    Nagraj.

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  4. Nagra

    Thanks for responding. I confess, given how polished your site looks, I just figured you were more advanced than a student. Otherwise I might have just written to you and not posted on my blog. As for what to refer to species that are the deepest branching within a group, I usually say just that "They are the deepest branching in the group." Another term people use is "most basal" although I prefer deepest branching.

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  5. That fine Mr.Eisen ........posting on ur blog or writing to me is same thing.............thanks for correcting me and also for answering to my question

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  6. Why can't we likewise say that the non-poriferan animals are 'primitive', while the sponges are the 'higher' form? I mean, at any given branching in the tree, there's no 'higher' or 'lower' branch, is there?

    We're all basal to the choanoflagellates, in some way. We just have a bias towards bigger things with more frill. Perhaps choanos would view us as evolutionary degenerates?

    And even if you use 'advanced' to mean 'more derived from common ancestral form', wouldn't most people object to the idea of parasites being some of the most advanced creatures on earth? The Myxosporidians are pretty damn derived!

    Your post should be forwarded throughout various biology departments -- too often I hear (especially from zoologists) all the 'primitiveness' crap. Of course they won't listen to me for I'm just an undergrad with a microbial fetish...

    But they can keep thinking that yeast and plants comprise a sensible grouping. Mwahaha. I'll just laugh as their predictions fail to make any sense and spiral down endlessly into the abyss of polyphyletic chaos...

    -Psi-

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  7. Amen. Even 'basal' and 'crown group' irk me, given their arbitrary dependence on lineage-differential sampling density. But least those terms are, when properly confined to a given tree, topologically informative and free of teleological overtones. Don't get me started on the infuriating 'coelocanths are living fossils' trip...

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  8. Good point Nat about basal. I never use it myself. And the species sampling issue is also important.

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