The Great Chain of Phylogenetic Wrongness

Phylogeny Friday -- 18 July 2008

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When they published the initial analysis of the complete platypus genome (doi:10.1038/nature06936), Nature, as they're wont to do, also put out a news item announcing the major findings (doi:10.1038/453138a). That news article included a phylogeny illustrating the evolutionary relationships of various animal species in various stages of having their complete genomes sequenced.

The problem with the illustration: they got some of the relationships wrong. This sparked a letter from Peter Ducey of SUNY Cortland (doi:10.1038/454027d), in which he wrote the following:

In your News story 'Top billing for platypus at end of evolution tree', the graphic depicting genome status presents a shocking new phylogeny of the Vertebrates -- with Archosaurs (birds and crocodilians) and Mammals forming a monophyletic group.

In all honesty, the tree is so craptacularly drawn that it's hard to say that Archosaurs and Mammals are monophyletic based on the illustration. It looks more like the branching order of most vertebrates (and most animals) is unresolved, but I can definitely understand how Ducey interprets the tree. Either way, Nature did a bad job with this illustration. Why did they suck so bad? Here's what Ducey thinks:

The bad news is that this dramatic new 'proposal' is completely adrift from the research Article by Wesley C. Warren and colleagues that the figurative tree is intended to illustrate, and it continues a persistent tendency in popular literature to portray all evolution as leading towards humans.

Totally! The idea of a great chain of being, starting with bacteria and progressing through animals to humans, permeates much of the popular discussion of evolution. And it's just not true. It's a phylogenetic fallacy.

Coincidentally, Ducey points out, Science repeated the same error Nature made. In their coverage of the platypus genome paper (doi:10.1126/science.320.5877.730), Science included this image:

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Ouch! At least the tree accompanying the Nature article was ambiguous enough that it could be interpreted to represent the true phylogeny. While the cladogram from Science does not perpetuate the great chain of being myth, it does represent a completely resolved phylogeny that also happens to be incorrect.

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Hey, Pete Ducey was my favorite professor at Cortland. He is a first rate teacher and researcher, it does not surprise me that he was the one to spot this error, he and I had some interesting discussions that really set me straight about "directionality" in evolution.

"The idea of a great chain of being, starting with bacteria and progressing through animals to humans, permeates much of the popular discussion of evolution. And it's just not true. It's a phylogenetic fallacy."

Actually it is true for any one extant animal species. There has to be a direct chain of descent leading from the earliest multi-celluar ancestor thru each modern species. It's just that you cannot scale this to the entire picture. I think that's where they get confused. Or maybe I'm just confused!

I'd argue that the Science graphic *does* perpetuate the great chain.

For example, look who's on top.

Perhaps it's my own ignorance and maybe it's customary to do this. But it seems to me that any order of stacking of that tree would be equally valid. As it is, it forms a staircase shape from bottom left to top right. Who's at the top right again?

I'm always amused when mere men attempt to explain the genetic code, delving into matters in which they are ill prepared to understand. Francis Collins pronounced quite profoundly that the DNA code was "the language of God." Mere men...

C. David Parsons, Author, The Quest for Right.

There has to be a direct chain of descent leading from the earliest multi-celluar ancestor thru each modern species.

I think you intended to write 'There has to be a direct chain of descent leading from the earliest multi-celluar ancestor to each modern species.' As it stands, it says there is a direct chain though amoeba, brine shimp, platypus, carnation, fig tree, starling, etc (presumably finishing with humans).

By Richard Simons (not verified) on 18 Jul 2008 #permalink

Perhaps the best way to look at evolution and descent is to consider the relative rate of change of a particular species to any other one. For example, how much has the cockroach changed in hundreds of millions of years compared to humans? Obviously humans have only been on the scene for roughly a million years, give or take several hundred thousand years. Both the cockroach and the human are "modern" species which have drifted genetically from their ancestors but how much genetic change had to occur to get to one versus the other?