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The great chain of being

In his talk on mental models of the structure of the world, Josh Tenenbaum talked about how people think of animals as being classified in a tree structure, and how this structure might differ from those implied by different scientific models. This kind of thing:

tree.png

Anyway, as an aside, Tenenbaum pointed out that, although the tree structure seems so natural to us, it doesn’t have to be this way. He noted that, traditionally, creatures have been organized into a linear “great chain of being” rather than as a tree structure. Then I realized . . . that’s how I think of the animal kingdom. It’s how we learned things in 9th grade biology. At the bottom are single-celled animals (amoebas and so forth), then gradually through the invertebrates, then the vertebrates, starting with the fish (with the sharks at the bottom because of their primitive structure), then amphibians, then reptiles (amphs are lower than reps because of being more fish-like and primitive, I think), then birds (higher because they’re warm-blooded), then animals, with primates at the top and, well, you know what’s the #1 primate . . .

Anyway, only when sitting in Tenenbaum’s talk did I realize that I’d swallowed this whole great-chain-of-being formulation without even thinking about it. The assumption is that every invertebrate is lower than every vertebrate, that the most complicated bird is lower than any mammal, that all plants are lower than all animals, etc. It’s still hard for me to shake this mode of thinking.

I guess it’s a good thing I’m not a biologist. (I did publish in the Journal of Theoretical Biology once, but we all know that knowledge of biology is not necessary to publish in that journal.)

2 Comments

  1. People often misinterpret evolutionary trees.

    T. Ryan Gregory, the author of the Genomicron blog, has written an excellent review, “Understanding Evolutionary Trees”. This paper, which appears in the new journal <a href="http://www.springer.com/life+sci/journal/12052&quot; rel="nofollow">Evolution: Education and Outreach, is available free online through 2008.

    Dr. Gregory writes:

    The arrangement of the tips is unimportant…, so long as the branching patterns are maintained. In this
    respect, phylogenies are like a baby’s mobile: every internal
    node can be rotated without any implications for the pattern
    in which the branches are connected.

    Dr. Gregory provides many good examples in his paper of how to avoid misinterpreting evolutionary trees.

  2. derek says:

    It's not necessarily a "tree" structure, as exactly the same mental model can be visualised as a "boxes within boxes" structure. Darwin's famous tree sketch is one way, but I feel sure that Linnaeus, who invented the biological naming system we still use today, used drawers as his mental picture.

    The graphical display technique called "treemap" also shows this ambiguity of possible views.