Notebooks

Biological Order, or Levels of Organization

11 Oct 1998 11:00

The whole notion that living things are organized in hierarchical levels (monomers inside macromolecules inside membranes inside organelles inside cells inside organs inside organisms) seems to have been thrust upon a willing world in the '20s and '30s by people like Joseph Needham (his Order and Life is a really classic exposition) and Waddington and Woodger (qv. below). This is now a thoroughly received notion, the sort of thing diagramed in high school biology texts, usually with a pair of arrows, one pointing up to show new properties emerging, and another pointing down to show components constrained by the level above ("downward causation"); "and another going nowhere at all just for show."

Probably it's pretty much correct, but I can't help thinking it's much too pat. Where, for instance, do bones fit in this? Or the endocrine system? Furthermore downward causation seems suspect to me. I'd be much more comfortable with lateral causation: the component molecules of (say) a membrance are constrained, yes, but by each other.

Suppose that instead of looking at spatial encapsulation, at what is inside what, we looked at causal isolation, so that item A in the hierarchy includes lower-level items B, C, D if B, C and D interact more strongly with each other than with the outside world, and this is no longer true is we remove any one of the three, or add a fourth. I think this comes closer to "dividing according to the natural formation, where the joint is, not breaking any part as a bad carver might" (Plato); in fact it seems to me that the spatial hierarchy always makes some kind of implicit appeal to autonomy to rule out arbitrary boundaries, which would say include half the liver, and a little bit of the left lung as well. (But the new scheme allows for such oddly shaped items, if their parts are strongly connected.) Furthermore, the causal version can be extended to machines, or societies of organisms, with less hand-waving and grimacing. --- This idea comes from thinking about Christopher Alexander's Notes on the Synthesis of Form, where drives home the point that, if you want to break a problem up into solvable pieces, spatial proximity is much less useful than causal links. (I'm agnostic about his specific algorithm for carving at the joints, and I think he's come to feel the same way.)

Now: does this give us any sort of handle on self-organization in general? It's easy to say, "Yes, if a new level of the hierarchy forms, that it self-organization," and there are cases where this seems to fit: spiral waves, say, or the more common objects in Life, are spatially well-defined and relatively autonomous. (In the lab, do we ever see any sort of pattern of spiral waves?) However,

  1. How can you tell when this happens?
  2. Is self-organization always the same as a new level of the hierarchy?
  3. Is there in fact only one hierarchy?
If anything, I'd guess that the reverse is true, that getting a good handle on self-organization would let us put some spine into these ideas of biological order; but maybe that's just my physicist's hubris speaking again.

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