The Bactra Review: Occasional and eclectic book reviews by Cosma Shalizi   4

The Evolution of Complexity, by Means of Natural Selection

by John Tyler Bonner

Princeton University Press, 1988
This is a delightful book, and rare in that it delivers what its title promises. Bonner begins with a short explanation of the orthodox neo-Darwinian "modern synthesis," to which he adheres, and then turns to complexity and its evolution. He defines the complexity of an organism as the number of types of cells in it. This is not likely to go over well with the professional complexity theorists, but, unlike any of the proposed measures of complexity coming from physics or computation, it can actually be calculated for living things, and it certainly is an indicator of complexity. The most serious grounds on which it can be faulted are the ambiguity of "cell type", and the fact that (a few Siberian mammoths and the like aside) we cannot apply it to fossils, which makes it hard to study the evolution of complexity. For the first, Bonner takes refuge in the authority of the cytologists. For the second, he proposes to look at the modern representatives of ancient taxa, a proposal uncomfortably reminiscent of the Great Chain of Being.

Having defined "complexity," Bonner presents his thesis: the complexity of the most complex living things has increased over time. The qualification is important, at often missing from similar theses propounded by, e.g., physicists and mathematicians. As Stephen Jay Gould puts it, the Age of Bacteria is not about to end any time soon. Bonner is fully aware of this --- indeed, as a specialist in the cellular slime molds, must have a most lively apprehension of it. Nonetheless, he does show that both plant and animal taxa which have evolved later have a greater number of cell types than their predecessors, and seeks an explanation acceptable to neo-Darwinism.

He finds it in the fact that complexity correlates very well with body-size, and for good reasons. To take what is perhaps the most fundamental point, if you are small enough, you can get oxygen to all your cells through diffusion, but above a certain size that won't work, and you need a specialized respiratory system. Similar things are true of digestion and other phases of metabolism, the control of movement, etc., etc., so if something is to get bigger, it will have to get more complex: and vice versa, increases in complexity (at least of certain kinds) allow an organism (or its descendants) to increase in size. Now there are solid and obvious advantages to being bigger --- you can exploit more resources (e.g., eat larger things), you are harder to kill, you have a more favorable surface/volume ratio: in short you are able to exploit new niches. If, later on, some of your descendants dwindle, to exploit a niche for small organisms, because of canalization they are generally stuck with your increased number of cell types, though Bonner points to a few instances which have become less complex: they are all, significantly, obligate parasites.

Bonner is logically led to consider the origins of multicellular life, and (what is separate) life-forms with specialized cell types. His speculations seem plausible, but are frankly beyond my competence to judge.

In summary, Bonner's argument goes as follows: Filling a previously empty niche does wonders for the reproductive success of an organism; variations which increase size make new niches available, and so are favorably selected. But, owing to constraints imposed by basic physics and chemistry, larger organisms must be more specialized internally, i.e. more complex, to be as efficient as smaller ones, or even just to survive, so the selection is especially favorable for larger and more complex organisms. Because of the way developmental processes work, this complexity will probably be retained even by later, smaller organisms in other niches. Voila: the evolution of complexity, by means of natural selection.

xii+260 pp., illus., bibliography, index
Evolution / Self-Organization, Complexity, etc.
Currently in print as a paperback, ISBN 0-691-08494-7, US$17.95 [Buy from Powell's], LoC QH371 B65
21 August 1995, rev. 28 August 1998 (with thanks to Nigel Snoad for keeping my rhetorical nose to the anti-teleological grindstone)