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about the apparent design of organisms. A good theory of design also ought to explain what kinds of entities are adapted and what kinds of complexity they show.

Organisms, together with man-made machines, seem to show a unique kind of complexity. We will call this the “complexity of purpose.” “Purpose,” as used here, is a metaphor, just as “natural selection” is a metaphor and has no real selector. This kind of complexity can even be used to define biological organisms. The organism is the consolidated unit of design or adaptation; almost everything in the organism seems built to further the individual’s survival and reproduction (Queller, 1997). Few parts of the organism are organized to gain at the expense of other parts, and few parts of the organism are organized to benefit other organisms (the chief exception being offspring).

The same cannot generally be said about groups of organisms. How does a flock of birds compare with the rocks and clocks? The parts of a flock of birds, the individual birds themselves, do not generally appear organized to benefit the flock. To the contrary, the members compete for food and mates, sometimes by physical fights, and they hide behind each other as shields against predation. Groups of organisms, e.g., flocks, populations, species, and communities, are not themselves clock-like or organismal.

Neither are they like the rock, because they are far more complex. But in contrast to the complexity of purpose shown by organisms, these aggregates have what we call the “complexity of cross-purpose.” The behavior of flocks, populations, and communities is extraordinarily rich, but not in a predictable and unified manner like the meshing of gears in a watch. Instead, much of the complexity stems from indifference of the parts to other parts and the apparent striving of each part to further its own survival and reproduction, if necessary at the expense of other parts.

Evolutionary theory has been addressing this issue of what kinds of units are adapted, and as it has done so, an interesting puzzle has emerged. The entities that we recognize as individual organisms actually originated as groups of lower-level units (Buss, 1987; Maynard Smith and Szathmáry, 1995). Somehow, the first cell assembled a group of components sufficient to sustain replication. The eukaryotic cell began as an assemblage of several prokaryotic cells, with at least the mitochondria and chloroplasts having independent origins. Larger organisms are groupings of cells. If groups show cross-purpose, how did they combine and make the transition to the unity of purpose of a single organism?

Social insect groups can give us special insight into this question. We will argue that social insect colonies are much like organisms, and we will show how their unity of purpose can arise through kin selection. We will also show that some cross-purpose remains, that colonies are not perfectly coherent. These remaining conflicts might be viewed as



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