From the introduction to: “The evolution of complexity without natural selection, a large-scale trend of the fourth kind,” D. McShea, Paleobiology (Supplement) 31:146-156.
There may be no marvel greater than the complex functionality of organisms: the precise coordination of their parts, their reliability in the face of environmental upset, and their ability to act appropriately in their own interests. And then there’s the sheer number of their parts and part types, across a range of physical scales, most obvious in the larger multicellular organisms. These properties together have been aptly called “adaptive complexity,” and the apparent increase in the adaptive complexity of organisms over the history of life is one of the great mysteries of biology. The mystery is half solved by breaking the notion of adaptive complexity into its two components, adaptation and complexity, and thinking about them separately. Notions like “coordination,” “reliability,” and “appropriate action” (from the first sentence) are all aspects of function, or adaptation. And function or adaptation in organisms is no mystery at all. Since Darwin, we have had a perfectly adequate explanation for it, natural selection.
Complexity is another matter. Suppose we do a thought experiment in which we subtract all aspects of adaptation from our understanding of an organism. In other words, we put aside everything we know about how the organism functions. What’s left is an assemblage of parts, diverse in their sizes, compositions, shapes, and orientations with respect to each other. To capture that diversity, we will call the organism complex, although keep in mind that this is what you might call ‘‘pure complexity,’’ divorced from any notion of function. For complexity in this sense, biology has no satisfactory explanation. (Continued: Trends of fourth kind-ZFEL)