Daniel W. McShea
Assistant Professor of Biology
Ph.D., University of Chicago, 1990
E-mail: dmcshea@duke.edu
Phone: (919) 660-7342

Research Interests -- Macroevolution, large-scale trends, complexity, biological hierarchies

Background (last updated: 2004) - A good case can be made that the complexity of organisms tends to increase in evolution. For one thing, it seems obvious that the earliest organisms (presumably bacteria) were simple, and their descendants (including humans) more complex, on average. Also, it is easy to imagine why such a trend might be expected. One argument is that more complex organisms might have more functional capabilities and therefore should be favored by natural selection, on average, over simpler organisms. These arguments are compelling but inconclusive. To compare ancient and modern organisms, we need first to specify what we mean by complexity, to say in what sense a modern organism is supposed to be more complex than an ancient one. Then we need to measure complexity to discover whether modern organisms are in fact more complex in the specified sense. Otherwise, the perceived difference could be an illusion. For example, modern organisms might just seem more complex because they are larger, on average, with parts that are easier to see at ordinary scales of observation. Further, even if the trend is real, it is not clear that there is any increasing tendency. For example, it could be that the first organisms arose near a kind of lower limit on complexity, in other words, that they were as simple as possible. If so, then as life diversified, new species arising would have had nowhere to go but up, and the group as a whole would have spread away from the lower limit. This spreading produces a trend, an increase in the mean complexity for life as a whole. But there is no increasing tendency in that, away from the lower boundary, increases and decreases occur equally often.

These have been live issues in evolutionary studies for two centuries. Darwin thought complexity increased. Indeed, he may have had something like complexity in mind when he wrote - in what is now a famous passage in the Origin - of "that vague yet ill-defined sentiment, felt by many palaeontologists, that organisation on the whole has progressed" (1859, p. 345). Nevertheless, treatment of complexity in the evolutionary literature since Darwin has consisted mostly of impressionistic assessments and casual theorizing, at least until recent decades. The main reason, I argue, is that complexity has been difficult to operationalize, to define in a way that enables us to assess or measure it. A major goal of my research is to help change this, to help transform the study of complexity in evolution into an empirical and rigorous research program.

Research Goals - My specific aims are:

1) To operationalize complexity, that is, to devise ways to understand it that enable us to assess it in real organisms, ideally using quantitative measures that can be applied to fossils as well to modern organisms.

2) To use these measures to test for trends in complexity in large groups of species, and where trends are found, to investigate the underlying "trend mechanism," or the pattern of change among lineages that accounts for a trend. In the technical language that has emerged for large-scale trends, the spreading away from a lower boundary is one type of what is called a "passive" mechanism, while a pervasive increasing tendency is a type of "driven" mechanism.

3) To investigate possible correlates of complexity in evolution. For example, the suggestion has been made that complexity is connected with size, that larger organisms tend to be more complex than smaller ones. Another candidate - my own hypothesis - is that a negative correlation exists between two types or senses of complexity, that as complexity in one sense increases, it decreases in another.

This work builds on past treatments of complexity by Herbert Spencer, Donald Campbell, and Herbert Simon, as well as a larger group of more recent researchers working on complexity, hierarchy, trends, and the historical and philosophical aspects of all of these. And it is interdisciplinary in two senses. First, it lies at the intersection of paleontology and biology, i.e., paleobiology. Second, the central issues are conceptual as well as empirical, and therefore it is part of the philosophy of biology as well.