The Evolution of Hierarchical Structure in Organisms: A Test for an Increasing Tendency

Duke University, Durham NC

Investigators

Abstract

The Evolution of Hierarchical Structure in Organisms: A Test for an Increasing Tendency Daniel McShea, Duke University EAR-0208156 ABSTRACT One of the clearest trends in the history of life is the increase in the hierarchical structure of organisms, that is, in the degree to which they are nested, parts within wholes. Well-known cases of hierarchy increase include the so-called "major transitions" in evolution, such as the various origins of multicellular organisms (e.g., plants and animals) from aggregates of free-living cells, and also at a higher level, the origins of colonies (e.g., social-insect colonies) from clones of multicellular individuals. The trend is clear, but we know little about the mechanism, about what drives the trend. One possibility is that there has been a bias, a positive tendency for hierarchy to increase, perhaps because greater nestedness is favored by natural selection. But it could also be that there is no such tendency, that hierarchy decreases as often as it increases, and that the trend is the result of the passive spread of evolutionary lineages across the hierarchy spectrum. To see how this might work, suppose that the first organisms (presumably bacteria) were hierarchically as shallow as possible, so that initially hierarchy could only increase. Then, as life diversified, with both increases and decreases occurring in various groups, the outer envelope of hierarchical structure would be pushed toward ever-higher levels. Stephen Jay Gould argued forcefully for this mechanism in his book, Full House. The evidence on this question - whether any increasing tendency exists - is equivocal. We know that losses of hierarchical structure have occurred, especially at the colony level, in the transitions back to solitary life. But their frequency, relative to increases, is unknown. The study proposed here is a first-ever attempt to answer the question in a rigorous way. One barrier to investigation has been the fact that too few major transitions are known to estimate relative frequencies of increase and decrease in a statistically robust way. This study solves the problem by employing a recently developed high-resolution hierarchy scale. The scale identifies intermediate levels between the major levels (e.g., between multicellular individual and colony), in effect interpolating a series of "minor transitions" within the major transitions and thereby increasing the number of transitions available to be studied. To test for an increasing tendency, the study will examine minor transitions across the spectrum of hierarchical organization, from bacteria to supercolony. Modern phylogenetic methods will be used to reconstruct the history of hierarchy with each group studied, to count numbers of increases and decreases in hierarchy, and to assess their relative probabilities.