Individuals, Hierarchies and the Levels of Selection: A Chapter in Stephen J. Gould’s Evolutionary Theory

Abstract

Darwin’s theory of natural selection was a theory about microevolution which implied (1) that individuals organisms can be unambiguously recognized, (2) that what identifies the level, or levels, of selection is interaction rather than inheritance, and (3) that levels of selection other than the individual organism are either nonexistent, or of little relevance. In The Structure of Evolutionary Theory, Gould explained why the units of selection must be identified, contra Dawkins, in the units of interaction rather than in the units of inheritance, and why the whole theory of selection (and evolution) can be developed by reference to a single hierarchy of levels, or units, of selection. If Gould and Eldredge’s model of punctuated equilibria represents the actual, or prevailing mode of evolution, species boundaries become less arbitrary than in Darwin’s own view, and species eventually emerge with an individuality that turns them into potential units of selection. Recent advances in fields as diverse as symbiosis, lateral gene transfer and the evolution of development suggest that to shoehorn biological systems into the levels of the so-called evolutionary hierarchy is an oversimplification. Even the concept of individual organism as a physically independent unit with its precise origin in time does not apply so easily and universally as generally accepted. Evolution, indeed, is not simply matter of change of ‘individuals’, at any and all levels of the gene-to-clade hierarchy, but also matter of change of the units (or levels) of selection and of the rules of change themselves.