Abstract

I address the controversy in evolutionary biology concerning which levels of biological entity (units) can and do undergo natural selection. I refine a definition of the unit of selection, first presented by William Wimsatt, that is grounded in the structure of natural selection models. I examine Elliott Sober's objection to this structural definition, the "homogeneous populations" problem; I find that neither the proposed definition nor Sober's own causal account can solve the problem. Sober, in his solution using his causal view, imports precisely the information needed to make the structural definition effective. Finally, I indicate how the proposed definition can clarify which sorts of evidence could be brought to bear on the controversial case of the Myxoma virus.