On the standardization of fitness and traits in comparative studies of phenotypic selection

Comparisons of the strength and form of phenotypic selection among groups provide a powerful approach for testing adaptive hypotheses. A central and largely unaddressed issue is how fitness and phenotypes are standardized in such studies; standardization across or within groups can qualitatively change conclusions whenever mean fitness differs between groups. We briefly reviewed recent relevant literature, and found that selection studies vary widely in their scale of standardization, but few investigators motivated their rationale for chosen standardization approaches. Here, we propose that the scale at which fitness should be relativized should reflect whether selection is likely to be hard or soft; that is, the scale at which populations (or hypothetical populations in the case of a contrived experiment) are regulated. We argue that many comparative studies of selection are implicitly or explicitly focused on soft selection (i.e., frequency and density-dependent selection). In such studies, relative fitness should preferably be calculated using within-group means, although this approach is taken only occasionally. Related difficulties arise for the standardization of phenotypes. The appropriate scale at which standardization should take place depends on whether groups are considered to be fixed or random. We emphasize that the scale of standardization is a critical decision in empirical studies of selection that should always warrant explicit justification.