Measuring shifts in function and evolutionary opportunity using variability profiles: A case study of the globins

Variability profiles measured over a set of aligned sequences can be used to estimate evolutionary freedom to vary. Differences in variability profiles between clades can be used to identify shifts in function at the molecular level. We demonstrate such a shift between the alpha and beta subunits of hemoglobin. We also show that the variability profiles for myoglobin are different between whales and primates and speculate that the differences between the two clades may reflect a shift associated with the novel oxygen storage demands in the lineage leading to whales. We discuss the relationship between sequence variability and "evolutionary opportunity" and explore the utility of Maynard Smith's multidimensional evolutionary opportunity space metaphor for exploring functional constraints, genetic redundancy, and the context dependency of the genotype-phenotype map. This work has implications for quantitatively defining and comparing protein function. Supplementary data is available from bioinfo.mbb.yale.edu/align.