There is still a great deal of debate over what counts as a constraint and about how to assess experimentally the relative importance of constraints and selection in evolutionary history. I will argue that the notion of a constraint on variation, and thus the selection-constraint distinction, depends on two specifications: (1) what counts as a variant -- constraints limit or bias the production of what? and (2) range of assessment -- over what range of times or conditions is the variation (...) assessed? Specifications 1 and 2 help us to understand empirical work on the relative importance of constraint and selection in evolution. (shrink)

George McGhee’s book “Convergent Evolution: limited forms most beautiful” provides an extensive survey of biological convergence. This paper has two main aims. First, it examines the theoretical claims McGhee makes about convergent evolution—specifically criticizing his use of a total morphospace to understand contingency and his assumption that functional constraints are non-contingent. Second, it sketches a group of important conceptual challenges facing researchers interested in convergence.