   Whether boundaries count as causes, and thereby explain evolutionary outcomes, is more problematic. A prevailing view within evolutionary biology is that constraints must count as causal influences, whatever our account of causation (whether it be by prevention or by omission, see Woodward 2003 for discussion). Gould and Lewontin (1979) adopted this position. Sansom (2003) argued that the debate over the evolutionary importance of constraint amounts to an issue about the relative causal contributions of constraint and natural selection to evolutionary outcomes. The causal contribution of constraint can be assessed by using comparative data in order to compare evolutionary outcomes of lineages with and without the constraint of interest. If there is a difference, then the constraint has an identifiable causal influence, all other things being equal. Pigliucci and Kaplan (2000, 2006) proposed a test protocol based on transition matrices in which probabilities of evolutionary change are specified under natural selection and under one or more competing constraint hypotheses, so as to test natural selection against constraint. More philosophical analysis is required to assess the viability of this approach.