   Developmental and genetic constraint hypotheses do not invoke causal processes in the same way that adaptive hypotheses do. Can such different hypotheses compete with one another? This aspect of the adaptationism debate connects biology to an important puzzle about the nature of causation and explanation. If explanation amounts to identifying causes, then constraints must be causes to explain evolutionary outcomes. But how can impediments have a causal influence on the evolutionary trajectory of a lineage? One possible answer is that they don't have any causal influence—they are merely the background against which natural selection operates. Such a view treats causes as part of a process, or involving some kind of energy transfer (Dowe 2008). This view of causation vindicates the adaptationist view that the constraint set is presupposed by any respectable adaptive hypothesis. For adaptationists, new constraints simply mean that we need to build new models. Testing is then just a matter of comparing different adaptive hypotheses that have different constraints. The other possible answer is that constraints do have a causal influence on evolution because they prevent evolutionary trajectories from attaining some outcomes. This view is consistent with counterfactual and interventionist approaches to causation, such as the comprehensive one defended by Woodward (2003), and vindicates the approach of treating constraints as genuine alternatives to adaptive hypotheses.