Theories of political opportunity structure (POS) have received ambiguous empirical support. Quite a few studies make a convincing case for POS, some find it to have no measurable effect, others show that it can have different effects in different contexts. The lack of a theory of the mechanisms that link POS to movement action is identified as the fundamental problem behind these divergent results. I suggest a dynamic and relational solution to the structureagency problem that employs an evolutionary mode of causal explanation. I show how such an approach helps us to understand why in spite of limited information and frequent errors of judgment, actors' choices may ultimately reflect structural opportunities. I also discuss how an evolutionary approach can help explain deviations from the predictions of POS theory, e.g., why adaptation to changes in POS is slow, why opportunities are sometimes missed, and why those that are perceived cannot always be seized. I conclude with a discussion of some methodological implications.

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