 However, interestingly, some behavioral ecologists have claimed that they do not use optimality models to test whether or not natural selection is acting on a trait, but instead assume the operation of natural selection in order to test hypotheses about the conditions and constraints acting on the trait (Parker and Maynard Smith, 1990). This could be problematic, if it means behavioral ecologists baldly accept the view that natural selection optimizes most traits and never properly test this claim. However, one interpretation of Parker and Maynard Smith (1990) is not that behavioral ecologists have an unfalsifiable commitment to the idea that natural selection is an optimizer. Instead, they are claiming that testing hypotheses about the operation of natural selection itself is not what the behavioral ecologists are focusing on. The behavioral ecologists are making the preliminary assumption that a trait is an adaptation in order to test other hypotheses about it—they “pretend as if” the trait were an adaptation, and use that assumption to detect deviations from optimality that might reveal interesting tradeoffs and constraints—the nature of the deviation can suggest what might be missing from the model. This of course has the upshot that when the models initially fail, the scientists do not look for alternative non-adaptationist hypotheses, but instead initially look for new constraints, tradeoffs and conditions under which selection might be operating. For example, Giraldeau (1997) describes deviations from optimality in the evolution of photopigments in the rods (as opposed to cones) in the retinas of fish that live in certain sorts of marine environments where a shift in photosensitivity would be useful; Giraldeau uses this deviation to suggest the presence of a molecular constraint on the evolution of the rod cells. Presumably, repeated failures to find ways to fill in the gaps in these optimality models could ultimately result in the scientists concerned considering alternative, non-adaptationist explanations. So in order for the behavioral ecologists do this work they do not need to be committed to the strong view that natural selection is always the most important force in the origin of traits; this heuristic requires at most the view that natural selection usually locally optimizes (i.e. optimizes only relative to possibly severe constraints).