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17. Gall flies lay their eggs in developing shoots of the goldenrod plant. The larvae produce chemicals that cause the plant tissue to swell into a mass called a gall. The size of the gall depends on the size of the larvae, which is genetically-based and variable. There are 2 main predators of gall fly larvae in the galls. Downy woodpeckers attack the largest galls and feed on those larvae inside. Parasitic wasps lays its own eggs inside the smallest galls, and the larvae that hatch parasitize and kill the gall fly larvae. Selective pressure from predators works against fly phenotypes that produce galls at both extremes, large and small. As a result, flies that produce middle-sized galls become more common. What type of selection is being described?

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