Butterfly wings, eyespots

The eyespots on butterfly wings are a recently derived evolutionary novelty that arose in a subset of the Lepidoptera and play an important role in predator avoidance. The production of the eyespot pattern is controlled by a developmental organizer called the focus, which induces the surrounding cells to synthesize specific pigments. The evolution of the developmental mechanisms that establish focus was therefore the key to the origin of butterfly eyespots (Keys et al., 1999, p.532). 

What Carroll s team discovered is that the genes and the entire regulatory pathway that integrates them and which control anterior/posterior wing development in the Drosophila (or its common ancestor with butterflies) have been recruited and modified to develop the eyespot focus. This discovery of the facility with which new developmental functions can evolve. .. within extant structures (Keys et al., 1999, p. 534) would have been impossible without the successful why-necessary answer to the proximate question of developmental biology. 

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