The physical characteristics of animals evolve because their genes change over successive generations. It is not always clear, though, which genes are involved. The genes that regulate embryonic or larval development are likely candidates, because they control how the animal develops its characteristic form and features. It is possible that natural selection might produce evolutionary change after the adjustment of just a few such switches on the genetic control panel of development. Writing in this issue, Sucena et al. and Gompel and Carroll provide evidence that this can indeed happen. They show that modifications at just a few developmental hotspots underlie 'parallel' evolutionary changes that occurred independently in different species. Tinkering with developmental genes is not necessarily an easy route to evolutionary change. For example, mutations in the anten-napedia gene―an important regulator of development in the fruitfly Drosophila―can produce a fly with legs growing on its head. This may be fascinating to developmental geneticists, but from the fly's point of view it is not helpful.
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