Homeotic, or Hox genes specify the identity of segments along the embryo's body axis and regulate the formation of major structures in every animal studied. But because laboratory mutations in these genes can cause monstrosities--such as a fly with legs where its antennae should be--many scientists doubted that natural variation of homeotic genes could underlie the incremental, survivable changes that accrued over eons as animals gradually evolved improved body parts. Today's finding marks the first time that changes in the control of homeotic genes have been shown to underlie an evolutionary trend leading to novel body structures.

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