In this video segment adapted from NOVA, evolutionary scientist Sean Carroll investigates why one species of fruit fly has wing spots while another genetically similar one does not. As the video explains, although two organisms may both possess the same gene linked to a particular trait, this trait may not be expressed unless the gene is turned on. This is done by an enhancer, a piece of noncoding DNA—previously thought of as "junk" DNA—that acts as a "switch."

This video is available in both English and Spanish audio, along with corresponding closed captions.