This animation shows how changes in the regulatory switch regions of the Pitx1 gene lead to morphological differences in stickleback fish.

During development, stickleback fish express Pitx1 in different parts of the body, including the pelvis. Most marine stickleback fish have a pelvis with a protective spine, but stickleback fish from certain freshwater populations have a missing or reduced pelvis. As shown in the animation, Pitx1 is activated in the pelvic region during development in marine fish. In fish with a missing or reduced pelvis, however, the regulatory switch that activates Pitx1 expression in the pelvic region is either missing or nonfunctional.

This animation is a clip from a 2005 Holiday Lecture Series, Evolution: Constant Change and Common Threads. Depending on students’ background, it may be helpful to pause the animation at various points to discuss different features.