Rapid evolution of a life history trait

The copepod Diaptomus sanguineus begins diapause in permanent ponds in late March as an adaptation to avoid summer fish predation. During a study of copepod populations in two Rhode Island ponds, a severe drought dried one pond killing all fish. The second (control) pond did not dry, and no fish were killed. Before the drought copepods in the two ponds entered diapause on nearly the same date. After the drought, the timing of diapause shifted to later in the year in the pond that had lost its fish, while no shift occurred in the control pond. The direction of this shift in the onset of diapause is that expected had the copepods been released from natural selection for early spring diapause imposed by summer fish predation.