Digest: Following clines along an Amazonian hybrid zone .

The shape and position of clines can provide crucial insights into the evolutionary forces at work in hybrid zones. In this issue, Del-Rio and colleagues applied cline theory to a hybrid zone between two antbird species in Amazonia. A narrow and displaced mitochondrial cline suggests that the selected genetic marker failed to track the northward movement of this hybrid zone, possibly due to reduced fitness of female hybrids.