Figure 1.

Three scenarios for changes in cline width and center following secondary contact. Sharp clines characteristic of tension zones are predicted if there is selection against hybrids due to underdominance or epistasis between interspecific genomic elements. In the absence of selection, neutral introgression will result in wider clines and cline centers will remain close to region of secondary contact. Adaptive introgression into one species is predicted to result in wide clines and displacement of the cline center. In each figure, the horizontal axis is geographic distance and the vertical axis is marker frequency.