Fig. 1.

Conceptual overview and tests for host-associated differentiation. Panel (A) illustrates our two alternative hypotheses of (H1) an ongoing sweep and (H2) polymorphism maintained by selection. Under the first hypothesis the inversion is young and in the process of sweeping; variation will not be maintained. Under the second hypothesis balancing selection promotes the long-term maintenance of inversion polymorphism. (B) Shows illustrations of Timema stick insects and their host plants, for the taxa studied here. Panel (C) summarizes genome-wide genetic differentiation for parapatric Timema populations on different hosts. Points denote mean F_ST for each of 13 T. cristinae linkage groups with horizontal lines extending to the 75th percentile of F_ST for that linkage group. Host abbreviations are A = Adenostoma, C = Ceanothus, P = Pseudotsuga menziesii (Douglas Fir), Pi = Pinus (pine), Q = Quercus (oak), and RW = Sequoia sempervirens (Redwood). See SI Appendix, Fig. S2 for more detailed patterns of genetic differentiation. Illustrations here and in other figures by Rosa Ribas.