Figure 1.

Map of the four paired sympatric collection sites for apple and hawthorn flies in the Midwestern United States. Also given are the ranges of the apple race and native hawthorn‐infesting populations of Rhagoletis pomonella in the United States and Mexico (see Supporting Information Table S2 for numerical designations of populations and site information). Note that the 430‐km transect through the Midwestern United States encompasses much of the latitudinal range of overlap of the apple and hawthorn host races in the region. The primary hawthorn host of R. pomonella in the Northeastern and Midwestern United States is Crataegus mollis. However, moving south from Urbana, apple is not infested and C. mollis becomes rare, although a variety of the species, C. mollis texana, exists in the state of Texas, United States (site 5). Other hawthorn species, with varying fruiting times, are the primary hosts of the fly in the southern United States and Mexico (see Figure 2a and Supporting Information Table S2; Lyons‐Sobaski & Berlocher, 2009; Rull et al., 2006). DNA sequencing data imply that hawthorn‐infesting R. pomonella populations from the Eje Volcánico Trans Mexicano (EVTM) and those in the Sierra Madre Oriental Mountains of Mexico (SMO) and United States have undergone cycles of allopatry followed by secondary contact and gene flow over the past ~1.5 My (Feder et al., 2005; Feder, Berlocher, et al., 2003; Michel et al., 2007; Xie et al., 2007), contributing to the creation and maintenance of geographic genetic variation in eclosion time in the fly