“…Early botanists recognized that ecologically differentiated plant populations represent intermediate stages in the speciation process (e.g., Clausen, ), and there is a sizeable body of the literature that examines the strengths and stages of reproductive isolating barriers within plant species, often in association with adaptation to contrasting environments. These studies of postzygotic reproductive isolation below the species boundary—or outbreeding depression (Price & Waser, )—reveal partial isolation within species across environmental gradients or ecotones (Fenster & Galloway, ; Grindeland, ; Montalvo & Ellstrand, ; Waser, Price, & Shaw, ), even in trees (Goto, Iijima, Ogawa, & Ohya, ; Stacy, ). In such cases, outbreeding depression appears to result from the disruption of local adaptation; however, partial reproductive isolation between conspecific populations can result also from physiological or intrinsic causes such as underdominance (heterozygote disadvantage), chromosomal rearrangements, or the breakup of coadapted gene complexes (Edmands, ; Price & Waser, ).…”