Effects of geographic distance, landscape features and host association on genetic differentiation of checkerspot butterflies
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Wright’s hypothesis of genetic isolation by distance (IBD) predicts that genetic differentiation increases with geographic distance. The use of different diets by allopatric populations may also contribute significantly to genetic isolation by restricting gene flow among populations using different hosts. The overarching objective is to ask whether and to what extent the degree of isolation between populations is associated with differences between them in host use after taking into account the effects of spatial isolation. AFLP markers were used to assess population genetic structure. In the first study, I ask if there is detectable host associated genetic differentiation between allopatric populations of the marsh fritillary butterfly Euphydryas aurinia. Each of the ten sampled populations utilizes one of two hosts, and an additional population utilizes both hosts. Substantial host associated genetic differentiation was found between allopatric populations (FCT = 0.12, P < 0.001), but no such association could be found within the dual host population. In the second study, I use a GIS to ask if whether and to what extent the degree of genetic differentiation between Euphydryas editha populations is associated with the type of intervening landscape matrix. Eight populations each from Sequoia National Park (SEKI) and Yosemite National Park (YOSE) were sampled. Genetic differentiation within SEKI and YOSE was found to be small but statistically significant (ΦST = 0.03, P < 0.001). At SEKI, no IBD was detected. In contrast, IBD was detected at YOSE (r = 0.66, P < 0.001). When landscape features were taken into account, the IBD relationship was strengthened (r = 0.77, P < 0.001). In the third study, an additional 26 populations were sampled to assess regional scale differentiation. There was substantial genetic differentiation between populations (ΦST = 0.19, P < 0.001) and significant IBD (r = 0.55, P < 0.001). Host association was also found to be significantly correlated with genetic differentiation (r = 0.17, P < 0.01). Populations on Castilleja exhibited a significant IBD relationship (r = 0.7, P < 0.001). Populations on Collinsia, on the other hand, appear to be recently colonized, and do not exhibit IBD.