A population of herbivorous insects that shifts to a novel host can experience selection pressures that result in adaptation to the new resource. Host race formation, considered an early stage of the speciation process, may result. Th e current study investigates host shifts and variation in traits potentially involved in the evolution of reproductive isolation among populations of the juniper hairstreak butterfl y, Mitoura gryneus . Mitoura are closely associated with their host trees (Cupressaceae) and exhibit host plant fi delity: in addition to larval development and oviposition, host trees support male leks and mating. Female oviposition preference for the natal host, and diff erential fi tness of larvae when reared on natal versus alternate hosts, are indications that specialization and local adaptation to the natal host plant are occurring. Populations with single host plant associations ( Juniperus ashei , J. pinchotii and J. virginiana ) as well as populations with multiple hosts (both J. ashei and J. pinchotii ) were examined. Concordance between female preference and larval performance was found for J. ashei -associated populations. Population-level variation in the patterns of female preference and larval performance, both within and among host associations, may refl ect diff erences in the timing and direction of colonization of hosts. For a single nominal species that otherwise exhibits no morphological or phenological diff erences, the experimental assessment of specialization and host fi delity in M. gryneus provides strong support for the hypothesis of ongoing host race formation in these butterfl ies.

Understanding the selective forces that shape dispersal strategies is a fundamental goal of evolutionary ecology and is increasingly important in changing, human‐altered environments. Sex‐biased dispersal (SBD) is common in dioecious taxa, and understanding variation in the direction and magnitude of SBD across taxa has been a persistent challenge. We took a comparative, laboratory‐based approach using 16 groups (species or strains) of bean beetles (genera Acanthoscelides, Callosobruchus, and Zabrotes, including 10 strains of one species) to test two predictions that emerge from dominant hypotheses for the evolution of SBD: (1) groups that suffer greater costs of inbreeding should exhibit greater SBD in favor of either sex (inbreeding avoidance hypothesis) and (2) groups with stronger local mate competition should exhibit greater male bias in dispersal (kin competition avoidance hypothesis). We used laboratory experiments to quantify SBD in crawling dispersal, the fitness effects of inbreeding, and the degree of polygyny (number of female mates per male), a proxy for local mate competition. While we found that both polygyny and male‐biased dispersal were common across bean beetle groups, consistent with the kin competition avoidance hypothesis, quantitative relationships between trait values did not support the predictions. Across groups, there was no significant association between SBD and effects of inbreeding nor SBD and degree of polygyny, using either raw values or phylogenetically independent contrasts. We discuss possible limitations of our experimental approach for detecting the predicted relationships, as well as reasons why single‐factor hypotheses may be too simplistic to explain the evolution of SBD.

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