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Abstract PremiseEvolutionary theory predicts polymorphism should be rare; however, intraspecific variation in floral color is common and can be attributed to genetic drift, plasticity, or variable selection. Examining floral color polymorphism both within contact zones and across a species' range can reveal the mechanisms maintaining this variation. Here, we used a multistep approach to investigate spatially heterogeneous variation in floral bract color inCastilleja coccinea. MethodsWe compared frequencies of color morphs, floral morphology, fitness, and genetic structure in regional populations and in a common garden. Next, we examined habitat differences, including edaphic factors, as potential drivers of variation. Lastly, we leveraged herbarium and iNaturalist occurrence data to investigate whether patterns were consistent at the landscape scale. ResultsBract color inC. coccineais genetically heritable, with yellow dominant over red, and is under selection. Populations are predominantly monomorphic, with color distance showing no correlation to genetic or geographic distance, despite significant genetic isolation by distance. Yellow morphs were associated with open wetlands, while red morphs occurred at drier sites with nearby tree cover. Red morphs demonstrated lower fitness in a common garden, suggesting trade‐offs associated with pleiotropic effects of floral color. ConclusionsDifferences in floral color between morphs are consistent with diversification associated with a shift in ecological niche. We identified variation in edaphic and habitat conditions as probable drivers of divergence in floral color. Additionally, variation in other floral traits suggests a combined role of pollinators and habitat differences acting in concert to maintain distinct floral color morphs.
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Incomplete reproductive isolation and low genetic differentiation despite floral divergence across varying geographic scales in Castilleja
PremiseDivergence depends on the strength of selection and frequency of gene flow between taxa, while reproductive isolation relies on mating barriers and geographic distance. Less is known about how these processes interact at early stages of speciation. Here, we compared population‐level differentiation in floral phenotype and genetic sequence variation among recently divergedCastillejato explore patterns of diversification under different scenarios of reproductive isolation. MethodsUsing target enrichment enabled by the Angiosperms353 probe set, we assessed genetic distance among 50 populations of fourCastillejaspecies. We investigated whether patterns of genetic divergence are explained by floral trait variation or geographic distance in two focal groups: the widespreadC. sessilifloraand the more restrictedC. purpureaspecies complex. ResultsWe document thatC. sessilifloraand theC. purpureacomplex are characterized by high diversity in floral color across varying geographic scales. Despite phenotypic divergence, groups were not well supported in phylogenetic analyses, and little genetic differentiation was found across targeted Angiosperms353 loci. Nonetheless, a principal coordinate analysis of single nucleotide polymorphisms revealed differentiation withinC. sessilifloraacross floral morphs and geography and less differentiation among species of theC. purpureacomplex. ConclusionsPatterns of genetic distance inC. sessiliflorasuggest species cohesion maintained over long distances despite variation in floral traits. In theC. purpureacomplex, divergence in floral color across narrow geographic clines may be driven by recent selection on floral color. These contrasting patterns of floral and genetic differentiation reveal that divergence can arise via multiple eco‐evolutionary paths.
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- Award ID(s):
- 1752785
- PAR ID:
- 10445276
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- American Journal of Botany
- Volume:
- 108
- Issue:
- 7
- ISSN:
- 0002-9122
- Format(s):
- Medium: X Size: p. 1270-1288
- Size(s):
- p. 1270-1288
- Sponsoring Org:
- National Science Foundation
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