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Combinations of correlated floral traits have arisen repeatedly across angiosperms through convergent evolution in response to pollinator selection to optimize reproduction. While some plant groups exhibit very distinct combinations of traits adapted to specific pollinators (so-called pollination syndromes), others do not. Determining how floral traits diverge across clades and whether floral traits show predictable correlations in diverse groups of flowering plants is key to determining the extent to which pollinator-mediated selection drives diversification. The North AmericanSilenesectionPhysolychnisis an ideal group to investigate patterns of floral evolution because it is characterized by the evolution of novel red floral color, extensive floral morphological variation, polyploidy, and exposure to a novel group of pollinators (hummingbirds). We test for correlated patterns of trait evolution that would be consistent with convergent responses to selection in the key floral traits of color and morphology. We also consider both the role of phylogenic distance and geographic overlap in explaining patterns of floral trait variation. Inconsistent with phenotypically divergent pollination syndromes, we find very little clustering of North AmericanSileneinto distinct floral morphospace. We also find little evidence that phylogenetic history or geographic overlap explains patterns of floral diversity in this group. White- and pink-flowering species show extensive phenotypic diversity but are entirely overlapping in morphological variation. However, red-flowering species have much less phenotypic disparity and cluster tightly in floral morphospace. We find that red-flowering species have evolved floral traits that align with a traditional hummingbird syndrome, but that these trait values overlap with several white and pink species as well. Our findings support the hypothesis that convergent evolution does not always proceed through comparative phenotypic divergence, but possibly through sorting of standing ancestral variation.
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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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