title: The geography of divergence with gene flow facilitates multitrait adaptation and the evolution of pollinator isolation in Mimulus aurantiacus.
abstract: Ecological adaptation is the driving force during divergence with gene flow and generates reproductive isolation early in speciation. Although gene flow opposes divergence, local adaptation can be facilitated by factors that prevent the breakup of favorable allelic combinations. We investigated how selection, genetic architecture, and geography have contributed to the maintenance of floral trait divergence and pollinator isolation between parapatric ecotypes of Mimulus aurantiacus. Combining greenhouse, field, and genomic studies, we show that sharp clines in floral traits are maintained by spatially varying selection. Although adaptation breaks down where the ecotypes co-occur, leading to the formation of a hybrid zone, the largely non-overlapping distributions of the ecotypes shield them from immigrant genes, facilitating divergence across most of the range. In contrast to the sharp genetic discontinuities observed across most hybrid zones, we observed a gradual cline in genome-wide divergence and a pattern of isolation by distance across the landscape. Thus, contrary to a long period of allopatry followed by recent re-contact, our data suggest that floral trait divergence in M. aurantiacus may have evolved with locally restricted, but ongoing gene flow. Therefore, our study reveals how the geographic distribution of an organism can contribute to the evolution of premating isolation in the early stages of divergence with gene flow.
authors.affiliation: Institute of Ecology and Evolution, University of Oregon, Eugene, Oregon, 97401. sstankow@uoregon.edu.
authors.forename: Sean
authors.initials: S
authors.lastname: Stankowski
authors.affiliation: Department of Biological Sciences, Binghamton University, Binghamton, New York, 13902.
authors.forename: James M
authors.initials: JM
authors.lastname: Sobel
authors.affiliation: Institute of Ecology and Evolution, University of Oregon, Eugene, Oregon, 97401.
authors.forename: Matthew A
authors.initials: MA
authors.lastname: Streisfeld
mesh_terms: D000818:Animals; D001717:Birds / Q000502:physiology; D035264:Flowers / Q000502:physiology*; D051456:Gene Flow; D049810:Genetic Speciation; D032241:Mimulus / Q000235:genetics / Q000502:physiology*; D009036:Moths / Q000502:physiology; D054817:Pollination; D020641:Polymorphism, Single Nucleotide; D060047:Reproductive Isolation
keywords: Cline analysis; Mimulus; divergence with gene flow; divergent selection; hybrid zone; reproductive isolation