title: Subspecies delineation amid phenotypic, geographic and genetic discordance in a songbird.
abstract: Understanding the processes that drive divergence within and among species is a long-standing goal in evolutionary biology. Traditional approaches to assessing differentiation rely on phenotypes to identify intra- and interspecific variation, but many species express subtle morphological gradients in which boundaries among forms are unclear. This intraspecific variation may be driven by differential adaptation to local conditions and may thereby reflect the evolutionary potential within a species. Here, we combine genetic and morphological data to evaluate intraspecific variation within the Nelson's (Ammodramus nelsoni) and salt marsh (Ammodramus caudacutus) sparrow complex, a group with populations that span considerable geographic distributions and a habitat gradient. We evaluated genetic structure among and within five putative subspecies of A. nelsoni and A. caudacutus using a reduced-representation sequencing approach to generate a panel of 1929 SNPs among 69 individuals. Although we detected morphological differences among some groups, individuals sorted along a continuous phenotypic gradient. In contrast, the genetic data identified three distinct clusters corresponding to populations that inhabit coastal salt marsh, interior freshwater marsh and coastal brackish-water marsh habitats. These patterns support the current species-level recognition but do not match the subspecies-level taxonomy within each species-a finding which may have important conservation implications. We identified loci exhibiting patterns of elevated divergence among and within these species, indicating a role for local selective pressures in driving patterns of differentiation across the complex. We conclude that this evidence for adaptive variation among subspecies warrants the consideration of evolutionary potential and genetic novelty when identifying conservation units for this group.
authors.affiliation: Fuller Evolutionary Biology Program, Cornell Laboratory of Ornithology, Ithaca, NY, 14850, USA.|Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, NY, 14853, USA.
authors.forename: Jennifer
authors.identifier: http://orcid.org/0000-0002-8146-7029
authors.initials: J
authors.lastname: Walsh
authors.affiliation: Fuller Evolutionary Biology Program, Cornell Laboratory of Ornithology, Ithaca, NY, 14850, USA.|Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, NY, 14853, USA.
authors.forename: Irby J
authors.initials: IJ
authors.lastname: Lovette
authors.affiliation: Department of Biology, Benedictine College, Atchison, KS, 66002, USA.
authors.forename: Virginia
authors.initials: V
authors.lastname: Winder
authors.affiliation: Ecology and Evolutionary Biology, University of Connecticut, Storrs, CT, 06269, USA.
authors.forename: Chris S
authors.initials: CS
authors.lastname: Elphick
authors.affiliation: School of Biology and Ecology, University of Maine, Orono, ME, 04469, USA.
authors.forename: Brian J
authors.initials: BJ
authors.lastname: Olsen
authors.affiliation: Department of Entomology and Wildlife Ecology, University of Delaware, Newark, DE, 19716, USA.
authors.forename: Gregory
authors.initials: G
authors.lastname: Shriver
authors.affiliation: Department of Natural Resources and the Environment, University of New Hampshire, Durham, NH, 03824, USA.
authors.forename: Adrienne I
authors.initials: AI
authors.lastname: Kovach
mesh_terms: D000818:Animals; D005075:Biological Evolution; D005828:Genetics, Population; D010641:Phenotype; D020641:Polymorphism, Single Nucleotide; D046370:Sparrows / Q000145:classification*; D053833:Wetlands
keywords: 
  Ammodramus caudacutus
  ; 
  Ammodramus nelsoni
  ; adaptation; conservation units; evolutionary potential; genetic structure