title: The temporal window of ecological adaptation in postglacial lakes: a comparison of head morphology, trophic position and habitat use in Norwegian threespine stickleback populations.
abstract: Studying how trophic traits and niche use are related in natural populations is important in order to understand adaptation and specialization. Here, we describe trophic trait diversity in twenty-five Norwegian freshwater threespine stickleback populations and their putative marine ancestor, and relate trait differences to postglacial lake age. By studying lakes of different ages, depths and distance to the sea we examine key environmental variables that may predict adaptation in trophic position and habitat use. We measured trophic traits including geometric landmarks that integrated variation in head shape as well as gillraker length and number. Trophic position (Tpos) and niche use (α) were estimated from stable isotopes (δ(13)C, δ(15)N). A comparison of head shape was also made with two North American benthic-limnetic species pairs.
  
  
  
  We found that head shape differed between marine and freshwater sticklebacks, with marine sticklebacks having more upturned mouths, smaller eyes, larger opercula and deeper heads. Size-adjusted gillraker lengths were larger in marine than in freshwater stickleback. Norwegian sticklebacks were compared on the same head shape axis as the one differentiating the benthic-limnetic North American threespine stickleback species pairs. Here, Norwegian freshwater sticklebacks with a more "limnetic head shape" had more and longer gillrakers than sticklebacks with "benthic head shape". The "limnetic morph" was positively associated with deeper lakes. Populations differed in α (mean ± sd: 0.76 ± 0.29) and Tpos (3.47 ± 0.27), where α increased with gillraker length. Larger fish had a higher Tpos than smaller fish. Compared to the ecologically divergent stickleback species pairs and solitary lake populations in North America, Norwegian freshwater sticklebacks had similar range in Tpos and α values, but much less trait divergences.
  
  
  
  Our results showed trait divergences between threespine stickleback in marine and freshwater environments. Freshwater populations diverged in trophic ecology and trophic traits, but trophic ecology was not related to the elapsed time in freshwater. Norwegian sticklebacks used the same niches as the ecologically divergent North American stickleback species pairs. However, as trophic trait divergences were smaller, and not strongly associated with the ecological niche, ecological adaptations along the benthic-limnetic axis were less developed in Norwegian sticklebacks.
authors.affiliation: Department of Biosciences, Centre for Ecological and Evolutionary Synthesis (CEES), University of Oslo, P. O. Box 1066, Blindern, N-0316, Oslo, Norway.|Faculty of Applied Ecology and Agricultural Sciences, Hedmark University of Applied Science, Campus Evenstad, NO-2480, Koppang, Norway.
authors.forename: Kjartan
authors.initials: K
authors.lastname: Østbye
authors.affiliation: Department of Physiological Ecology, Max Planck Institute for Limnology, Postfach 165, D-24302, Plön, Germany.|Fish and Stable Isotope Ecology Laboratory, Instituto de Ciencias Naturales Alexander von Humbolt, Universidad de Antofagasta, Avenida Angamos 601, Antofagasta, Chile.
authors.forename: Chris
authors.initials: C
authors.lastname: Harrod
authors.affiliation: Multiconsult, P.O. Box 265 Skøyen, N-0213, Oslo, Norway.
authors.forename: Finn
authors.initials: F
authors.lastname: Gregersen
authors.affiliation: Department of Biology, Aquatic Behavioural Ecology Research group, University of Bergen, P. O. Box 7800, N-5020, Bergen, Norway.
authors.forename: Tom
authors.initials: T
authors.lastname: Klepaker
authors.affiliation: , August-Bebel-Strasse 46, 16816, Neuruppin, Germany.
authors.forename: Michael
authors.initials: M
authors.lastname: Schulz
authors.affiliation: Department of Zoology, University of British Columbia, Vancouver, British Columbia, V6T 1Z4, Canada.
authors.forename: Dolph
authors.initials: D
authors.lastname: Schluter
authors.affiliation: Department of Biosciences, Centre for Ecological and Evolutionary Synthesis (CEES), University of Oslo, P. O. Box 1066, Blindern, N-0316, Oslo, Norway. asbjorn.vollestad@ibv.uio.no.
authors.forename: Leif Asbjørn
authors.initials: LA
authors.lastname: Vøllestad
mesh_terms: D000064:Acclimatization; D000222:Adaptation, Physiological; D000818:Animals; D017753:Ecosystem; D005260:Female; D006257:Head / Q000033:anatomy & histology; D060106:Lakes; D008297:Male; D009656:North America; D009664:Norway; D010641:Phenotype; D023701:Smegmamorpha / Q000033:anatomy & histology*
keywords: Adaptive radiation; Benthic-limnetic stickleback species pairs; Ecological niche; Holocene; Isostatic uplift; Natural selection; Pleistocene ice sheet; Stable isotopic analysis