585 © 2020 The Linnean Society of London, Biological Journal of the Linnean Society, 2020, 131, 585–599 Biological Journal of the Linnean Society, 2020, 131, 585–599. With 7 figures. Intraspecific variation and structuring of phenotype in a lake-dwelling species are driven by lake size and elevation J. PETER KOENE 1,2, * , , KATHRYN R. ELMER 2, , and COLIN E. ADAMS 1,2, 1 Scottish Centre for Ecology and the Natural Environment (SCENE), University of Glasgow, Rowardennan, Glasgow G63 0AW, UK 2 Institute of Biodiversity, Animal Health and Comparative Medicine, College of Medical, Veterinary and Life Science, University of Glasgow, Glasgow G12 8QQ, UK Received 15 May 2020; revised 4 August 2020; accepted for publication 7 August 2020 The fragmented, heterogeneous and relatively depauperate ecosystems of recently glaciated lakes present contrasting ecological opportunities for resident fish. Across a species, local adaptation may induce diverse and distinct phenotypic responses to various selection pressures. We tested for intraspecific phenotypic structuring by population in a common native lake-dwelling fish species across a medium-scale geographic region with considerable variation in lake types. We investigated potential lake-characteristic drivers of trophic morphology. Using geometric morphometric techniques, we quantified the head shapes of 759 adult brown trout (Salmo trutta L.) from 28 lakes and reservoirs across Scotland. Multivariate statistical analyses showed that almost all populations differed from one another. Trout from larger and deeper lakes had deeper, but shorter heads, and smaller eyes. Higher elevation lakes were associated with fish with shorter heads and jaws. Within-population shape variation also differed by population, and was positively correlated with lake surface area and depth. Trout within reservoirs differed subtly from those in natural lakes, having larger eyes, shorter jaws and greater variability. This study documents an extraordinary morphological variation between and within populations of brown trout, and demonstrates the role of the extrinsic environment in driving phenotypic structuring over a medium-scale and varied geographic area. ADDITIONAL KEYWORDS: brown trout – ecosystem-size – elevation – morphology – natural lake – phenotype – reservoir – structuring – trophic. INTRODUCTION Variation is ubiquitous in nature: individuals differ from one another, even within the same species, in the phenotypic traits of morphology, physiology, behaviour and life history. This is often, but not necessarily, preceded by genetic differentiation (Levis & Pfennig, 2016). Although within-species variations in phenotype and genotype may be continuous in nature ( Hendry et al. , 2009), it is evident that a number of species display measurable genetic and/ or phenotypic discontinuities, resulting in more or less clearly defined groupings of organisms within a species (Skúlason & Smith, 1995). This results in multimodal genetic or phenotype frequency peaks (Hendry et al., 2007; Elmer et al., 2010). Collectively, these intraspecific genetic and/or phenotypic patterns may be termed ‘intraspecific structuring’. Interest in structuring is based on the assumption that it is driven by evolution, and that closer examination of its patterns may beget insights into evolutionary processes (Schluter, 2000; Nosil, 2012). Structuring of phenotype may reflect differential and localized evolutionary responses to environments ( Bolnick et al. , 2011 ). This is particularly true when structuring manifests between discrete habitats. Such intraspecific structuring based upon differential niche use is relatively common amongst the major vertebrate groups ( Skúlason & Smith, 1995), but appears to be more prevalent in aquatic systems than in terrestrial, in freshwater systems more than marine, in lakes more than rivers, and in lakes that *Corresponding author. E-mail: j.peter.koene@gmx.at Downloaded from https://academic.oup.com/biolinnean/article/131/3/585/5937293 by guest on 14 January 2022