<b>Abstract</b><br/>Ecologically, morphologically, and genetically distinct populations within single taxa often co-exist in postglacial lakes and have provided important model systems with which to investigate ecological and evolutionary processes such as niche partitioning and ecological speciation. Within the Salmonidae, these species complexes have been well studied, particularly within the Coregonus clupeaformis-C. laveratus (lake and European whitefish, respectively) group, but the phenomenon has been less well documented in the other whitefish genera, Prosopium and Stenodus. Here, we examined the morphology, feeding biology, and genetic structure of three putative forms of the pygmy whitefish, Prosopium coulterii, that were first reported from Chignik Lake, southwestern Alaska, over 40 years ago. Field collections and morphological analyses resolved a shallow water (< 5 m depth) low gill raker count form (< 15 first arch gill rakers), a deepwater (> 30 m), low gill raker form, and a deepwater, high gill raker count (> 15 gill rakers) form. The two low gill raker count forms fed almost exclusively on benthic invertebrates (mostly chironomids), while the deepwater, high gill raker count form fed almost exclusively on zooplankton, differences in diet that were also reflected in differences both in δ13C and δ15N stable isotopes. All three forms were characterized by the same major mitochondrial DNA clade that has been associated with persistence in, and postglacial dispersal from, a Beringian glacial refugium. Analysis of variation at nine microsatellite DNA loci indicated low, but significant differentiation amongst forms, especially between the two low gill raker count forms and the high gill raker count form. The extent of differentiation along phenotypic (considerable) and genetic (subtle) axes amongst the Chignik Lake forms is similar to that found amongst distinct taxa of Prosopium found in pre-glacial Bear Lake (Utah-Idaho, USA) which is probably at least ten times older than Chignik Lake. Our analyses illustrate the potential for the postglacial differentiation in traits subject to divergent natural selection across variable environments.

<b>Abstract</b><br/>The evolution of population structure in marine organisms and its relevance to conservation has recently received increasing attention. We tested the degree of genetic subdivision among ten populations of copper rockfish (Sebastes caurinus) representing paired samples of outer coast and the heads of five replicate sounds on the west coast of Vancouver Island, British Columbia using 17 microsatellite DNA loci. Overall, subdivision (FST) was low (FST = 0.031), but consistently higher between paired coast and head of inlet sites (mean FST = 0.047) compared to among five coast sites (mean FST = -0.001) or among the five head of inlet sites (mean FST = 0.026). Heterozygosity, allelic richness, and estimates of effective population size were also consistently lower in head of inlet sites than in coast sites. Bayesian analysis of population structure identified two genetic groups across all samples, a single genetic group amongst only coast samples, two genetic groups amongst head of inlet samples, and two genetic groups within each sound analysed separately. Head of inlet copper rockfish were also consistently shorter with lower condition factors, and grew more slowly than fish collected from coast sites. Our results implicate coast- head of inlet habitat transitions in driving the evolution of population structure, likely resulting from reduced physical connectivity and selection against immigrants in contrasting environments. Coast sites appear to be well served by existing marine protected areas. By contrast, head of inlet sites may require more specific local conservation measures as they appear to be less well connected to adjacent coast sites as well as to each other.

<b>Abstract</b><br/>Ecological speciation occurs when populations evolve reproductive isolation as a result of divergent natural selection. This isolation can be influenced by many potential reproductive barriers, including selection against hybrids, selection against migrants, and assortative mating. How and when these barriers act and interact in nature is understood for relatively few empirical systems. We used a mark-recapture experiment in a contact zone between lake and stream three-spined sticklebacks (Gasterosteus aculeatus, Linnaeus) to evaluate the occurrence of hybrids (allowing inferences about reproductive isolation), the inter-annual survival of hybrids (allowing inferences about selection against hybrids), and the shift in lake-like versus stream-like characteristics (allowing inferences about selection against migrants). Genetic and morphological data suggest the occurrence of hybrids and no selection against hybrids in general, a result contradictory to a number of other studies of sticklebacks. However, we did find selection against more lake-like individuals, suggesting a barrier to gene flow from the lake into the stream. Combined with previous work on this system, our results suggest that multiple (most weakly and often asymmetric) barriers must be combining to yield substantial restrictions on gene flow. This work provides evidence of a reproductive barrier in lake-stream sticklebacks and highlights the value of assessing multiple reproductive barriers in natural contexts.

<b>Abstract</b><br/>Ecological speciation is the evolution of reproductive isolation as a direct or indirect consequence of divergent natural selection. Reduced performance of hybrids in nature is thought to be an important process by which natural selection can favour the evolution of assortative mating and drive speciation. Benthic and limnetic sympatric species of threespine stickleback (Gasterosteus aculeatus) are adapted to alternative trophic niches (bottom browsing versus open water planktivory, respectively) and reduced feeding performance of hybrids is thought to have contributed to the evolution of reproductive isolation. We tested this “hybrid-disadvantage hypothesis” by inferring growth rates from otoliths sampled from wild, free-ranging benthic, limnetic, and hybrid sticklebacks from two lakes. There were significant differences in growth rate between lakes, life-history stages, and among years (maximum P = 0.02), as well as interactions between most factors, but not between hybrid and parental species sticklebacks in most comparisons. Our results provide little evidence of a growth disadvantage in hybrid sticklebacks when free-ranging in nature. While trophic ecology per se may contribute less to ecological speciation than envisioned, it may act in concert with other aspects of stickleback biology, such as interactions with parasites, predators, competitors and/or sexual selection, to present strong multifarious selection against hybrids.

<b>Abstract</b><br/>The Nooksack dace (Pisces: an undescribed putative taxon within Rhinichthys) and longnose dace (Rhinichthys cataractae) are two forms within the R. cataractae species complex that are distinguishable from one another by mitochondrial (mt) DNA divergence of 2–3%, as well as by subtle morphological differences. The two forms are found in allopatry in south-eastern British Columbia (BC), Canada, and adjacent areas of western Washington, USA, and are sympatric in three streams in the lower Fraser River valley, BC, and may represent cryptic species. We assayed 12 morphometric traits and two meristic characters (N = 582; 23 sampling locations) to test for diagnosability of the two dace, as well as to test for morphological differentiation by mtDNA type in sympatry. We then employed a 10-locus microsatellite DNA assay (N = 374; 12 sampling locations) to test for genetic distinction between Nooksack dace and longnose dace in sympatry. We found that the two dace could not be reliably differentiated morphologically: there was overlap in all characters measured, and sampling location had a much larger effect on morphology than mtDNA group. Microsatellite analysis showed no distinction by mtDNA type in localities with sympatric dace, indicating complete admixture between the sympatric Nooksack dace and longnose dace. The Nooksack dace and longnose dace provide an example of ‘ephemeral speciation’: two lineages that, despite an estimated 1.1 Myr of isolation, have developed no apparent barriers to reproduction and appear to have collapsed into a single interbreeding population where they come into secondary contact. Nonetheless, the zone of secondary contact and the diagnosability of the Nooksack dace in terms of mtDNA represent significant aspects of the evolutionary legacy within R. cataractae and support its conservation importance.

<b>Abstract</b><br/>Range expansion in north-temperate fishes subsequent to the retreat of the Wisconsinan glaciers has resulted in the rapid colonization of previously unexploited, heterogeneous habitats and, in many situations, secondary contact among conspecific lineages that were once previously isolated. Such ecological opportunity coupled with reduced competition likely promoted morphological and genetic differentiation within and among post-glacial fish populations. Discrete morphological forms existing in sympatry, for example, have now been described in many species, yet few studies have directly assessed the association between morphological and genetic variation. Morphotypes of Lake Trout, Salvelinus namaycush, are found in several large-lake systems including Great Bear Lake (GBL), Northwest Territories, Canada, where several shallow-water forms are known. Here, we assess microsatellite and mitochondrial DNA variation among four morphotypes of Lake Trout from the five distinct arms of GBL, and also from locations outside of this system to evaluate several hypotheses concerning the evolution of morphological variation in this species. Our data indicate that morphotypes of Lake Trout from GBL are genetically differentiated from one another, yet the morphotypes are still genetically more similar to one another compared with populations from outside of this system. Furthermore, our data suggest that Lake Trout colonized GBL following dispersal from a single glacial refugium (the Mississippian) and support an intra-lake model of divergence. Overall, our study provides insights into the origins of morphological and genetic variation in post-glacial populations of fishes and provides benchmarks important for monitoring Lake Trout biodiversity in a region thought to be disproportionately susceptible to impacts from climate change.

<b>Abstract</b><br/>Aims: We investigated post-glacial recolonization of the North American Arctic by Arctic char (Salvelinus alpinus) and examined potential hybridization between different glacial lineages upon secondary contact. Location: North American Arctic and adjacent areas. Methods: We collected mtDNA sequence data from 1355 individuals from 110 sampling locations and data from nine microsatellite loci from 931 individuals from 37 locations. We assessed the phylogenetic relationships and geographical distribution of mtDNA haplotypes and conducted historical demographic analyses. We used a Bayesian clustering analysis method to detect potential hybridization between glacial lineages. Results: Two highly divergent mtDNA lineages were identified in the Arctic region with distinct but overlapping geographic distributions: one in Beringia and the other over the entire Arctic Archipelago and coastal mainland east of Alaska. The microsatellite data also implied the existence of these two lineages. Evidence of hybridization was detected between the Arctic lineage and an Atlantic lineage in eastern North America. Main conclusions: Our data suggested survival and recolonization from two northern glacial refugia: one in Beringia and another in a smaller refugium, perhaps in the Arctic Archipelago itself or a separate refugium within Beringia. Patterns of hybridization detected supported the presence of a secondary contact zone between glacial lineages in the eastern Canadian Arctic.

<b>Abstract</b><br/>Census population size (Nc) is crucial to the development of resource management strategies, however, monitoring the effective population size (Ne) of managed populations has proliferated because of this parameter’s relationship to the short-term impacts of genetic stochasticity and long-term population viability. Thus, having a sound understanding of both Nc and Ne, including population connectivity, provides valuable insights into both the demographic and genetic risks to extinction. Here, we assessed microsatellite DNA variation in four (of five known) anadromous northern Dolly Varden (NDV, Salvelinus malma malma) populations from Canada’s western Arctic region, to estimate Ne using both temporal-based and single-sample estimators and to test for associations between Ne and Nc. We also employed approximate Bayesian computation (ABC) to evaluate several evolutionary scenarios that have potentially shaped contemporary population structure in this species, focusing particularly on population size and connectivity. We found evidence for moderate to large contemporary and historical Ne, suggesting that short- and long-term extinction risks are low for these populations. Estimates of contemporary and long-term Ne were variable within and among populations and overall estimates could not be reliably linked with Nc or available spawning habitat. The overall estimate of Ne/Nc, was 0.152 and ranged from 0.024 to 0.442 when including errors around the estimate of Ne and Nc. Finally, ABC analyses suggest that NDV had a common origin followed by divergence in isolation while maintaining large effective sizes, but also that these populations were bottlenecked in the past, likely the result of post-glacial colonization processes. These results corroborate indications of limited gene flow at present, indicating independent demographic and evolutionary trajectories that imply NDV is best managed on a per-river-population basis. Overall, the results of this study further our general understanding of Ne, Ne/Nc and demographic independence in NDV, and provide a comprehensive and quantitative assessment of the potential genetic and demographic risk status of Arctic anadromous salmonids, including baselines for future monitoring.

<b>Abstract</b><br/>The Northern Dolly Varden (Salvelinus malma malma) displays variable life-history types and occupies freshwater habitats with varying levels of connectivity. Here, we assayed microsatellite DNA variation in Northern Dolly Varden from the western Canadian Arctic to resolve landscape and life history variables driving variation in genetic diversity and population structure. Overall, genetic variation was highest in anadromous populations and lowest in those isolated above waterfalls with stream-resident forms intermediate between the two. Anadromous and isolated populations were genetically divergent from each other while no genetic differentiation was detectable between sympatric anadromous and stream-resident forms. Population structure was stable over 25 years, hierarchically organized and conformed to an isolation-by-distance pattern, but stream-isolated forms often deviated from these patterns. Gene flow occurred primarily among Yukon North Slope populations and between sympatric anadromous and resident forms. These results were sex-dependent to some extent, but were influenced more by reproductive status and life history. Our study provides novel insights into the life history, population demographic and habitat variables that shape the distribution of genetic variation and population structure in Arctic fluvial habitats while providing a spatial context for management and conservation.

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