Parallel divergence of sympatric genetic and body size forms of Arctic charr,Salvelinus alpinus,from two Scottish lakes

F _ST and R_ST estimates for Arctic charr from six microsatelite markers collected from two neighbouring Scottish lakes, Loch Maree and Loch Stack, confirm the presence of two distinct genetic groupings representing separate populations within each lake. In both lakes, there was also a clear body size dimorphism, with large and small body size forms that segregated according to genetic grouping. There was evidence of only subtle foraging ecology differences between morphs, with the small body size morph in both lakes being more generalist in its foraging in the summer (consuming mostly plankton but also some macrobenthos) than the large body size morph, which specialized on planktonic prey. Trophic morphology (head and mouth shape) did not differ significantly between morphs (although the small sample size for Maree makes this a preliminary finding). Cluster analysis of the microsatelite data and the presence of private alleles showed that morphologically similar forms in different lakes were not genetically similar, as would be expected under a multiple invasion hypothesis. Thus, the data do not support a hypothesis of a dual invasion of both lakes by two common ancestors but instead suggest an independent origin of the two forms in each lake. Thus parallel sympatric divergence as a result of common selection pressures in both lakes is the most parsimonious explanation of the evolutionary origin of these polymorphisms. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 95 , 748–757.     

Variation in scale shape among alternative sympatric phenotypes of Arctic charr Salvelinus alpinus from two lakes in Scotland

Landmark‐based geometric morphometric analysis was used to detect differences in scale shape between ecologically distinct phenotypes of Arctic charr Salvelinus alpinus coexisting in the same lake. Relative warp analysis and standard multivariate analyses of the partial warps, obtained after a Procrustes superimposition, showed that scale landmarks were efficient in discriminating among two closely related alternative phenotypes within each of the two lakes. In Loch Tay, S. alpinus exhibited a bimodal body size‐frequency distribution among sexually mature fish, whereas in Loch Awe, S. alpinus are unimodal in body size but segregated into two distinct spawning phenotypes. In both lakes, alternative phenotypes showed significant differences in foraging ecology, habitat use and life history. It is probable that differences in scale shape reflect differences in ecology of these forms.     

Effects of population,family, and diet on craniofacial morphology of Icelandic Arctic charr (Salvelinus alpinus)

We evaluated hypotheses of intralacustrine diversification and plastic responses to two diet environments in Icelandic Arctic charr (Salvelinus alpinus). Full‐sib families of progeny of wild polymorphic charr from two lakes where morphs vary in their degree of phenotypic and ecological divergence were split, with half of the offspring reared on a benthic and half on a limnetic type of diet to estimate family norms of reaction. We focused on variation in craniofacial traits because they are probably functionally related to diet and complement a previous study of body shape in these charr. A hierarchical analysis of phenotypic variation between lakes, pairs of morphs within each lake, and two families within each morph found that phenotypic variation partitioned between families relative to morphs was reduced in the more ecologically diversified population, which is consistent with adaptive diversification. The effect size of plastic responses between lake populations was similar, suggesting little difference in the degree of canalization in contrast to a previous analysis of body form plasticity. Thus, the role that plastic morphological responses play in the adaptive diversification of morphs and different lake populations of Arctic charr may depend on the trait. © 2013 The Linnean Society of London     

Trophic polymorphism amongst Arctic charr from Loch Rannoch, Scotland

This paper describes the results of a multivariate and univariate morphometric analysis of three groups of Arctic charr Salvelinus alpinus from Loch Rannoch, Perthshire, Scotland, distinguished a priori on the basis of coloration (bright v. cryptic) and site of capture (from two locations 12 km apart). The analysis produced a clear and highly significant distinction, not only between brightly and cryptically coloured charr, but also between the two cryptically coloured groups, one of which was more robust in terms of several size-independent head measurements. Stomach analysis showed that the brightly coloured charr fed entirely on zooplankton, the less robust, cryptically coloured fish, fed on benthic macroinvertebrates, whereas a significant proportion of the diet of the more robust cryptically-coloured form consisted of other fish. Differences in length at age also distinguished the three forms, with the robust, piscivorous charr, which live longer and potentially reach a larger final size, attaining smaller sizes at a given age. These data clearly confirm the previous identification of a distinct planktivorous morph of Arctic charr in Loch Rannoch and extend the morph analysis by distinguishing two additional morphs, a benthivorous morph and a piscivorous morph that are morphologically and ecologically distinct. The results are discussed in the context of other systems in which sympatric morphs of Arctic charr have been described.     

Incipient speciation driven by phenotypic plasticity? Evidence from sympatric populations of Arctic charr

Recent models suggest that the existence of environmentally induced polymorphisms within a single population (especially those related to foraging) facilitates the process of evolutionary divergence within a single gene pool by generating distinct phenotypic modes that are exposed to differential selection. In order to test a prediction of the phenotypic plasticity model of divergence, we used a well-documented polymorphism to disentangle the relative effects of morph and rearing environment in generating phenotypic variance. We reared first-generation offspring of two sympatric morphs of Arctic charr Salvelinus alpinus in the laboratory and compared their head morphology with that of their wild parents. Morphological characters with a known functional role in foraging were highly plastic. Rearing environment accounted for the largest component of the variation in expressed phenotype, but this environmental effect overlaid a clear (but small) genetic effect. We conclude that phenotypic plasticity has played a significant role in the evolution of this trophic polymorphism, but that the evolutionary process has progressed to the point that the gene pool is now segregated.  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society , 2004, 81 , 611–618.     

Morphometric analysis of two ecologically distinct forms of Arctic charr, Salvelinus alpinus (L.), in Loch Rannoch, Scotland

Gill–netted samples of Arctic charr from Loch Rannoch, Scotland were bimodal when tested by univariate and multivariate morphometric analyses. The separation into two morphs corresponded very closely (95–98%) with fish classified subjectively in the field as benthic or pelagic, based largely on colour differences and ecological observations. The benthic charr had relatively longer heads, larger eyes and more powerful jaws than the pelagic charr. Unlike sympatric morphotypes described from Scandinavia and Greenland, neither Rannoch morph is dwarfed.     

Stable isotope analysis reveals ecological segregation in a bimodal size polymorphism in Arctic charr from Loch Tay, Scotland

The serendipitous discovery of a body-size dimorphism amongst the sexually mature Arctic charr Salvelinus alpinus of Loch Tay is described. Sexually mature Arctic charr, collected by gill netting on spawning areas, showed a clear and distinct bimodal size distribution with no overlap in fork length distributions. The upper (19–29 cm L _F) and lower modes (8–16 cm) were not solely the result of sex or age differences. Analysis of stable isotope ratios of C and N in muscle showed highly significant differences in mean δ¹³C and δ¹⁵N between populations, demonstrating a difference in trophic ecology between the two body-size morphs. Overlap in the range of δ¹³C and δ¹⁵N values for the two morphs, however, suggested that they occasionally shared a common diet. Data from other studies strongly indicated that the proximate and ultimate mechanisms that control body-size dimorphisms in Arctic charr differed between sites. Clear differences in trophic ecology in the Loch Tay Arctic charr suggested that the available feeding opportunity may differ for the two morphs. The most likely proximate mechanism resulting in this dimorphism is growth rate differences resulting from differences in food availability for the two subgroups occupying alternative foraging niches in Loch Tay.     

The functional significance of inherited differences in feeding morphology in a sympatric polymorphic population of Arctic charr

Artificially fertilised eggs from wild-caught Arctic charr parents of two sympatric morphs (benthivorous and planktivorous) from Loch Rannoch, Scotland were reared in the laboratory under identical conditions. During the subsequent 2 years, aspects of their trophic anatomy and feeding behaviour were compared. As previously described for wild-caught fish, charr derived from the benthivorous morph had an increasingly wider mouth gape for a given body length than those derived from the planktivorous morph. The functional significance of these differences in gape was tested by comparing the maximum size of prey that could be handled by each of the two morphs. In both forms, a larger gape enabled larger food particles to be eaten, but the elevation of the regression of maximum prey size on gape was higher in the benthivorous form, indicating the existence of additional morphological and/or behavioural differences influencing the size of prey consumed. When offered a choice between a typical benthic prey item and a typical pelagic food item, charr of benthivorous origin were more likely to feed on the former, whereas those of planktivorous origin were more likely to feed on the latter. Thus inherited differences in gape place constraints on foraging ability and are associated with inherited differences in dietary preference. We conclude that the functional significance of the foraging specialisations indicate a strong selection pressure for the evolution of the divergence and propose that heterochronic growth is the mechanism resulting in the divergence of tropic anatomy.     

Morphological divergence between three Arctic charr morphs – the significance of the deep‐water environment

Morphological divergence was evident among three sympatric morphs of Arctic charr (Salvelinus alpinus (L.)) that are ecologically diverged along the shallow‐, deep‐water resource axis in a subarctic postglacial lake (Norway). The two deep‐water (profundal) spawning morphs, a benthivore (PB‐morph) and a piscivore (PP‐morph), have evolved under identical abiotic conditions with constant low light and temperature levels in their deep‐water habitat, and were morphologically most similar. However, they differed in important head traits (e.g., eye and mouth size) related to their different diet specializations. The small‐sized PB‐morph had a paedomorphic appearance with a blunt head shape, large eyes, and a deep body shape adapted to their profundal lifestyle feeding on submerged benthos from soft, deep‐water sediments. The PP‐morph had a robust head, large mouth with numerous teeth, and an elongated body shape strongly related to their piscivorous behavior. The littoral spawning omnivore morph (LO‐morph) predominantly utilizes the shallow benthic–pelagic habitat and food resources. Compared to the deep‐water morphs, the LO‐morph had smaller head relative to body size. The LO‐morph exhibited traits typical for both shallow‐water benthic feeding (e.g., large body depths and small eyes) and planktivorous feeding in the pelagic habitat (e.g., streamlined body shape and small mouth). The development of morphological differences within the same deep‐water habitat for the PB‐ and PP‐morphs highlights the potential of biotic factors and ecological interactions to promote further divergence in the evolution of polymorphism in a tentative incipient speciation process. The diversity of deep‐water charr in this study represents a novelty in the Arctic charr polymorphism as a truly deep‐water piscivore morph has to our knowledge not been described elsewhere.     

Extensive genetic differentiation between recently evolved sympatric Arctic charr morphs

The availability of diverse ecological niches can promote adaptation of trophic specializations and related traits, as has been repeatedly observed in evolutionary radiations of freshwater fish. The role of genetics, environment, and history in ecologically driven divergence and adaptation, can be studied on adaptive radiations or populations showing ecological polymorphism. Salmonids, especially the Salvelinus genus, are renowned for both phenotypic diversity and polymorphism. Arctic charr (Salvelinus alpinus) invaded Icelandic streams during the glacial retreat (about 10,000 years ago) and exhibits many instances of sympatric polymorphism. Particularly, well studied are the four morphs in Lake Þingvallavatn in Iceland. The small benthic (SB), large benthic (LB), planktivorous (PL), and piscivorous (PI) charr differ in many regards, including size, form, and life history traits. To investigate relatedness and genomic differentiation between morphs, we identified variable sites from RNA‐sequencing data from three of those morphs and verified 22 variants in population samples. The data reveal genetic differences between the morphs, with the two benthic morphs being more similar and the PL‐charr more genetically different. The markers with high differentiation map to all linkage groups, suggesting ancient and pervasive genetic separation of these three morphs. Furthermore, GO analyses suggest differences in collagen metabolism, odontogenesis, and sensory systems between PL‐charr and the benthic morphs. Genotyping in population samples from all four morphs confirms the genetic separation and indicates that the PI‐charr are less genetically distinct than the other three morphs. The genetic separation of the other three morphs indicates certain degree of reproductive isolation. The extent of gene flow between the morphs and the nature of reproductive barriers between them remain to be elucidated.     

Characterizing neutral and adaptive genomic differentiation in a changing climate: The most northerly freshwater fish as a model

Arctic freshwater ecosystems have been profoundly affected by climate change. Given that the Arctic charr (Salvelinus alpinus) is often the only fish species inhabiting these ecosystems, it represents a valuable model for studying the impacts of climate change on species life‐history diversity and adaptability. Using a genotyping‐by‐sequencing approach, we identified 5,976 neutral single nucleotide polymorphisms and found evidence for reduced gene flow between allopatric morphs from two high Arctic lakes, Linne'vatn (Anadromous, Normal, and Dwarf) and Ellasjøen (Littoral and Pelagic). Within each lake, the degree of genetic differentiation ranged from low (Pelagic vs. Littoral) to moderate (Anadromous and Normal vs. Dwarf). We identified 17 highly diagnostic, putatively adaptive SNPs that differentiated the allopatric morphs. Although we found no evidence for adaptive differences between morphs within Ellasjøen, we found evidence for moderate (Anadromous vs. Normal) to high genetic differentiation (Anadromous and Normal vs. Dwarf) among morphs within Linne'vatn based on two adaptive loci. As these freshwater ecosystems become more productive, the frequency of sympatric morphs in Ellasjøen will likely shift based on foraging opportunities, whereas the propensity to migrate may decrease in Linne'vatn, increasing the frequency of the Normal morph. The Dwarf charr was the most genetically distinct group. Identifying the biological basis for small body size should elucidate the potential for increased growth and subsequent interbreeding with sympatric morphs. Overall, neutral and adaptive genomic differentiation between allopatric and some sympatric morphs suggests that the response of Arctic charr to climate change will be variable across freshwater ecosystems.     

Genetic divergence between sympatric Arctic charr Salvelinus alpinus morphs in Gander Lake,Newfoundland: roles of migration,mutation and unequal effective population sizes

A suite of 10 microsatellite loci was used to examine genetic divergence between two sympatric morphs of Arctic charr Salvelinus alpinus (‘dark’ and ‘pale’) inhabiting Gander Lake, Newfoundland. Results can be summarized as follows: (1) the morphs are strongly reproductively isolated – gene flow–migration estimates were consistently low in long and short‐term evolutionary timescales of analysis; (2) intermorph divergence based on allele size (R_ST) was significantly larger than those based on allele state (θ) implying a cumulative effect of stepwise‐like mutations; (3) historical (coalescent) and current (linkage disequilibrium) point estimates of effective population size (N_e) were consistently higher for dark than for pale S. alpinus. The first and second findings lend support to the hypothesis that divergence between forms may have preceded the last glacial period (ending c. 12 000 years bp ). The third finding argues for significant differences in habitat quantity and quality between morphs, which were emphasized in a previous study. Overall, these analyses underscore the importance of genetic assessment and monitoring in the conservation of fish diversity, with emphasis on ‘rare’ or under‐represented forms among temperate species pairs.     

Macrophyte development in unimpacted lowland rivers in Poland

Three ecologically and morphologically distinct forms of Arctic charr (Salvelinus alpinus L.) have been identified in Loch Rannoch, Scotland, whose evolutionary status and origins are incompletely understood. A study was made of restriction fragment length polymorphism (RFLPs) detected variation in the D-loop, ND1 and cytochrome b regions of the mitochondrial genome, encompassing >3500 bp. Eight RFLP haplotypes were identified that clustered into three distinct clans based on restriction differences and into four clans based on sequence differences. Significant differences in RFLP frequencies were found among all morph groups. The pelagic morph was highly divergent from the two benthic forms, with the benthic forms having variants from only one genetic clan while the pelagic was dominated by a single variant from another clan. The relative divergence observed among benthic and pelagic forms is ~10 fold greater when nucleotide divergence among the haplotypes, as well as haplotype frequency differences, is taken into account. Sequence divergence between haplotypes in the two main clans is of a similar order to that between haplotypes in these clans and a charr from North America. In contrast, divergence among the two benthic morphs relates entirely to differences in haplotype frequencies. The study confirms the genetic distinctiveness of the pelagic and benthic forms as well as of the two benthic forms. It strongly supports previous evidence that the genetic divergence between the pelagic and benthic populations is allopatric in origin. Additionally, the results strongly suggest that the two benthic populations have undergone peripatric divergence through the sequential colonisation of the two basins by one lineage, followed by their spatial separation and reproductive isolation.     

Epigenetic regulation of trophic morphology through feeding behaviour in Arctic charr, Salvelinus alpinus

Several models of speciation suggest that in species that are phenotypically plastic, selection can act on phenotypic variation that is environmentally induced in the earliest stages of divergence. One trait that could be subject to this process is foraging behaviour, where discrete foraging strategies are common. One species which is highly plastic in the expression of phenotype, the Arctic charr, Salvelinus alpinus (L.), is characterized by discrete variation in the anatomy of the head and mouthparts. These traits have been shown to have a functional significance, but the expression of which is thought to be at least partly phenotypically plastic. Here we test the hypothesis that foraging behaviour may regulate the anatomy of the head and mouthparts in Arctic charr. In a dyad experiment, size‐matched pairs of fish from a mixed family group were fed a diet of either Mysis (a hard‐bodied shrimp) or Chironomid larvae. Nine morphometric measures of head dimensions that describe wild trophic morphs were measured at the start of the experiment and 24 weeks later. Principal component scores of size‐corrected morphometric measures showed highly significant differences between fish exposed to the two diets. Univariate ANOVA analysis of the head morphometric variables showed that fish fed on Chironomids developed longer, wider jaws, longer heads and a larger eye for a given body length than did those fish fed upon Mysis. We conclude that foraging anatomy in Arctic charr is phenotypically plastic and that variation in foraging behaviour that results in feeding specialization in the wild could induce variation in head anatomy. This in turn could reinforce foraging specialization. Very rapid epigenetic divergence into distinct feeding morphs (as demonstrated here) would allow selection to act at more than one mode and thus could promote rapid evolutionary divergence, initially prior to genetic segregation, in species which are highly plastic. © 2003 The Linnean Society of London, Biological Journal of the Linnean Society , 2003, 78 , 43–49.     

The trade‐off between fecundity and egg size in a polymorphic population of Arctic charr (Salvelinus alpinus (L.)) in Skogsfjordvatn,subarctic Norway

Reproductive traits differ between intralacustrine Arctic charr morphs. Here, we examine three sympatric lacustrine Arctic charr morphs with respect to fecundity, egg size and spawning time/site to assess reproductive investments and trade‐offs, and possible fitness consequences. The littoral omnivore morph (LO‐morph) utilizes the upper water for feeding and reproduction and spawn early in October. The large profundal piscivore morph (PP‐morph) and the small profundal benthivore morph (PB‐morph) utilize the profundal habitat for feeding and reproduction and spawn in December and November, respectively. Females from all morphs were sampled for fecundity and egg‐size analysis. There were large differences between the morphs. The PB‐morph had the lowest fecundity (mean = 45, SD = 13) and smallest egg size (mean = 3.2 mm, SD = 0.32 mm). In contrast, the PP‐morph had the highest fecundity (mean = 859.5, SD = 462) and the largest egg size (mean = 4.5 mm, SD = 0.46 mm), whereas the LO‐morph had intermediate fecundity (mean = 580, SD = 225) and egg size (mean = 4.3, SD = 0.24 mm). Fecundity increased with increasing body size within each morph. This was not the case for egg size, which was independent of body sizes within morph. Different adaptations to feeding and habitat utilization have apparently led to a difference in the trade‐off between fecundity and egg size among the three different morphs.     

Polymorphism and speciation in Arctic charr

The Arctic charr Salvelinus alpinus exhibits 1–4 sympatric morphs in postglacial lakes, of which one or two are epibenthic zoobenthos feeders, one is a limnetic planktivore and one is a piscivorous form. In addition, northern rivers support partly migratory populations with anadromous and freshwater resident fish. The morphs vary in their coloration, morphology, life history, behaviour and genetic characteristics. The morphs usually differentiate according to their ontogenetic stage at maturity, which parallels paedomorphism in amphibians. The young usually start as epibenthic zoobenthivores, but may become pelagic at a certain size according to the predation risk experienced at that time. From a length of > 20–25 cm, charr start to become piscivorous. The proportion of piscivorous fish increases with increasing body size. In partly anadromous populations, fish that mature before smolting become freshwater resident, the others anadromous. In some rivers, the morphs occupy separate niches (epibenthic and limnetic), from emergence onwards. The morphs exhibit different degrees of reproductive isolation that vary from a high degree of interbreeding to complete isolation. Usually, they spawn within morph (assortative mating), but alternative male mating behaviour (sneaking, fighting) may occur in stream‐spawning populations and at great depths in lakes. Morphologically specialized morphs appear to feed more effectively than intermediate forms, and selection according to feeding mode, site fidelity and associated assortative mating are prerequisites for the evolution of the different morphs. Charr morphs develop into stable feeding niches under conditions of intense intraspecific competition when there is little competition with other species. Sympatric morphs exhibit different degrees of speciation, but similar morphs in different systems are not individual species because of (1) their polyphyletic origin, (2) the supporting systems are often young, transient environments making the future situation for the populations uncertain, and (3) the genetic differentiation among morphs is low. Sympatric morphs may interbreed and produce fertile hybrids. Nevertheless, sympatric charr morphs should be managed as separate species. Changes in the natural conditions or human impacts to which the morphs are adapted will have a strong influence on the persistence and survival of each different morph.     

Evidence for genetic differences in the offspring of two sympatric morphs of Arctic charr

The sub-arctic Lake Fjellfrøsvatn, northern Norway, has two morphs of Arctic charr that are reproductively isolated because they spawn 5 months apart. The smaller morph (≤14 cm L_F ) is confined to the profundal zone of the lake and the larger morph is mainly littoral. Three hypotheses were tested: (i) the offspring of the profundal Arctic charr grow slower than the offspring of the littoral Arctic charr under identical conditions, thus indicating a genetic basis for the slow growth of the profundal Arctic charr in the wild; (ii) the wild phenotypes of the two morphs are morphometrically different and the differences are persistent in the offspring; (iii) the offspring of the two morphs have different behaviour traits under similar treatments. The first hypothesis was rejected; offspring of the profundal morph grew slightly better than offspring of the littoral morph at 10° C in the laboratory. The second and third hypotheses were supported by the data. Wild-caught fish of the two morphs differed in several morphometric characters and most of the differences persisted in the offspring. In the laboratory, offspring of the littoral morph were more active, more aggressive and more pelagic than offspring of the profundal morph and naive offspring of the profundal morph were more effective in eating live chironomid larvae than were offspring of the littoral morph. The data for morphometry and behaviour, but not growth, provide evidence for genetic differences between the two Arctic charr morphs of Fjellfrøsvatn.     

Molecular genetic examination of the polymorphic Arctic charr Salvelinus alpinus of Thingvallavatn, Iceland

Thingvallavatn, Iceland contains two sympatric morphotypes (benthic and limnetic) of Arctic charr Salvelinus alpinus. Each morphotype is composed of two morphs and these differ markedly in ecology, behaviour and life history. We used molecular genetic approaches to test whether (i) genetic heterogeneity exists among morphs and (ii) if morphs arose in allopatry and came into secondary contact or arose sympatrically within the lake through genetic segregation and/or phenotypic plasticity. Direct sequencing of 275 bp of the mitochondrial DNA (mtDNA) control region, mtDNA restriction fragment length polymorphisms and single locus minisatellite analyses detected insufficient variation to test our hypotheses. Analysis of multilocus minisatellite band sharing detected no significant differences between morphs within the same morphotype. However, significant differences among morphs belonging to different morphotypes suggest some genetic heterogeneity in Thingvallavatn charr. Limnetic charr from Thingvallavatn were more similar to sympatric benthic charr than to allopatric limnetics from two other Icelandic lakes. This suggests that the Thingvallavatn morphs arose sympatrically within the lake rather than in allopatry followed by secondary contact.     

Evidence for divergence and adaptive isolation in post‐glacially derived bimodal allopatric and sympatric rainbow smelt populations

The roles of ecological speciation and reinforcement in the formation of contemporary diversity remain contentious. In the present study, we contrast phenotypic and molecular divergence within morphologically diverged bimodal sympatric and allopatric pairs of rainbow smelt, Osmerus mordax. We hypothesize that, in sympatry, evidence of selection associated with resource partitioning will be visible through strong divergence, reinforcement, and greater character displacement. Parallel morphological divergence was observed between the two trophic forms (macrophagous and microphagous), with several examples of greater trait divergence in sympatry than allopatry. Mitochondrial DNA sequence analysis indicated no association between historical clades and morphology; however, Bayesian clustering using microsatellites supported the isolation of these morphs under both allopatry and sympatry. Estimates of genetic isolation were one order of magnitude lower than measures of morphological divergence, consistent with a hypothesis of strong contemporary selection. Using experimental crosses, we obtained similar rates of fertilization success among the allopatric hybrid and pure crosses; whereas, in the sympatric hybrid crosses, fertilization rates dropped by 30–50%, suggesting a clear role for reinforcement through prezygotic incompatibilities. The present study supports the hypothesis that processes of post‐glacial radiation and diversification differ between sympatry and allopatry, and indicates a role for reinforcement and ecological processes in recent sympatric diversification. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 101 , 583–594.     

Incipient speciation through niche expansion: an example from the Arctic charr in a subarctic lake

Two reproductive isolated morphs of Arctic charr (Salvelinus alpinus), termed profundal and littoral charr according to their different spawning habitats, co-occur in the postglacial lake Fjellfr?svatn in North Norway. All profundal charr live in deep water their entire life and have a maximum size of 14cm, while the littoral charr grow to 40cm. Some small and young littoral charr move to the profundal zone in an ontogenetic habitat shift in the ice-free season and the rest of the population remains in epilimnic waters. The two morphs had different diet niches in the profundal zone: the profundal charr ate typical soft-bottom prey (chironomid larvae, pea mussels and benthic copepods), while the young littoral charr mainly consumed crustacean zooplankton. In four other lakes without a profundal morph (i.e. monomorphic populations), young charr also performed ontogenetic habitat shifts to the profundal zone and fed on zooplankton. The profundal morph of Fjellfr?svatn therefore utilize a food resource niche that neither the littoral morph nor comparable monomorphic populations exploit. This suggests that intraspecific resource competition has driven incipient ecological speciation of the profundal charr of Fjellfr?svatn. The exploitation of the soft-bottom resources by the profundal charr supports earlier experimental findings that the profundal morph is genetically different in trophic behaviour and morphology. The sympatric ecological divergence within the profundal habitat is possible because unexploited food resources (soft-bottom profundal prey) are available. Apparently, this represents a case of incipient segregation by expansion to new resource types (niche invasion), and not by subdivision of one broad ancestral niche.