Ecological speciation in postglacial European whitefish: rapid adaptive radiations into the littoral,pelagic, and profundal lake habitats

Understanding how a monophyletic lineage of a species diverges into several adaptive forms has received increased attention in recent years, but the underlying mechanisms in this process are still under debate. Postglacial fishes are excellent model organisms for exploring this process, especially the initial stages of ecological speciation, as postglacial lakes represent replicated discrete environments with variation in available niches. Here, we combine data of niche utilization, trophic morphology, and 17 microsatellite loci to investigate the diversification process of three sympatric European whitefish morphs from three northern Fennoscandian lakes. The morphological divergence in the gill raker number among the whitefish morphs was related to the utilization of different trophic niches and was associated with reproductive isolation within and across lakes. The intralacustrine comparison of whitefish morphs showed that these systems represent two levels of adaptive divergence: (1) a consistent littoral–pelagic resource axis; and (2) a more variable littoral–profundal resource axis. The results also indicate that the profundal whitefish morph has diverged repeatedly from the ancestral littoral whitefish morph in sympatry in two different watercourses. In contrast, all the analyses performed revealed clustering of the pelagic whitefish morphs across lakes suggesting parallel postglacial immigration with the littoral whitefish morph into each lake. Finally, the analyses strongly suggested that the trophic adaptive trait, number of gill rakers, was under diversifying selection in the different whitefish morphs. Together, the results support a complex evolutionary scenario where ecological speciation acts, but where both allopatric (colonization history) and sympatric (within watercourse divergence) processes are involved.     

Inter- and intra-morph patterns in helminth communities of sympatric whitefish morphs

Infection patterns of trophically transmitted helminth parasites were compared with feeding ecology in two sympatric whitefish Coregonus lavaretus morphs from two lake systems in northern Norway. In both lakes, the pelagic morph was an obligate zooplanktivore, while the benthic morph utilized both the benthivore and zooplanktivore trophic niches. The differences in niche utilization between the two morphs were associated with differences in trophic morphology (gill raker numbers), suggesting that they were genetically dissimilar and reproductively isolated. The benthic morph had the highest number of helminth species, probably because they exhibited a broader niche width compared to the pelagic morph. In both lakes, the species composition and intensities of helminths reflected the trophic diversification of the whitefish ecotypes with respect to different habitat choice (benthic v . pelagic) and dietary specialization (benthivore v . zooplanktivore feeding strategies within the benthic whitefish morph). Zooplanktivorous fish from both morphs acquired parasites mainly from pelagic copepods and in almost equal quantities. The benthivore feeders within the benthic morph had the highest proportion of parasites with transmission stages from benthic organisms. Host feeding behaviour seemed to be a major determinant of the helminth community structure, and helminths appeared to be useful indicators of long-term trophic specialization of whitefish ecotypes.     

Genomic insights into the vulnerability of sympatric whitefish species flocks

The erosion of habitat heterogeneity can reduce species diversity directly but can also lead to the loss of distinctiveness of sympatric species through speciation reversal. We know little about changes in genomic differentiation during the early stages of these processes, which can be mediated by anthropogenic perturbation. Here, we analyse three sympatric whitefish species (Coregonus spp) sampled across two neighbouring and connected Swiss pre‐alpine lakes, which have been differentially affected by anthropogenic eutrophication. Our data set comprises 16,173 loci genotyped across 138 whitefish using restriction‐site associated DNA sequencing (RADseq). Our analysis suggests that in each of the two lakes, the population of a different, but ecologically similar, whitefish species declined following a recent period of eutrophication. Genomic signatures consistent with hybridization are more pronounced in the more severely impacted lake. Comparisons between sympatric pairs of whitefish species with contrasting ecology, where one is shallow benthic and the other one more profundal pelagic, reveal genomic differentiation that is largely correlated along the genome, while differentiation is uncorrelated between pairs of allopatric provenance with similar ecology. We identify four genomic loci that provide evidence of parallel divergent adaptation between the shallow benthic species and the two different more profundal species. Functional annotations available for two of those loci are consistent with divergent ecological adaptation. Our genomic analysis indicates the action of divergent natural selection between sympatric whitefish species in pre‐alpine lakes and reveals the vulnerability of these species to anthropogenic alterations of the environment and associated adaptive landscape.     

Resource partitioning between pelagic coregonids in a subarctic watercourse following a biological invasion

If a pelagic ecosystem is invaded by an efficient planktivorous fish, competition theory predicts that habitat and/or diet segregation should occur if the invader and native planktivores are to co-exist. We compared the diet and pelagic habitat use between invading vendace (Coregonus albula) and two native morphs of whitefish (Coregonus lavaretus) in three subarctic lakes located within the same watercourse. No clear vertical habitat segregation was found as vendace prevailed throughout the entire depth range. Zooplankton generally dominated the diet of all three coregonids. No obvious resource partitioning was found in two of the lakes, while in the third lake the diets of whitefish and vendace differed significantly from each other. Here, the predominant pelagic whitefish morph fed extensively on water mites whereas vendace mainly consumed surface insects and to some extent small fish, apparently as a result of highly depleted zooplankton resources. The results suggest that various levels of resource depletion following the vendace invasion caused the observed differences between the lakes, indicating that different levels of resource partitioning were associated with dissimilar competition intensities. At present, coexistence of these congeneric species seems to be possible, although the vendace invasion has apparently resulted in a challenging and unstable situation for the planktivorous whitefish.     

Discrete foraging niches promote ecological, phenotypic, and genetic divergence in sympatric whitefish (Coregonus lavaretus)

Natural populations often vary in their degree of ecological, morphological and genetic divergence. This variation can be arranged along an ecological speciation continuum of increasingly discrete variation, with high inter-individual variation at one end and well defined species in the other. In postglacial fishes, evolutionary divergence has commonly resulted in the co-occurrence of a pelagic and a benthic specialist. We studied three replicate lakes supporting sympatric pelagic and benthic European whitefish (Coregonus lavaretus (L.)) morphs in search for early signs of possible further divergence into more specialized niches. Using stomach content data (recent diet) and stable isotope analyses (time-integrated measure of trophic niche use), we observed a split in the trophic niche within the benthic whitefish morph, with individuals specializing on either littoral or profundal resources. This divergence in resource use was accompanied by small but significant differences in an adaptive morphological trait (gill raker number) and significant genetic differences between fish exploiting littoral and profundal habitats and foraging resources. The same pattern of parallel divergence was found in all three lakes, suggesting similar natural selection pressures driving and/or maintaining the divergence. The two levels of divergence (a clear and robust benthic – pelagic and a more subtle littoral – profundal divergence) observed in this study apparently represent different stages in the process of ecological speciation.     

CORRELATED TROPHIC SPECIALIZATION AND GENETIC DIVERGENCE IN SYMPATRIC LAKE WHITEFISH ECOTYPES (COREGONUS CLUPEAFORMIS): SUPPORT FOR THE ECOLOGICAL SPECIATION HYPOTHESIS

There is ample empirical evidence that phenotypic diversification in an adaptive radiation is the outcome of divergent natural selection related to differential resource use. In contrast, the role of ecological forces in favoring and maintaining reproductive isolation in nature remains poorly understood. If the same forces driving phenotypic divergence are also responsible for speciation, one would predict a correlation between the extent of trophic specialization (reflecting variable intensity of divergent natural selection) and that of reproductive isolation being reached in a given environment. We tested this hypothesis by comparing the extent of morphological and genetic differentiation between sympatric dwarf and normal whitefish ecotypes (Coregonus sp.) from six lakes of the St. John River basin (eastern Canada and northern Maine). Eight meristic variables, 19 morphometric variables, and six microsatellite loci were used to quantify morphological and genetic differentiation, respectively. Dwarf and normal ecotypes in each lake differed primarily by traits related to trophic specialization, but the extent of differentiation varied among lakes. Significant but variable genetic divergence between ecotypes within lakes was also observed. A negative correlation was observed between the extent of gene flow between ecotypes within a lake and that of their morphological differentiation in trophic-related traits. The extent of reproductive isolation reached between dwarf and normal whitefish ecotypes appears to be driven by the potential for occupying distinct trophic niches and, thus, by the same selective forces driving tropic specialization in each lake. These results therefore support the hypothesis of ecological speciation.     

Role of epibenthic resource opportunities in the parallel evolution of lake whitefish species pairs (Coregonus sp.)

Parallel evolution of a dwarf and normal whitefish has been documented in six post-glacial lakes. Here, we relate the structure and seasonal variations of the epibenthic invertebrate communities to the extent of phenotypic differentiation in these species pairs. The highest phenotypic differentiation occurs in lakes characterized by less overlap in size distribution between limnetic and epibenthic prey which could represent enhanced ecological opportunities for trophic specialization and adaptive divergence. Differences in community assemblages and seasonal variation of biotic and abiotic conditions may also play a role. Accumulating evidence indicates that strong directional selection acting on dwarf whitefish may be more important than divergent selection acting on both sympatric forms in driving whitefish phenotypic divergence and ultimately, ecological speciation. Along with Landry et al. (2007), this study supports the general hypothesis that parallelism in divergence among sympatric dwarf and normal whitefish is associated with parallelism in limnological adaptive landscape.     

Parallelism in the oxygen transport system of the lake whitefish: the role of physiological divergence in ecological speciation

In North America, populations of lake whitefish (Coregonus clupeaformis) have evolved sympatric 'dwarf' and 'normal' ecotypes that are associated with distinct trophic niches within lakes. Trophic specialization should place diverging physiological demands on individuals, and thus, genes and phenotypes associated with energy production represent ideal candidates for studies of adaptation. Here, we test for the parallel divergence of traits involved in oxygen transport in dwarf and normal lake whitefish from Québec, Canada and Maine, USA. We observed significant differences in red blood cell morphology between the ecotypes. Specifically, dwarfs exhibited larger nuclei and a higher nucleus area/total cell area than normal whitefish in all of the lakes examined. In addition, isoelectric focusing gels revealed variation in the haemoglobin protein components found in whitefish. Dwarf and normal whitefish exhibited a similar number of protein components, but the composition of these components differed, with dwarf whitefish bearing a greater proportion of cathodic components compared to the normals. Furthermore, dwarf whitefish showed significant haemoglobin gene upregulation in the brain compared with the levels shown in normals. Together, our results indicate that metabolic traits involved in oxygen transport differ between the whitefish ecotypes and the strong parallel patterns of divergence observed across lakes implicates ecologically driven selection pressures. We discuss the function of these traits in relation to the differing trophic niches occupied by the whitefish and the potential contributions of trait plasticity and genetic divergence to energetic adaptation.     

The genetic architecture of ecological speciation and the association with signatures of selection in natural lake whitefish (Coregonus sp. Salmonidae) species pairs

Adaptive evolutionary change is contingent on variation and selection; thus, understanding adaptive divergence and ultimately speciation requires information on both the genetic basis of adaptive traits as well as an understanding of the role of divergent natural selection on those traits. The lake whitefish (Coregonus clupeaformis) consists of several sympatric "dwarf" (limnetic) and normal (benthic) species pairs that co-inhabit northern postglacial lakes. These young species pairs have evolved independently and display parallelism in life history, behavioral, and morphological divergence associated with the use of distinct trophic resources. We identified phenotype-environment associations and determined the genetic architecture and the role of selection modulating population genetic divergence in sympatric dwarf and normal lake whitefish. The genetic architecture of 9 adaptive traits was analyzed in 2 hybrid backcrosses individually phenotyped throughout their life history. Significant quantitative trait loci (QTL) were associated with swimming behavior (habitat selection and predator avoidance), growth rate, morphology (condition factor and gill rakers), and life history (onset of maturity and fecundity). Genome scans among 4 natural sympatric pairs, using loci segregating in the map, revealed a signature of selection for 24 loci. Loci exhibiting a signature of selection were associated with QTL relative to other regions of the genome more often than expected by chance alone. Two parallel QTL outliers for growth and condition factor exhibited segregation distortion in both mapping families, supporting the hypothesis that adaptive divergence contributing to parallel reductions of gene flow among natural populations may cause genetic incompatibilities. Overall, these findings offer evidence that the genetic architecture of ecological speciation is associated with signatures of selection in nature, providing strong support for the hypothesis that divergent natural selection is currently maintaining adaptive differentiation and promoting ecological speciation in lake whitefish species pairs.     

MULTIPLE MODES OF SPECIATION INVOLVED IN THE PARALLEL EVOLUTION OF SYMPATRIC MORPHOTYPES OF LAKE WHITEFISH (COREGONUS CLUPEAFORMIS,SALMONIDAE)

We performed a phylogenetic analysis of mtDNA variation among seven sympatric pairs of dwarf and normal morphotypes of whitefish from northern Québec and the St. John River drainage to address three questions relevant to understanding their radiation. Are all sympatric pairs reproductively isolated? Do phylogenetic analyses confirm that sympatric whitefish morphotypes found in eastern North America represent the outcome of polyphyletic evolutionary events? If so, did all sympatric pairs from the St. John River drainage originate from the same scenario of allopatric divergence and secondary contact? The hypothesis of genetic differentiation was supported for all sympatric pairs from the St. John River drainage, whereas lack of mtDNA diversity precluded any test of reproductive isolation for northern Québec populations. Patterns of mtDNA variation confirmed that dwarf and normal morphotypes evolved in parallel among independent, yet closely related, lineages, thus providing indirect evidence for the role of natural selection in promoting phenotypic radiation in whitefish. Patterns of mtDNA diversity among sympatric pairs of the St. John River indicated a complex picture of whitefish evolution that implied sympatric divergence and multiple allopatric divergence/secondary contact events on a small geographic scale. These results suggests that ecological opportunities, namely trophic niche availability, may promote population divergence in whitefish.     

Convergent evolutionary processes driven by foraging opportunity in two sympatric morph pairs of Arctic charr with contrasting post‐glacial origins

The expression of two or more discrete phenotypes amongst individuals within a species (morphs) provides multiple modes upon which selection can act semi‐independently, and thus may be an important stage in speciation. In the present study, we compared two sympatric morph systems aiming to address hypotheses related to their evolutionary origin. Arctic charr in sympatry in Loch Tay, Scotland, exhibit one of two discrete, alternative body size phenotypes at maturity (large or small body size). Arctic charr in Loch Awe segregate into two temporally segregated spawning groups (breeding in either spring or autumn). Mitochondrial DNA restriction fragment length polymorphism analysis showed that the morph pairs in both lakes comprise separate gene pools, although segregation of the Loch Awe morphs is more subtle than that of Loch Tay. We conclude that the Loch Awe morphs diverged in situ (within the lake), whereas Loch Tay morphs most likely arose through multiple invasions by different ancestral groups that segregated before post‐glacial invasion (i.e. in allopatry). Both morph pairs showed clear trophic segregation between planktonic and benthic resources (measured by stable isotope analysis) but this was significantly less distinct in Loch Tay than in Loch Awe. By contrast, both inter‐morph morphological and life‐history differences were more subtle in Loch Awe than in Loch Tay. The strong ecological but relatively weak morphological and life‐history divergence of the in situ derived morphs compared to morphs with allopatric origins indicates a strong link between early ecological and subsequent genetic divergence of sympatric origin emerging species pairs. The emergence of parallel specialisms despite distinct genetic origins of these morph pairs suggests that the effect of available foraging opportunities may be at least as important as genetic origin in structuring sympatric divergence in post‐glacial fishes with high levels of phenotypic plasticity. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ?? , ??–??.     

Variable extent of parallelism in respiratory,circulatory, and neurological traits across lake whitefish species pairs

Parallel adaptive radiation events provide a powerful framework for investigations of ecology's contribution to phenotypic diversification. Ecologically driven divergence has been invoked to explain the repeated evolution of sympatric dwarf and normal lake whitefish (Coregonus clupeaformis) species in multiple lakes in eastern North America. Nevertheless, links between most putatively adaptive traits and ecological variation remain poorly defined within and among whitefish species pairs. Here, we examine four species pairs for variation in gill, heart, and brain size; three traits predicted to show strong phenotypic responses to ecological divergence. In each of the species pairs, normals exhibited larger body size standardized gills compared to dwarfs – a pattern that is suggestive of a common ecological driver of gill size divergence. Within lakes, the seasonal hypoxia experienced in the benthic environment is a likely factor leading to the requirement for larger gills in normals. Interestingly, the morphological pathways used to achieve larger gills varied between species pairs from Québec and Maine, which may imply subtle non‐parallelism in gill size divergence related to differences in genetic background. There was also a non‐significant trend toward larger hearts in dwarfs, the more active species of the two, whereas brain size varied exclusively among the lake populations. Taken together, our results suggest that the diversification of whitefish has been driven by parallel and non‐parallel ecological conditions across lakes. Furthermore, the phenotypic response to ecological variation may depend on genetic background of each population.     

GENETIC EVIDENCE FOR REPRODUCTIVE ISOLATION AND MULTIPLE ORIGINS OF SYMPATRIC TROPHIC ECOTYPES OF WHITEFISH (COREGONUS)

We assessed variation in mitochondrial DNA (mtDNA) by restriction fragment length polymorphism (RFLP) analysis and in nuclear genes by allozyme analysis among sympatric pairs of limnetic and benthic ecotypes of whitefish (Coregonus) coexisting in three lakes of southern Yukon to address three evolutionary questions regarding their origins. Are sympatric low and high gill-raker count ecotypes genetically differentiated? Are they issued from monophyletic or polyphyletic evolutionary events? If they are polyphyletic in origins, did they originate from multiple allopatric speciation events or intralacustrine radiation? Our results corroborated previous genetic and ecological studies of these ecotypes, indicating that they represent genetically distinct reproductive units, and therefore refuting the hypothesis of phenotypic polymorphism within a single population. However, the amount of gene flow between ecotypes varied among lakes, correlating with the extent of morphological differentiation and the potential for premating reproductive isolation. The results indicated a polyphyletic origin of ecotypes whereby each of them have been expressed independently more than once. In the two lakes of Squanga Creek drainage, the existence of sympatric pairs was best explained by the secondary contact of two monophyletic whitefish groups that evolved in allopatry during the last glaciation events. In Dezadeash L. of Alsek R. drainage, our results could not verify either sympatric or allopatric (or microallopatric) origin of ecotypes. Regardless of the mode of speciation involved in their origins, these sympatric whitefish populations provided further evidence that Pleistocene glaciation events created conditions favoring rapid divergence and phenotypic differentiation among northern freshwater fishes.     

Trophic polymorphism, habitat and diet segregation in Percichthys trucha (Pisces: Percichthyidae) in the Andes

Divergent natural selection affecting specific trait combinations that lead to greater efficiency in resource exploitation is believed to be a major mechanism leading to trophic polymorphism and adaptive radiation. We present evidence of trophic polymorphism involving two benthic morphs within Percichthys trucha , a fish endemic to temperate South America. In a series of lakes located in the southern Andes, we found two morphs of P. trucha that could be distinguished on the basis of gill raker length and five other morphological measures, most of which are likely associated with the use of food resources. The differences were consistent across all lakes examined, and were correlated with habitat use and diet. Individuals with longer gill rakers were more abundant in the littoral zone (littoral morph) while the short gill-raker morph was more abundant at 10 m depth and deeper (deep benthic morph). Both morphs fed primarily on benthic invertebrates, but the littoral morph fed more on larval Anisoptera than did the deep benthic morph. Phenotypic correlations among traits were high for the littoral morph, but low and non-significant for the deep-benthic morph. We suggest that gill raker length may influence the relative efficiency of suction feeding for the two morphs. This is the first evidence of trophic polymorphism in fishes from temperate South America.     

The interaction of resource use and gene flow on the phenotypic divergence of benthic and pelagic morphs of Icelandic Arctic charr (Salvelinus alpinus)

Conceptual models of adaptive divergence and ecological speciation in sympatry predict differential resource use, phenotype–environment correlations, and reduced gene flow among diverging phenotypes. While these predictions have been assessed in past studies, connections among them have rarely been assessed collectively. We examined relationships among phenotypic, ecological, and genetic variation in Arctic charr (Salvelinus alpinus) from six Icelandic localities that have undergone varying degrees of divergence into sympatric benthic and pelagic morphs. We characterized morphological variation with geometric morphometrics, tested for differential resource use between morphs using stable isotopes, and inferred the amount of gene flow from single nucleotide polymorphisms. Analysis of stable isotopic signatures indicated that sympatric morphs showed similar difference in resource use across populations, likely arising from the common utilization of niche space within each population. Carbon isotopic signature was also a significant predictor of individual variation in body shape and size, suggesting that variation in benthic and pelagic resource use is associated with phenotypic variation. The estimated percentage of hybrids between sympatric morphs varied across populations (from 0% to 15.6%) but the majority of fish had genotypes (ancestry coefficients) characteristic of pure morphs. Despite evidence of reduced gene flow between sympatric morphs, we did not detect the expected negative relationship between divergence in resource use and gene flow. Three lakes showed the expected pattern, but morphs in the fourth showed no detectable hybridization and had relatively low differences in resource use between them. This coupled with the finding that resource use and genetic differentiation had differential effects on body shape variation across populations suggests that reproductive isolation maintains phenotypic divergence between benthic and pelagic morphs when the effects of resource use are relatively low. Our ability to assess relationships between phenotype, ecology, and genetics deepens our understanding of the processes underlying adaptive divergence in sympatry.     

Empirical evaluation of phenotype–environment correlation and trait utility with allopatric and sympatric whitefish, Coregonus lavaretus (L.), populations in subarctic lakes

Adaptive phenotypic divergence of sympatric morphs in a single species may have significant evolutionary consequences. In the present study, phenotypic impacts of predator on zooplankton prey populations were compared in two northern Finnish lakes; one with an allopatric whitefish, Coregonus lavaretus (L.), population and the other with three sympatric whitefish populations. First, we examined whether there were phenotypic associations with specific niches in allopatric and sympatric whitefish. Second, trait utility (i.e. number of gillrakers) of allopatric and sympatric whitefish in utilizing a pelagic resource was explored by comparing predator avoidance of prey, prey size in environment, and prey size in predator diet. The allopatric living large sparsely rakered (LSR) whitefish morph, was a generalist using both pelagic and benthic niches. In contrast, sympatric living whitefish morphs were specialized: LSR whitefish was a littoral benthivore, small sparsely rakered whitefish was a profundal benthivore and densely rakered (DR) whitefish was a pelagic planktivore. In the lake with allopatric whitefish, zooplankton prey did not migrate vertically to avoid predation whereas, in the lake with sympatric whitefish, all important prey taxa migrated significantly. Trait utility was observed as significantly smaller size of prey in environment and predator diet in the lake with DR whitefish than in the lake with only LSR whitefish.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 92 , 561–572.     

Trophic polymorphism in a riverine fish: morphological, dietary, and genetic analysis of mountain whitefish

Trophic polymorphisms are a prominent form of phenotypic diversification in many animal taxa. Northern temperate lakes have become model systems for the investigation of sympatric speciation due to trophic polymorphisms. Many examples of niche-based phenotypic variation occur in temperate lakes, whereas northern rivers offer few such examples. To further investigate the conditions under which trophic polymorphisms are likely to evolve, the present study examined phenotypic variation related to snout size and shape in the mountain whitefish (Salmonidae: Prosopium williamsoni ), which has been hypothesized to exhibit a rare example of reproductively isolated trophic morphs in a northern river-dwelling fish species. Variation in snout size and shape increased greatly with body size and, although this variation was continuously distributed, individuals in the largest size class tended to lie at phenotypic extremes. At one extreme were individuals with a large bulbous snout and a sloping forehead ('pinocchio'), and at the other were individuals that lack the bulbous snout and have a concave forehead ('normal'). The pinocchio trait may result from a stage-specific developmental switch that occurs late in ontogeny. Consistent differences were found with respect to diet between individuals with extreme snout morphologies, but no evidence was found for assortative mating within populations at seven microsatellite loci. The explosive mating system of this species may be responsible for this lack of assortative mating. The present study highlights the influence of ecological factors in shaping phenotypic and behavioural diversification due to trophic morphology.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 92 , 253–267.     

Patterns of morphological changes and hybridization between sympatric whitefish morphs (Coregonus spp.) in a Swiss lake: a role for eutrophication?

Whitefish, genus Coregonus, show exceptional levels of phenotypic diversity with sympatric morphs occurring in numerous postglacial lakes in the northern hemisphere. Here, we studied the effects of human‐induced eutrophication on sympatric whitefish morphs in the Swiss lake, Lake Thun. In particular, we addressed the questions whether eutrophication (i) induced hybridization between two ecologically divergent summer‐spawning morphs through a loss of environmental heterogeneity, and (ii) induced rapid adaptive morphological changes through changes in the food web structure. Genetic analysis based on 11 microsatellite loci of 282 spawners revealed that the pelagic and the benthic morph represent highly distinct gene pools occurring at different relative proportions on all seven known spawning sites. Gill raker counts, a highly heritable trait, showed nearly discrete distributions for the two morphs. Multilocus genotypes characteristic of the pelagic morph had more gill rakers than genotypes characteristic of benthic morph. Using Bayesian methods, we found indications of recent but limited introgressive hybridization. Comparisons with historical gill raker data yielded median evolutionary rates of 0.24 haldanes and median selection intensities of 0.27 for this trait in both morphs for 1948–2004 suggesting rapid evolution through directional selection at this trait. However, phenotypic plasticity as an alternative explanation for this phenotypic change cannot be discarded. We hypothesize that both the temporal shifts in mean gill raker counts and the recent hybridization reflect responses to changes in the trophic state of the lake induced by pollution in the 1960s, which created novel selection pressures with respect to feeding niches and spawning site preferences.