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Phenotypic differences can exist between species, between local populations of the same species and between individuals within single local populations. At all scales, phenotypic differences can be either adaptive or non-adaptive. Using natural selection to explain differences between closely related species was controversial during the 1940s but had become common by the 1960s. Similarly, the adaptive nature of differences between local populations was initially controversial but had become widely accepted by the 1980s. The interpretation of differences at the finest scale-between individuals within single populations-is still unresolved. This paper reviews studies of adaptive individual differences in resource use and response to risk. A general conceptual framework for thinking about adaptive individual differences within populations can unite subjects as seemingly different as speciation and personality psychology.

The study examines the basic morphological and ecological features of Dolly Varden from Lake Kronotskoe (Russia, Kamchatka). Seven valid morphs different in head proportions, feeding, timing, and place of spawning have been determined in this ecosystem. The basic morphometric characteristics clearly separate Lake Kronotskoe morphs from each other, as well as from its potential ancestor (Dolly Varden). According to CVA analysis, the most notable morphological characteristics determining the mouth position are the length of a lower jaw and rostrum. Furthermore, five of seven morphs inhabit different depth zones of the lake and feed on different food resources. Our data suggest that reproductive isolation may be maintained by temporal/spatial isolation for two morphs with lacustrine spawning, and by spatial isolation only for the rest of the morphs with riverine spawning. The sympatric diversity of the Lake Kronotskoe charrs is exceptionally wide, and there are no other examples for seven sympatric morphs of genus Salvelinus to coexist within a single ecosystem. This study puts forward a three‐step hypothetical model of charr divergence in Lake Kronotskoe as a potential ground for future studies. The basic morphological and ecological features are examined for Dolly Varden from Lake Kronotskoe (Russia, Kamchatka). It was found that sympatric diversity of Lake Kronotskoe charrs is exceptionally wide, and there are no other examples for seven sympatric morphs of genus Salvelinus to coexist within a single ecosystem. In conclusion, on the basis of morphological and ecological features, we suggest a possible way of three‐step sympatric speciation of charrs in this lake.

Theory predicts intraguild predation (IGP) to be unstable despite its ubiquity in nature, prompting exploration of stabilizing mechanisms of IGP. One of the many ways IGP manifests is through inducible trophic polymorphisms in the intraguild (IG) predator, where a resource-eating predator morph competes with the intraguild (IG) prey for the shared resource while a top predator morph consumes the IG prey. Cannibalism is common in this type of system due to the top predator morph’s specialization on the trophic level below it, which includes the resource-eating predator morph. Here, we explore the consequences of inducible trophic polymorphisms in cannibal predators for IGP stability using an IGP model with and without cannibalism. We employ linear stability analysis and identify regions of coexistence based on the top predator morph’s preference for conspecifics vs. heterospecifics and the IG prey’s competitive ability relative to the resource-eating morph. Our findings reveal that preferential cannibalism (i.e., the preferential consumption of conspecifics) stabilizes the system when the IG prey and resource-eating morph have similar competitive abilities for the shared resource. Though original IGP theory finds that the IG prey must be a superior resource competitor as a general criterion for coexistence, this is not typically the case when the predator has an inducible trophic polymorphism and the resource-eating morph is specialized in resource acquisition. Preferential cannibalism may therefore be a key stabilizing mechanism in IGP systems with a cannibalistic, trophic polymorphic IG predators, providing further insight into what general mechanisms stabilize the pervasive IGP interaction.

Predators are increasingly recognized as key elements in food webs because of their ability to link the fluxes of nutrients and energy between spatially separated food chains. However, in the context of food web connectivity, predator populations have been mainly treated as homogeneous units, despite compelling evidence of individual specialization in resource use. It is conceivable that individuals of a predatory species use different resources associated with spatially separated food chains, thereby decoupling cross-habitat linkages. We tested whether intrapopulation differences in habitat use in the generalist freshwater predator Eurasian perch (Perca fluviatilis) led to long-term niche partitioning and affected the degree of ecological habitat coupling. We evaluated trophic niche variability at successively larger timescales by analyzing gut contents and stable isotopes (δ¹³C and δ¹⁵N) in liver and muscle, tissues that provide successively longer integration of trophic activity. We found that the use of distinct habitats in perch led to intrapopulation niche partitioning between pelagic and littoral subpopulations, consistent through the various timescales. Pelagic fish showed a narrower niche, lower individual specialization, and more stable trophic behavior than littoral fish, as could be expected from inhabiting a relatively less diverse environment. This result indicated that substantial niche reduction could occur in a generalist predator at the subpopulation level, consistent with the use of a habitat that provides fewer chances of individual specialization. We showed that intrapopulation niche partitioning limits the ability of individual predators to link spatially separated food chains. In addition, we suggest a quantitative, standardized approach based on stable isotopes to measure the degree of habitat coupling mediated by a top predator.

Adaptive radiation in fishes entering novel post-glacial lakes ubiquitously gives rise to a set of typical forms specializing under the pressure of the same ecological gradients. The world’s most diverse flock of salmonids from Lake Kronotskoe (Kamchatka, N-E Asia) serves as a good model to study evolutionary divergence modes. Eight forms with different feeding types and habitat preferences are known to originate from Salvelinus malma. To determine the phasing of the flock radiation we analyzed morphometry, muscle isotope content, parasite fauna abundance and microsatellite DNA polymorphism in the sympatric forms inhabiting the lake. We found that seven out of eight morphologically independent forms can be distinguished by a stable feeding niche through comparing the stomach content, isotopic status and parasite fauna abundance. Five eco-morphs were found to be genetically independent, while three other morphs fell into one cluster without any significant restriction of the gene flow among the members. Partitioning of the resources in pelagic—benthic, epilimnetic—deepwater and lacustrine—riverine environments leads to divergence of the most genetically and morphologically distant forms. Smaller genetic and morphometric distances were found for the form pairs diverged along one or two of these gradients. The divergence of the last three eco-morphs in feeding tactics within the coastal zone is not associated with differentiation in microsatellite polymorphism suggesting a more recent and/or subtle specialization within this environment.

Resource partitioning in polymorphic fish species is expected to be altered by human exploitation, as individual specialization is density dependent in many vertebrates. We tested this hypothesis using the ballan wrasse Labrus bergylta as a model species. We compared the isotope niches of the plain and spotted morphs of the species in a marine protected area (MPA) and in adjoining areas open to fishing, both off Galicia (NW Spain). Underwater visual census confirmed a 3-fold increase in the biomass of ballan wrasse off the Cíes Islands compared to areas open to recreational fishing, thus demonstrating that populations outside MPAs are well below carrying capacity. The stable isotope ratio of C revealed differences in the resource use patterns of plain and spotted ballan wrasses both in areas open and closed to fishing, as plain wrasses were always depleted in 13C compared to sympatric spotted ones. The stable isotope ratio of N showed that plain ballan wrasses foraged consistently at a higher trophic level than spotted ones in areas of high population density closed to recreational fishing, whereas differences did not exist or were reversed in areas open to fishing. These results demonstrate that the pattern of trophic resource partitioning between 2 morphs of the ballan wrasse is density dependent and that plain and spotted ballan wrasses likely had different ecological niches in pristine ecosystems.

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. Our study assesses the contribution of differential resource use, phenotype–environment correlations, and reductions in gene flow to adaptive divergence of benthic–pelagic morphs of Icelandic Arctic charr in sympatry. We integrate a variety of analytical approaches to test our hypotheses using geometric morphometrics, stable isotopic signatures, and single nucleotide polymorphisms. Resource use appears to be the agent of selection driving phenotypic divergence along a benthic–pelagic axis and reproductive isolation may maintain this divergence when the intensity of divergent selection is weak. ​

Charrs of the genus Salvelinus form distinct trophic morphs living in sympatry in numerous postglacial lakes. Here we demonstrate that charrs can diversify into amphipod foraging specialists and sedentary macrobenthos consumers in the shallow-water ecosystems. This pattern was revealed in three out of six lakes under exploration supported by differences in stomach content, trophic-transmitted parasite, and stable isotope ratio analyzes. The body shape and growth rate comparison indicates that this kind of trophic-based diversification emerges at a juvenile stage and is maintained throughout the whole life. The restriction in gene flow found between the morphs allows to propose the possibility for the hereditable-based specialization to evolve. We found that those diversification phenomena are possible only in the lakes situated in vicinity of the ocean coastline, while no evidence of this split was found for inland mountain lakes. We suggest that the trophic-based diversification in the littoral ecosystems is accounted for the heterogeneity in the ecological conditions and food resources’ distribution linked to coastal wind action. This phenomenon was previously reported for the charr in Lake Fjellfrosvatn, Scandinavia, so it seems to be some universal yet poorly described kind of disruptive selection pressure for northern latitude fishes.

The niche variation hypothesis (NVH) predicts that populations with wider niches are phenotypically more variable than populations with narrower niches, which is frequently used to explain diversifying processes such as ecological release. However, not all empirical evidence supports the NVH. Furthermore, a relationship between population phenotypic variation and niche width can be caused by sexual selection or environmental gradients, which should be carefully considered along with competition in explaining niche variation. In this study, we used eight populations of a generalist passerine species, Paradoxornis webbianus (vinous‐throated parrotbill), to test the NVH. We assessed evidence of ecological sexual dimorphism and environmental gradients in bill morphology of P. webbianus. A total of 170 P. webbianus from eight sites ranging 24–2668 m in altitude were included in this study. We used two principal components to quantify bill morphology: one describes bill size and the other describes bill slenderness. We used stable carbon and nitrogen isotope values of bird feathers to quantify trophic positions, and we estimated population trophic niche width using Bayesian standardized ellipse area. Paradoxornis webbianus with larger and more slender bills fed at higher trophic levels and population trophic niche width tended to increase with bill‐size variation, supporting the NVH. The males had larger bills and marginally higher nitrogen isotope values than the females, suggesting ecological sexual dimorphism. Despite a positive correlation between bill size and wing length indicating sexual selection for larger male size, only three of the eight populations showed both male‐biased bill size and male‐biased wing length. Sexual dimorphism explained 13%–64% of bill‐size variation across sites, suggesting its role in niche variation could vary greatly among populations. The variation in bill slenderness in P. webbianus increased with elevation. However, neither bill‐size variation nor trophic niche width changed with elevation. Therefore, environmental gradients that could be reflected in the elevation are not likely to drive the observed morphological and niche variation. This study provides an empirical case for the NVH and highlights the importance of investigating sexual dimorphism and environmental gradients in studies of niche dynamics.

This study identifies one possible mechanism whereby gene flow is interrupted in populations undergoing evolutionary divergence in sympatry; this is an important issue in evolutionary biology that remains poorly understood. Variation in trophic morphology was induced in three-spined stickleback by exposing them from an early age either to large benthic or to small pelagic prey. At sexual maturity, females given a choice between two breeding males, showed positive assortative mate choice for males raised on the same diet as themselves. The data indicate that this was mediated through a preference for males with trophic morphology similar to that of fish with which the females were familiar (from their pre-testing holding tanks). In trials where the female did not choose the most familiar male, the evidence suggests that either she had difficulty discriminating between two similar males or was positively choosing males with more extreme morphologies (more benthic-like or pelagic-like). This study has shown for the first time that expression of a plastic trait induced at an early age, not only results in specialisation for local foraging regimes but can also play a significant role in mate choice. This is equivalent to an environmentally induced, plastic version of the “magic traits” that promote ecologically-driven divergence in sympatry, hence the proposed descriptor “plastic magic trait”.