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Habitats can select for specialized phenotypic characteristics in animals. However, the consistency of evolutionary responses to particular environmental conditions remains difficult to predict. One trait of great ecological importance is brain form, which is expected to vary between habitats that differ in their cognitive requirements. Here, we compared divergence in brain form and oral jaw size across a common littoral-pelagic ecological axis in two sunfishes at both the intraspecific and interspecific levels. Brain form differed between habitats at every level of comparison; however, divergence was inconsistent, despite consistent differences in oral jaw size. Pumpkinseed and bluegill species differed in cerebellum, optic tectum and olfactory bulb size. These differences are consistent with a historical ecological divergence because they did not manifest between littoral and pelagic ecotypes within either species, suggesting constraints on changes to these regions over short evolutionary time scales. There were also differences in brain form between conspecific ecotypes, but they were inconsistent between species. Littoral pumpkinseed had larger brains than their pelagic counterpart, and littoral bluegill had smaller telencephalons than their pelagic counterpart. Inconsistent brain form divergence between conspecific ecotypes of pumpkinseed and bluegill sharing a common littoral-pelagic habitat axis suggests that contemporary ecological conditions and historic evolutionary context interact to influence evolutionary changes in brain form in fishes.

Mechanisms that generate brain size variation and the consequences of such variation on ecological performance are poorly understood in most natural animal populations. We use a reciprocal-transplant common garden experiment and foraging performance trials to test for brain size plasticity and the functional consequences of brain size variation in Pumpkinseed sunfish (Lepomis gibbosus) ecotypes that have diverged between nearshore littoral and offshore pelagic lake habitats. Different age-classes of wild-caught juveniles from both habitats were exposed for 6 months to treatments that mimicked littoral and pelagic foraging. Plastic responses in oral jaw size suggested that treatments mimicked natural habitat-specific foraging conditions. Plastic brain size responses to foraging manipulations differed between ecotypes, as only pelagic sourced fish showed brain size plasticity. Only pelagic juveniles under 1 year-old expressed this plastic response, suggesting that plastic brain size responses decline with age and so may be irreversible. Finally, larger brain size was associated with enhanced foraging performance on live benthic but not pelagic prey, providing the first experimental evidence of a relationship between brain size and prey-specific foraging performance in fishes. The recent post-glacial origin of these ecotypes suggests that brain size plasticity can rapidly evolve and diverge in fish under contrasting ecological conditions.

Ecological opportunities, such as access to a novel habitat or the extirpation of a key competitor, can lead to adaptive divergence by exposing populations to diversifying selection. Typically, effects of ecological opportunity on adaptive divergence are inferred from macroevolutionary patterns rather than tested in populations undergoing contemporary divergence. This limits our insight on how ecological conditions contribute to adaptive divergence. Pumpkinseed sunfish (Centrarchidae: Lepomis gibbosus) have recently and repeatedly colonized a ‘novel’ pelagic habitat in postglacial lakes, and subsequently undergone phenotypic diversification. We investigated whether ecological opportunity has contributed to diversification in a pumpkinseed population that has diversified between lake habitats. We used a between-year mark-recapture study to evaluate whether (1) the novel pelagic habitat represents an ecological opportunity by supporting a large, high-density population, and (2) connectivity between ecotype populations is restricted by limited adult dispersal. We found that phenotypic variation is spatially structured between habitats, similar to prior studies. Submerged shoals in the pelagic habitat do sustain a large adult population at a density seven times greater than the ancestral littoral habitat. Additionally, body condition and size of pelagic pumpkinseed is similar to littoral pumpkinseed. This suggests the pelagic habitat provides an ecological opportunity to pumpkinseed in the form of abundant, available resources. Furthermore, strong between-year habitat fidelity suggests aspects of the ecological opportunity have reduced adult dispersal and could limit gene flow. In combination with prior evidence indicating diversifying selection between habitats, these results provide an example of how ecological opportunity might contribute to contemporary adaptive divergence.

Adaptive autotomy is the self-amputation of an appendage in response to external stimuli that benefits survival. Variation in the ease of appendage removal among populations suggests that autotomy performance is under selection, evolves, or is phenotypically plastic, although the latter has never been experimentally tested. We model an autotomy threshold that optimally balances how the benefits of surviving predator attack versus the costs of losing an appendage vary with predator presence. We test for functional plasticity in autotomy threshold in the caudal lamellae of Enallagma damselfly larvae by experimentally manipulating non-lethal cues from predatory dragonfly larvae. Predator cues lead to functional plastic responses in the form of smaller lamellar joints that required lower peak breaking force. This is the first experimental demonstration of functional plasticity in autotomy to cues from a grasping predator, a novel form of indirect predator effects on prey, realized through plasticity in morphological traits that govern the autotomy threshold. This supports the model of optimized autotomy performance and provides a novel explanation for variation in performance among populations under different predator conditions. Plastic autotomy responses that mitigate costs in the face of variation in mortality risks might be a form of inducible defense.

A functional relationship between relative brain size and cognitive performance has been hypothesized. However, the influence of ontogenetic niche shifts on cognitive performance is not well understood. Increases in body size can affect niche use but distinguishing nonecologically relevant brain development from effects associated with ecology is difficult. If survival is enhanced by functional changes in ecocognitive performance over ontogeny, then brain size development should track ontogenetic shifts in ecology. We control for nonecologically relevant brain size development by comparing brain growth between two ecotypes of Pumpkinseed sunfish whose ecologies diverge over ontogeny from a shared juvenile niche. Brain size differs between ecotypes from their birth year onwards even though their foraging ecology appears to diverge at age 3. This finding suggests that the eco-cognitive requirements of adult niches shape early life brain growth more than the requirements of juvenile ecology.

Harvest can change phenotypic traits of populations through immediate demographic consequences, evolutionary responses to harvest selection, or developmental responses by individuals. This study investigated the plastic phenotypic effects of harvest on size and age at maturity in a commercially exploited freshwater fish. We tested an individual growth and life history plasticity model using lagged correlations incorporating how harvesting fish ages 2 and older influenced the abundance of juvenile fish, resource availability, individual growth rates, and carry-over responses in age and size at maturity. Our test used cohort data for Lake Erie yellow perch (Perca flavescens). Age and size at maturity fluctuated widely and rapidly across 23 cohorts between 1991 and 2013, suggesting phenotypic plasticity contributed strongly to maturation dynamics. The changes in maturity could not be explained by responses to harvest, as expected under the plasticity model. In Lake Erie, age and size at maturity in yellow perch appear to be responding to other drivers, such as harvest-induced dynamics of other fish stocks or ecosystem changes that are independent of harvest.

Behavioral interference between species, such as territorial aggression, courtship, and mating, is widespread in animals. While aggressive and reproductive forms of interspecific interference have generally been studied separately, their many parallels and connections warrant a unified conceptual approach. Substantial evidence exists that aggressive and reproductive interference have pervasive effects on species coexistence, range limits, and evolutionary processes, including divergent and convergent forms of character displacement. Alien species invasions and climate change-induced range shifts result in novel interspecific interactions, heightening the importance of predicting the consequences of species interactions, and behavioral interference is a fundamental but neglected part of the equation. Here, we outline priorities for further theoretical and empirical research on the ecological and evolutionary consequences of behavioral interference.

Many morphological, behavioral, physiological, and life-history traits covary across the biological scales of individuals, populations, and species. However, the processes that cause traits to covary also change over these scales, challenging our ability to use patterns of trait covariance to infer process. Trait relationships are also widely assumed to have generic functional relationships with similar evolutionary potentials, and even though many different trait relationships are now identified, there is little appreciation that these may influence trait covariation and evolution in unique ways. We use a trait-performance-fitness framework to classify and organize trait relationships into three general classes, address which ones more likely generate trait covariation among individuals in a population, and review how selection shapes phenotypic covariation. We generate predictions about how trait covariance changes within and among populations as a result of trait relationships and in response to selection and consider how these can be tested with comparative data. Careful comparisons of covariation patterns can narrow the set of hypothesized processes that cause trait covariation when the form of the trait relationship and how it responds to selection yield clear predictions about patterns of trait covariation. We discuss the opportunities and limitations of comparative approaches to evaluate hypotheses about the evolutionary causes and consequences of trait covariation and highlight the importance of evaluating patterns within populations replicated in the same and in different selective environments. Explicit hypotheses about trait relationships are key to generating effective predictions about phenotype and its evolution using covariance data.

Harvested marine fish stocks often show a rapid and substantial decline in the age and size at maturation. Such changes can arise from multiple processes including fisheries‐induced evolution, phenotypic plasticity, and responses to environmental factors other than harvest. The relative importance of these processes could differ systematically between marine and freshwater systems. We tested for temporal shifts in the mean and within‐cohort variability of age‐ and size‐based maturation probabilities of female yellow perch (Perca flavescens Mitchill) from four management units (MUs) in Lake Erie. Lake Erie yellow perch have been commercially harvested for more than a century, and age and size at maturation have varied since sampling began in the 1980s. Our analysis compared probabilistic maturation reaction norms (PMRNs) for cohorts when abundance was lower and harvest higher (1993–1998) to cohorts when abundance was higher and harvest lower (2005–2010). PMRNs have been used in previous studies to detect signs of evolutionary change in response to harvest. Maturation size threshold increased between the early and late cohorts, and the increases were statistically significant for the youngest age in the western MU1 and for older ages in the eastern MU3. Maturation envelope widths, a measure of the variability in maturation among individuals in a cohort, also increased between early and late cohorts in the western MUs where harvest was highest. The highest rates of change in size at maturation for a given age were as large or larger than rates reported for harvested marine fishes where declines in age and size at maturation have been observed. Contrary to the general observation of earlier maturation evolving in harvested stocks, female yellow perch in Lake Erie may be rapidly evolving delayed maturation since harvest was relaxed in the late 1990s, providing a rare example of possible evolutionary recovery. publishedVersion

In the original publication of the article, the sample size reported in the article differs from the sample size in the Electronic Supplementary material 2 dataset. The corrections in the text and the revised supplementary data set are provided in this correction.

Variation in spatial complexity and foraging requirements between habitats can impose different cognitive demands on animals that may influence brain size. However, the relationship between ecologically related cognitive performance and brain size is not well established. We test whether variation in relative brain size and brain region size is associated with habitat use within a population of pumpkinseed sunfish composed of different ecotypes that inhabit either the structurally complex shoreline littoral habitat or simpler open-water pelagic habitat. Sunfish using the littoral habitat have on average 8.3% larger brains than those using the pelagic habitat. We found little difference in the proportional sizes of five brain regions between ecotypes. The results suggest that cognitive demands on sunfish may be reduced in the pelagic habitat given no habitat-specific differences in body condition. They also suggest that either a short divergence time or physiological processes may constrain changes to concerted, global modifications of brain size between sunfish ecotypes.

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

Behaviors toward heterospecifics and conspecifics may be correlated because of shared mechanisms of expression in both social contexts (nonadaptive covariation) or because correlational selection favors adaptive covariation. We evaluated these hypotheses by comparing behavior toward conspecifics and heterospecifics in brook stickleback (Culaea inconstans) from three populations sympatric with and three allopatric from a competitor, the ninespine stickleback (Pungitius pungitius). Behavioral traits were classified into three multivariate components: overt aggression, sociability, and activity. The correlation of behavior between social contexts for both overt aggression and activity varied among populations in a way unrelated to sympatry with ninespine stickleback, while mean aggression was reduced in sympatry. Correlations in allopatric populations suggest that overt aggression and activity may genetically covary between social contexts for nonadaptive reasons. Sociability was rarely correlated in allopatry but was consistently correlated in sympatry despite reduced mean sociability, suggesting that correlational selection may favor a sociability syndrome in brook stickleback when they coexist with ninespine stickleback. Thus, interspecific competition may impose diversifying selection on behavior among populations, although the causes of correlated behavior toward conspecifics and heterospecifics and whether it can evolve in one social context independent of the other may depend on the type of behavior.

Stable isotope analysis is frequently used to infer resource use in natural populations of fishes. Studies have examined factors, other than diet, that influence δ15N and δ13C including tissue-specific rates of equilibration and starvation. Most such studies completed under laboratory conditions tightly control food quantity and its isotopic composition, but it is also necessary to evaluate the influence of these factors under more natural conditions. Using pumpkinseed sunfish (Lepomis gibbosus) we evaluated whether restricted rations below minimum daily requirements affects tissue equilibration to a change in diet by holding fish on two treatments that often reflect divergent resource use in natural populations (pelagic zooplankton or littoral macroinvertebrates). Over 42 days, δ15N values increased while δ13C values did not change, additionally neither were related to diet treatment. Increased δ15N values were negatively related to body condition while δ13C values were not, indicating that stable isotope values were more affected by decreasing body condition than by diet. Additionally, δ15N values changed more in the blood and liver tissues than in white muscle tissue, indicating that restricting food availability had greater effects on tissues with greater metabolic activity. We hypothesize that stable isotope values of consumers are subject to a tissue-specific trade-off between sensitivity to changes in resource use and resistance to the effects of low resource availability. This trade-off may require consideration in stable isotope studies of wild populations facing periodic limitations of food availability.

Autotomy, the discarding of a prey appendage grasped by a predator, may evolve when the benefits of immediate survival outweigh the costs of appendage loss. In larval damselflies, joints connecting lamellae to the abdomen vary in size and shape within and among taxa suggesting that they may evolve under selection by invertebrate predators, such as dragonfly larvae. Assuming that joint width is proportional to the force required for autotomy, we tested if invertebrate predation favours smaller lamellar joints for autotomy or larger joints for structural support of lamellae for swimming. We compared the maximum joint widths of larval Lestes and Enallagma among ponds that varied in risk of invertebrate predation. Relative predation risk estimated as the frequency of regenerated lamellae within ponds was weakly and positively related to the relative abundance of larval dragonflies. The allometry of lamellar joint size decreased with increasing risk of invertebrate predation across ponds after controlling for variation in body size in Lestes congener but not in Enallagma species. Both species of Lestes had larger joint sizes than the five species of Enallagma, suggesting that the ancestral divergence of lamellar joints between these genera may influence contemporary responses to invertebrate predation.

Historical and contemporary evolutionary processes can both contribute to patterns of phenotypic variation among populations of a species. Recent studies are revealing how interactions between historical and contemporary processes better explain observed patterns of phenotypic divergence than either process alone. Here, we investigate the roles of evolutionary history and adaptation to current environmental conditions in structuring phenotypic variation among polyphenic populations of sunfish inhabiting 12 postglacial lakes in eastern North America. The pumpkinseed sunfish polyphenism includes sympatric ecomorphs specialized for littoral or pelagic lake habitats. First, we use population genetic methods to test the evolutionary independence of within-lake phenotypic divergences of ecomorphs and to describe patterns of genetic structure among lake populations that clustered into three geographical groupings. We then used multivariate analysis of covariance (MANCOVA) to partition body shape variation (quantified with geometric morphometrics) among the effects of evolutionary history (reflecting phenotypic variation among genetic clusters), the shared phenotypic response of all populations to alternate habitats within lakes (reflecting adaptation to contemporary conditions), and unique phenotypic responses to habitats within lakes nested within genetic clusters. All effects had a significant influence on body form, but the effects of history and the interaction between history and contemporary habitat were larger than contemporary processes in structuring phenotypic variation. This highlights how divergence can be better understood against a known backdrop of evolutionary history.

Moles MD, Johnston TA, Robinson BW, Bernard AM, Wilson CC, Wiegand MD, Leggett WC. Reproductive divergence between growth forms of Lake Winnipeg walleye (Sander vitreus). Ecology of Freshwater Fish 2011: 20: 52–66. © 2010 John Wiley & Sons A/S Abstract – Growth polymorphisms occur in many fish species, particularly in northern temperate freshwater lakes. The only known growth polymorphism in walleye (Sander vitreus) is in Lake Winnipeg, Canada, where slow-growing (dwarf) and fast-growing (normal) morphotypes co-exist. We examined differences in reproductive traits between forms to determine whether divergence in growth rates was accompanied by divergence in reproductive strategies and also assessed the genetic composition of both forms using mitochondrial and microsatellite DNA to determine their origin and potential reproductive isolation. Both sexes of the dwarf form produced relatively larger gonads than the normal form. Female dwarfs also had a lower liver lipid concentration, a higher ovary lipid concentration, and produced relatively larger eggs compared to the normal form. There was also some indication that dwarf and normal females differed in the fatty acid profiles of their egg lipids. Population genetic analysis based on partial sequencing of the mitochondrial control region and genotyping nine microsatellite loci indicated a single interbreeding population and sympatric origin for both morphotypes. The dwarf form appears to make a relatively larger reproductive investment at a given body size, and the differences between the two forms may result from plastic developmental responses to subtle differences in niche use.

Ecologically related polymorphisms occur in many northern freshwater fish populations and are distinguished by alternate phenotypes consistently associated with differential use of habitats or resources. We tested for polymorphism in walleye (Sander vitreus (Mitchill, 1818)) in the south basin of Lake Winnipeg, where evidence suggests that “dwarf” and “normal” growth forms may coexist. Morphometric comparisons of these growth forms at similar size demonstrated significant differences in the relative sizes of head, snout, eye, and mouth, collectively suggesting dwarf individuals inhabit a more benthic niche than normal morphs. Stable isotope analysis did not support this morphometric interpretation, however, as δ13C and δ15N isotope signatures were similar between similar-sized individuals of both forms, but differed from those of large normal walleye. Walleye from Lake Winnipeg do not seem to exhibit a simple resource polymorphism where different ecotypes are associated with alternate habitats throughout their life history, nor do they exhibit characteristics of a simple mating polymorphism. We suggest the walleye from Lake Winnipeg represent two growth forms sharing a common juvenile habitat followed by the normal form shifting into an alternative adult niche.

Direct interactions among conspecific and heterospecific animals are often mediated by aggressive behavior. We analyze the ecology and evolution of resourc-related heterospecific aggression (HA) by reviewing and meta-analysing 126 studies, contrasting HA with conspecfic aggression (CA), and discussing terminological confusions and conceptual models. HA occurred in 78% of tests (n = 459), suggesting a high prevalence and potential effect on niche use and community structure. The benefits of both CA and HA are linked to resource defensibility and abundance, yet HA can change independently of CA. Ecological inferences about HA are often weak because they assume that interference always results from resource competition, and evolutionary inferences made by comparing HA to CA are also weak because they usually ignore history. We believe that comparisons between situations where a focal species is allopatric from and sympatric with a heterospecfic competitor provide better opportunities to test hypotheses about HA. In general, according to our data set, aggression was higher with increased resource overlap as expected, both because CA was greater than HA, and HA was greater within compared to between genera. Progress in understanding HA requires distinguishing traits (aggressive behavior) from interactions (agonism, interference), as well as from the ecological and evolutionary causer (competition, ancestry) and consequences (dominance, territoriality, exclusion) of those interactions.

A key requirement of effective fisheries management is a better understanding of the mechanisms driving recruitment variability. Recent research suggests that the quantity and quality of gametes produced by fish is closely linked to the availability of surplus energy, usually stored as lipid. We sampled mature walleye (Sander vitreus) from seven populations across Canada to determine if indices of reproductive effort were positively related to indices of adult nutrient reserves, primarily body lipid concentration. Populations varied greatly in growth rates, body lipid con- tents, and reproductive traits. Within-population trends between indices of reproductive effort and parental body traits were highly variable. Body lipid concentration was most closely related to gonad lipid concentration, but not in all populations. Among populations, relative fecundity and ovary lipid concentrations were positively related, and egg size was negatively related to whole-body lipid concentrations; no trends were evident for males. Our results suggest that relationships between walleye reproductive effort and body lipid reserves may be more complex than previously thought. Resume : Un amenagement halieutique efficace necessite une meilleure comprehension des mecanismes explicatifs de la variabilite du recrutement. Des etudes recentes laissent croire que la quantite et la qualite des gametes produits par un poisson sont intimement liees a la disponibilite d'energie excedentaire, generalement accumulee sous forme de lipi- des. Nous avons echantillonne des dores (Sander vitreus) matures de sept populations provenant de divers sites d'un bout a l'autre du Canada afin de determiner si les indices d'effort reproductif sont en relation positive avec les indices d'accumulation de nutriments chez les adultes, en particulier avec le contenu lipidique du corps. Il y a parmi les popu- lations une forte variation dans les taux de croissance, les contenus lipidiques du corps et les caracteristiques reproduc- tives. Les tendances a l'interieur de chaque population sont tres variables entre les indices d'effort reproductif et les caracteristiques corporelles des parents. La concentration de lipides dans le corps est plus souvent reliee a la concentra- tion de lipides dans les gonades, mais pas dans toutes les populations. Dans l'ensemble des populations, il y a une cor- relation positive entre la fecondite relative et le contenu lipidique des ovaires, ainsi qu'une correlation negative entre la taille des oeufs et le contenu lipidique du corps entier; aucune tendance ne ressort chez les mâles. Nos resultats indi- quent que les relations entre l'effort reproductif du dore et les reserves corporelles de lipides sont plus complexes qu'on ne le croyait.

Resource use is widely thought to influence adaptive phenotypic divergence, whereas other ecological factors, such as predation, are frequently overlooked, particularly in studies of polyphenism in fishes. Juvenile pumpkinseed sunfish (Lepomis gibbosus) reared with predatory walleye (Sander vitreus) increase body depth and dorsal spine length, indicating that developmental responses to predation can shape phenotype. Body form responses to the same predator cues though have also evolutionarily diverged between sunfish ecomorphs that coexist in single lake populations by inhabiting either littoral or pelagic habitats, suggesting that predation risk varies between habitats. Here, we test if prior exposure to predator cues influences the development of behavior in juvenile pumpkinseed sunfish, if behavioral responses to the same predator cues vary between ecomorphs, and if induced phenotypic variation affects survival under predation. Behavior depended strongly on prior exposure to predator cues, but this effect varied between sunfish ecomorphs, indicating that ecomorphs have different responses to the same predator cues. Predator-induced phenotypes had higher survival than control phenotypes under simulated littoral but not pelagic conditions. Predator-induced phenotypic responses are candidate-inducible defenses, and divergent responses between ecomorphs suggest that they can evolve in response to selection imposed by differences in habitat-specific predation risk. Key words: evolution, induced defense, Lepomis gibbosus, phenotypic plasticity, polymorphism, predation. [Behav Ecol]

Phylogenetic hierarchies are often composed of younger diverging lineages nested within older diverging lineages. Comparing phenotypic variation among several hierarchical levels can be used to test hypotheses about selection, phenotypic evolution and speciation. Such hierarchical comparisons have only been performed in threespine stickleback, and so here we use a hierarchical pattern of divergences between near-shore littoral and off-shore pelagic habitats to test for selection on the evolution of body form in Lepomis sunfish in lakes. We compare variation in external body form between fish from littoral and pelagic habitats at three levels: among ecomorphs within individual lake populations (intrapopulation), among populations of the same species in different lakes (interpopulation), and between bluegill and pumpkinseed sunfish species (interspecifically). Using geometric morphometric methods, we first demonstrate that interpopulation variation in mean body form of pumpkinseed sunfish varies with the presence of pelagic habitat. We then incorporate these results with existing data in order to test the similarity of phenotypic divergence between littoral and pelagic habitats at different hierarchical levels. Parallel relationships between certain body form traits (head length, caudal length and pectoral length) and habitat occur at all three levels suggesting that selection persistently acts at all levels to diversify these traits and so may contribute to species formation. For other traits (caudal depth and pectoral altitude), divergence between habitats is inconsistent at different hierarchical levels. Thus, nested biological variation in Lepomid body form reflects a history of deterministic selection and historical contingency, and also identifies traits that likely have likely influenced fitness and so serve important functions.

Predation has important ecological and evolutionary consequences. Evolutionary responses to diversifying selection include genetic differentiation, the evolution of adaptive phenotypic plasticity, and the genetic differentiation of plastic responses between populations. We tested if pumpkinseed sunfish (Lepomis gibbosus) respond to predation cues by changing their external body form in functionally sensible ways. We then asked whether predation has influenced the divergence of coexisting littoral and pelagic ecomorphs, by testing for divergent predator-induced responses. Juvenile L. gibbosus of both ecomorphs were reared with and without predation cues supplied by walleye (Sander vitreus) feeding on L. gibbosus. Predation cues stimulated increased body depth and dorsal spine length, but no increase in anal spine length or pectoral fin size. The dorsal spines of pelagic ecomorphs also grew longer than did those of littoral ecomorphs, while positive body depth responses were similar in both ecomorphs. This is the second fish taxa in which predator-induced morphological responses have been found, and the first in which divergent responses have been detected between ecomorphs. This suggests that the developmental systems of L. gibbosus ecomorphs have diverged under selection related to predation. We propose that other 'resource polymorphisms' in fishes have evolved under selection arising from a variety of factors, including predation, and not just selection related to resource use.

Experimental biologists use reciprocal transplant experiments (RTEs) involving divergent forms to test hypotheses about fitness trade-offs across, and local adaptation to, native environments. Additional evolutionary hypotheses about diversifying selection, the evolution of specialization, and the coexistence of specialists and generalists are only testable when the RTE also includes intermediate (or alternatively generalist) forms. Environmental variation makes such RTEs challenging, and so strategies that increase their effectiveness are useful. Here, we focus on improvements to the efficiency of RTEs involving intermediate forms with respect to the experimental design and the analysis of the resulting data. We provide a likelihood ratio-based test that offers increased statistical power and robustness relative to another test involving nonlinear regression, when used both for simulated data sets and for data from a study of two divergent fish species and their hybrids transplanted between two lake habitats. The test can be used with unequal numbers of observations, unequal variances, and binomial-type survival data and other nonnormal data. Simulations suggest that having equal numbers of experimental units in each phenotype-environment combination is reasonable. The intentional pairing of observations between environmental conditions (by using clones, full sibs, or half-sibs) is beneficial when paired observations have fitnesses that are negatively related between conditions but is detrimental with positive relatedness. Our methods can be extended to study more than two divergent forms.

Body shape is a difficult, but important, trait to quantify. Researchers have traditionally used multivariate analysis of several linear measures ('trusses') across the body form to quantify shape. Newer geometric morphometric methods claim to better estimate shape because they analyze the geometry among the locations of all landmarks simultaneously rather than the linear distances between pairs of landmarks. We tested this claim by comparing the results of several traditional morphometric analyses against a newer geometric analysis involving thin-plate splines (TPS), all applied to a common data set of morphologically variable new world cichlids Amphilophus citrinellus and A. zaliosus. The TPS method yielded slightly stronger evidence of morphological differences among forms, although traditional methods also distinguished the two species. Perhaps our most important result was the idiosyncratic interpretation of shape variation among the traditional truss-based methods, whereas the generation of deformation grids using the TPS approach yielded clear and visually interpretable figures. Our results indicate that geometric morphometrics can be a more effective way to analyze and interpret body form, but also that traditional methods can be relied upon to provide statistical evidence of shape differences, although not necessarily accurate information about the nature of variation in shape.

The phenotypic diversity exhibited within and among populations of freshwater fishes in postglacial lakes has intrigued biologists for two reasons: (i) their high phenotypic variation and (ii) the apparently recent and rapid divergence of forms. Genetic and ecological studies of these taxa are shedding new light on mechanisms of divergence and species formation. Surprisingly, the roles of phenotypic plasticity in the origins, maintenance, and generation of phenotypic diversity in this system are rarely directly addressed. We synthesize the available literature on morphological plasticity in these fishes and, using a meta-analysis, test for adaptive plasticity. We conclude that (i) morphological plasticity is common in at least six families of northern freshwater fishes, (ii) plastic responses can often be induced by conditions related to littoral and pelagic lake environments, (iii) plasticity often represents adaptive responses to conditions in these habitats, and (iv) that although rarely tested, heritable variation in morphological plasticity is present. The rich amount of phenotypic plasticity has not constrained recent adaptive divergence and species formation in postglacial fishes, and instead plasticity may play a role in the notably high rates of divergence observed in these and other fishes currently undergoing adaptive radiation.

The ecological character displacement hypothesis assumes that the effects of interspecific resource competition cause divergent selection to favour phenotypes that exploit non-shared resources. This model predicts that interspecific competition declines with increased divergence. Direct tests of this decline are rare despite much comparative evidence for character displacement. We tested this prediction using a pair of divergent brook sticklebacks. Brook sticklebacks sympatric with ninespine sticklebacks have diverged from local allopatric brook populations, and so the two types of brook sticklebacks potentially represent pre- and post-displacement forms. We used enclosures placed in a lake to compare short-term fitness (growth) of sympatric (post-displacement) and allopatric (pre-displacement) brook forms in the presence and absence of ninespine sticklebacks. Brook sticklebacks grew less in the presence vs. absence of ninespine sticklebacks, indicating that interspecific competition occurred. As expected, allopatric brook forms had lower growth than sympatric forms when ninespine sticklebacks were present. This result suggests that ecological character displacement has occurred.

The evolution of ‘adaptive’ growth rate and its influence on how other life history traits evolve is a neglected topic in biology. Growth rate influences life history because size strongly influences age-specific survival and fecundity, and because growth rate defines the relationship between age and size. Improved predictions about the evolution of life history traits may be possible with a greater understanding of the factors that influence the evolution of growth rate. We experimentally tested the hypothesis that a trade off may exist between growth rate and developmental stability in freshwater threespine sticklebacks, Gasterosteus aculeatus. We compared the degree of developmental instability (measured as fluctuating asymmetry = FA) in four lateral plate and two fin traits of fish reared under a high vs. low growth regime in response to food ration and temperature. We found evidence that symmetry was reduced (FA increased) in fast growing compared to slow growing fish, suggesting that a trade off between developmental stability and growth is possible. FA plausibly reflected developmental instability because of significant associations between rank FA levels across traits in individuals. These results are preliminary because of the possible confounding effects of temperature and food ration on asymmetry, and because we do not know if this trade off has fitness or other life history consequences. Our results also do not support the hypothesis of honest signaling sometimes invoked in studies of sexual selection because greater symmetry was found under poorer rather than better resource levels.

In larval damselflies, the self-amputation (autotomy) of the caudal lamellae permits escape from predatory larval dragonflies. Lamellar joint size declines among populations with increasing risk of dragonfly predation, but the breaking force required for autotomy and the biomechanical factors that influence breaking force are unknown. If autotomy enhances survival in larval damselflies, then predation by larval dragonflies should select for joints that require less force to break. We test this adaptive hypothesis by evaluating whether breaking force is negatively related to local predation risk from larval dragonflies. We also test a cuticle structure hypothesis, which predicts that breaking force is positively related to joint size and to joint cuticle thickness because of a structural support relationship between joint and lamella. The peak force necessary for lamellar autotomy was assessed on individual larval Enallagma damselflies collected from populations that varied in risk of predation. Easier lamellar autotomy occurred in larvae from sites with higher predation risk because damselflies from fishless ponds (where predatory larval dragonflies are likely more abundant) had lower breaking forces than those from ponds with fish (where larval dragonfly predation is likely reduced). Furthermore, breaking force was a positive function of joint size and also of total cuticle cross-sectional area after controlling for joint size. This suggests that autotomy may evolve in larval damselflies under selection from small grasping predators such as larval dragonflies by favouring smaller joint size or reduced cuticle area of lamellar joints.

I assessed habitat-specific foraging efficiency of shallow-water and open-water forms from a morphologically variable population of threespine sticklebacks. Individuals sampled from open-water or shallow-water habitats tend to be morphologically divergent, although the morphology of the modal form is intermediate between the better-known benthic and limnetic pairs of stickleback species that coexist in other local lakes. I tested two hypotheses about this intraspecific variation. First, that heritable genetic variation in body form exists between forms. Morphological differences occurred between progeny reared under common laboratory conditions indicating that heritable genetic variation contributes to morphological variation. Second, that open-water and shallow-water forms face trade offs involving foraging on habitat-specific prey. The foraging efficiency of both forms was measured in two types of habitat (benthos and open-water) simulated in lab aquaria. Foraging efficiency was related to morphological differences between morphs. Relative morph efficiencies were reversed between habitats in a predictable fashion. More streamlined open-water forms consumed Artemia nauplii at a higher rate and with fewer bites than more robust shallow-water morphs. Conversely, shallow-water morphs required fewer bites to capture and consume more and larger amphipods than open-water forms. An asymmetry in the trade offs indicates that shallow-water types may be the more specialized form. These results are consistent with a hypothesis that trade offs in habitat use efficiency can be involved early in the adaptive divergence of sticklebacks into different lake environments. Similar trade offs exist for the coexisting pairs of stickleback species, suggesting that trade offs are more likely a cause, rather than a consequence, of speciation in sticklebacks that colonize lakes.

Classical Darwinian adaptation to a change in environment can ensue when selection favours beneficial genetic variation. How plastic trait responses to new conditions affect this process depends on how plasticity reveals to selection the influence of genotype on phenotype. Genetic accommodation theory predicts that evolutionary rate may sharply increase when a new environment induces plastic responses and selects on sufficient genetic variation in those responses to produce an immediate evolutionary response, but natural examples are rare. In Iceland, marine threespine stickleback that have colonized freshwater habitats have evolved more rapid individual growth. Heritable variation in growth is greater for marine full-siblings reared at low versus high salinity, and genetic variation exists in plastic growth responses to low salinity. In fish from recently founded freshwater populations reared at low salinity, the plastic response was strongly correlated with growth. Plasticity and growth were not correlated in full-siblings reared at high salinity nor in marine fish at either salinity. In well-adapted lake populations, rapid growth evolved jointly with stronger plastic responses to low salinity and the persistence of strong plastic responses indicates that growth is not genetically assimilated. Thus, beneficial plastic growth responses to low salinity have both guided and evolved along with rapid growth as stickleback adapted to freshwater.

Adaptive variation can exist at a variety of scales in biological systems, including among species, among local populations of a single species and among individuals within a single population. Trophic or resource polymorphisms in fishes are a good example of the lowest level of this hierarchy. In lakes without bluegill sunfish (Lepomis macrochirus), pumpkinseed sunfish (Lepomis gibbosus) can be trophically polymorphic, including a planktivorous limnetic form found in the pelagic habitat, in addition to the usual benthic form found in the littoral zone. In this paper we examine the degree to which morphological differences between the two forms are caused by genetic differences versus phenotypic plasticity. Adults from pelagic and littoral sites in Paradox Lake, NY, were bred separately and their progeny were raised in cages both in the open water and shallow water habitats of an artificial pond. The experimental design permitted two tests of genetic differences between the breeding stocks (in open and shallow water cages, respectively) and two tests of phenotypic plasticity (in the limnetic and benthic offspring, respectively). Limnetic progeny were more fusiform than benthic progeny raised in the same habitat. In addition, progeny of both stocks displayed limnetic-type characteristics when raised in the open water and benthic-type characteristics in the shallow water. Thus, genetic differences and phenotypic plasticity both contributed to the trophic polymorphism. Phenotypic plasticity and genetic differentiation accounted for 53 and 14%, respectively, of the variation in morphology. This study addresses the nature of subtle phenotypic differences among individuals from a single population that is embedded within a complex community, a condition that is likely to be the norm for most natural populations, as opposed to very large differences that have evolved in relatively few populations that reside in species-poor environments.

Most empirical studies of specialists and generalists have focused on com- parisons among different species. Interspecific comparisons have recently been criticized because phenotypic trade-offs can evolve after ecological specialization. As a result, it has been suggested that intraspecific variation is the appropriate level to demonstrate functional trade-offs. We demonstrate functional trade-offs for two components of fitness in pump- kinseed sunfish (Lepomis gibbosus) from a single trophically polymorphic population: con- dition factor and growth rate. Condition factor was estimated using somatic lipid content, while growth rate was determined from scale measurements. Fish with extreme benthic and limnetic morphologies are on average superior to fish with intermediate morphologies, especially in the pelagic habitat. Our study is a particularly stringent test of functional trade-offs at the intraspecific level because the morphological variation is very subtle in this population.

A brief period of drawdown can stimulate wetland productivity and enhance the attractiveness of a site for breeding Trumpeter Swans (Cygnus buccinator) by providing a nutrient pulse. Drawdown of a pond in Aurora, Ontario, lasting about 8 weeks in late summer and fall 2009 followed by re-flooding increased the abundance of invertebrates, especially snails, in the following year. This response was ephemeral, lasting 1 year. Wild Trumpeter Swans and their cygnets responded by selective feeding the year after drawdown, despite the risk of predation by Snapping Turtles (Chelydra serpentina). There was a strong correlation between the feeding activity of two cygnets and the local abundance of snails in the pond in 2010. The nutritional content, especially protein, calcium, phosphorus, and magnesium, of a variety of abundant foods satisfied the requirements for skeletal growth and development and was higher than that of available commercial duck grower rations. The responsive feeding behaviours of the cygnets are typical of specific appetitive behaviour and suggest that swans rapidly exploit unpredictable nutrient fluxes in their local environment.

Biologists concerned with the evolutionary consequences of competition seem unaware of a large literature on freshwater fishes that bears directly on the debates over ecological character displacem...

Historical and contemporary evolutionary processes can both contribute to patterns of phenotypic variation among populations of a species. Recent studies are revealing how interactions between historical and contemporary processes better explain observed patterns of phenotypic divergence than either process alone. Here, we investigate the roles of evolutionary history and adaptation to current environmental conditions in structuring phenotypic variation among polyphenic populations of sunfish inhabiting 12 postglacial lakes in eastern North America. The pumpkinseed sunfish polyphenism includes sympatric ecomorphs specialized for littoral or pelagic lake habitats. First, we use population genetic methods to test the evolutionary independence of within-lake phenotypic divergences of ecomorphs and to describe patterns of genetic structure among lake populations that clustered into three geographical groupings. We then used multivariate analysis of covariance (MANCOVA) to partition body shape variation (quantified with geometric morphometrics) among the effects of evolutionary history (reflecting phenotypic variation among genetic clusters), the shared phenotypic response of all populations to alternate habitats within lakes (reflecting adaptation to contemporary conditions), and unique phenotypic responses to habitats within lakes nested within genetic clusters. All effects had a significant influence on body form, but the effects of history and the interaction between history and contemporary habitat were larger than contemporary processes in structuring phenotypic variation. This highlights how divergence can be better understood against a known backdrop of evolutionary history.

Over the last three decades, sunfish of the familyCentrarchidae have become recognized as a model system in which the ecological consequences of species interactions can be observed and tested. The evolutionary consequences of species interactions in sunfish have received less attention. Bluegill (Lepomis machrochirus) and pumpkinseed (Lepomis gibbosus) sunfish are two common and well-studied species that occupy separate ecological niches. Adult bluegill are generalists that feed in the open water on zooplankton during much of the year, while adult pumpkinseeds specialize on crushing hard-bodied prey such as snails. These species coexist over much of their geographical ranges, but bluegill are historically absent from several large drainage basins in the northeastern US. Here we show that pumpkinseeds from an Adirondack lake without bluegills have differentiated into two morphological forms, one of which is planktivorous. Differentiation is independent of sex and occurs over a broad range of sizes. Thus, the ecological diversity that exists between the bluegill and pumpkinseeds in sympatry has been replaced by a comparable degree of diversity within pumpkinseeds in allopatry.

Species that appear highly specialized on the basis of their phenotype (e.g., morphology, behavior, and physiology) also sometimes act as ecological generalists. This apparent paradox has been used to argue against the importance of competition as a diversifying evolutionary force. We provide an alternative explanation based on optimal foraging theory. Some resources are intrinsically easy to use and are widely preferred, while others require specialized phenotypic traits on the part of the consumer. This asymmetry allows optimally foraging consumers to evolve phenotypic specializations on nonpreferred resources without greatly compromising their ability to use preferred resources. The evolution of phenotypic specialization on nonpreferred resources can be driven by competition, but the specialists act as ecological generalists whenever their preferred resources are available. Our model identifies at least three different concepts of specialization that need to be distinguished, based on diet, prey utilization efficiencies, and phenotypic adaptations. The relationships among these concepts are complex and often counterintuitive. Specialists should often reject the very resources that they have evolved traits to use. The most extreme phenotypic specializations should occur in the absence of a trade-off between using preferred and nonpreferred resources. Our model may explain why extreme phenotypic-specializations evolve more often in fish communities than in terrestrial vertebrate communities and provides a mechanism whereby species can coexist in stable communities despite common preferences for some resources.

We have estimated the phenotypic correlation between growth rate and interactive behavior in a laboratory population of hybrid tilapia (O. mossambicus × hornorum). Observations were made on growth and several measures of interactive behavior (in both high and low competitive feeding situations) in group-reared juveniles over 52 days. Growth was not food-limited. Four feeding behavior variables: BITES, BOUTS, LATENCY, and FFI plus one agonistic behavior variable, DONOR, were significantly correlated with size-specific growth rate. The total proportion of growth rate variance “explained” by the observed behavior (R2) was high, ranging from 0.69 to 0.82. The correlations were quantitatively, but not qualitatively influenced by the size of the individual relative to its competitors. Growth was positively correlated with measures of the intensity of feeding activity, and negatively correlated with agonistic behavior. Sex accounted for significant growth rate variation but for almost no behavioral variation. On the basis of these phenotypic correlations we suggest that agonistic interaction (aggression) in this tilapia is unlikely to be indirectly selected for during simple growth rate selection of fish reared under adequate food resource conditions.

Agonistic behavior between heterospecifics, in which individuals of one species attack another, may cause a subordinate species to shift resource or habitat use. Subsequent evolutionary responses to selection may mimic shifts expected under ecological character displacement, but with no role played by exploitative competition. Alternatively, aggressive behavior can evolve when fitness is improved by excluding members of a coexisting species from a defendable resource through interference. We tested whether heterospecific agonistic behavior has evolved in brook stickleback (Culaea inconstans) by comparing replicate allopatric populations to those sympatric with ninespine stickleback (Pungitius pungitius). We also tested for heritable variation in heterospecific aggressive behavior by rearing family groups in a common environment. Allopatric populations of brook stickleback were more aggressive than ninespine stickleback, suggesting that pre-existing aggression in brook stickleback contributed to niche shifts by ninespine stickleback. In addition, sympatric adult brook stickleback were more aggressive toward ninespine stickleback than brook stickleback from allopatric populations. Overt heterospecific aggressive behaviors were heritable, and aggression in juvenile brook stickleback increased with age in sympatric but not in allopatric populations reared in a common environment. Brook stickleback have evolved increased aggression when they coexist with ninespine stickleback. These stickleback communities have been structured by both evolved and pre-existing variation in heterospecific aggressive behavior in brook stickleback.

Morphological plasticity can influence adaptive divergence when it affects fitness components such as foraging performance. We induced morphological variation in pumpkinseed sunfish (Lepomis gibbosus) ecomorphs and tested for effects on foraging performance. Young-of-year pumpkinseed sunfish from littoral and pelagic lake habitats were reared each on a ‘specialist diet’ representing their native habitat-specific prey, or a ‘generalist diet’ reflecting a combination of native and non-native prey. Specialist and generalist diets, respectively, induced divergent and intermediate body forms. Specialists had the highest capture success on their native prey whereas generalist forms were inferior. Specialists faced trade-offs across prey types. However, pelagic specialists also had the highest intake rate on both prey types suggesting that foraging trade-offs are relaxed when prey are abundant. This increases the likelihood of a resource polymorphism because the specialized pelagic form can be favoured by directional selection when prey are abundant and by diversifying selection when prey resources are restricted.

Colonization of a novel environment is expected to result in adaptive divergence from the ancestral population when selection favors a new phenotypic optimum. Local adaptation in the new environment occurs through the accumulation and integration of character states that positively affect fitness. The role played by plastic traits in adaptation to a novel environment has generally been ignored, except for variable environments. We propose that if conditions in a relatively stable but novel environment induce phenotypically plastic responses in many traits, and if genetic variation exists in the form of those responses, then selection may initially favor the accumulation and integration of functionally useful plastic responses. Early divergence between ancestral and colonist forms will then occur with respect to their plastic responses across the gradient bounded by ancestral and novel environmental conditions. To test this, we compared the magnitude, integration, and pattern of plastic character responses in external body form induced by shallow versus open water conditions between two sunfish ecomorphs that coexist in four postglacial lakes. The novel sunfish ecomorph is present in the deeper open water habitat, whereas the ancestral ecomorph inhabits the shallow waters along the lake margin. Plastic responses by open water ecomorphs were more correlated than those of their local shallow water ecomorph in two of the populations, whereas equal levels of correlated plastic character responses occurred between ecomorphs in the other two populations. Small but persistent differences occurred between ecomorph pairs in the pattern of their character responses, suggesting a recent divergence. Open water ecomorphs shared some similarities in the covariance among plastic responses to rearing environment. Replication in the form of correlated plastic responses among populations of open water ecomorphs suggests that plastic character states may evolve under selection. Variation between ecomorphs and among lake populations in the covariance of plastic responses suggests the presence of genetic variation in plastic character responses. In three populations, open water ecomorphs also exhibited larger plastic responses to the environmental gradient than the local shallow water ecomorph. This could account for the greater integration of plastic responses in open water ecomorphs in two of the populations. This suggests that the plastic responses of local sunfish ecomorphs can diverge through changes in the magnitude and coordination of plastic responses. Although these results require further investigation, they suggest that early adaptive evolution in a novel environment can include changes to plastic character states. The genetic assimilation of coordinated plastic responses could result in the further, and possibly rapid, divergence of such populations and could also account for the evolution of genes of major effect that contribute to suites of phenotypic differences between divergent populations.

Ecological character displacement (ECD) provides opportunities to test how resource competition generates diversifying selection that results in adaptive divergence. We quantify an association between phenotypic and ecological divergence between two similar small fishes, brook (Culaea inconstans) and ninespine (Pungitius pungitius) sticklebacks, in replicate northern Ontario lakes, Canada. The two species partition resources and habitat, where they coexist, and brooks that coexist with ninespines are more benthically specialized in body form and diet than brooks from local allopatric populations. Here we test various explanations for this pattern. Chance is unlikely to have been the primary cause because divergence is replicated in three separate populations. Preliminary comparisons indicate that resource availability and a variety of abiotic ecological conditions are generally similar between sympatric and allopatric sites, and so do not readily account for the divergence. Biased colonization or extinction is less likely to account for the divergence because character values in sympatry tend to exceed those in allopatry, as expected if they have repeatedly evolved under diversifying selection. Recent studies have also demonstrated that these two species compete, and that competitive effects are more severe for allopatric compared to sympatric brook forms, as predicted if divergence reflects the ghost of competition past. Ongoing studies indicate heritable variation in this system. Our results suggest that even small amounts of character shifts can influence competition and hence relative fitness, further implicating a role for ECD in the evolution of biodiversity.

Colonization of a novel environment is expected to result in adaptive divergence from the ancestral population when selection favors a new phenotypic optimum. Local adaptation in the new environment occurs through the accumulation and integration of character states that positively affect fitness. The role played by plastic traits in adaptation to a novel environment has generally been ignored, except for variable environments. We propose that if conditions in a relatively stable but novel environment induce phenotypically plastic responses in many traits, and if genetic variation exists in the form of those responses, then selection may initially favor the accumulation and integration of functionally useful plastic responses. Early divergence between ancestral and colonist forms will then occur with respect to their plastic responses across the gradient bounded by ancestral and novel environmental conditions. To test this, we compared the magnitude, integration, and pattern of plastic character responses in external body form induced by shallow versus open water conditions between two sunfish ecomorphs that coexist in four postglacial lakes. The novel sunfish ecomorph is present in the deeper open water habitat, whereas the ancestral ecomorph inhabits the shallow waters along the lake margin. Plastic responses by open water ecomorphs were more correlated than those of their local shallow water ecomorph in two of the populations, whereas equal levels of correlated plastic character responses occurred between ecomorphs in the other two populations. Small but persistent differences occurred between ecomorph pairs in the pattern of their character responses, suggesting a recent divergence. Open water ecomorphs shared some similarities in the covariance among plastic responses to rearing environment. Replication in the form of correlated plastic responses among populations of open water ecomorphs suggests that plastic character states may evolve under selection. Variation between ecomorphs and among lake populations in the covariance of plastic responses suggests the presence of genetic variation in plastic character responses. In three populations, open water ecomorphs also exhibited larger plastic responses to the environmental gradient than the local shallow water ecomorph. This could account for the greater integration of plastic responses in open water ecomorphs in two of the populations. This suggests that the plastic responses of local sunfish ecomorphs can diverge through changes in the magnitude and coordination of plastic responses. Although these results require further investigation, they suggest that early adaptive evolution in a novel environment can include changes to plastic character states. The genetic assimilation of coordinated plastic responses could result in the further, and possibly rapid, divergence of such populations and could also account for the evolution of genes of major effect that contribute to suites of phenotypic differences between divergent populations.

While the importance of competition in adaptive divergence has recently been demonstrated in a number of systems, its interaction with other ecological factors has been neglected. We attempt a more pluralistic approach in this comparative study of di- vergence in pumpkinseed sunfish (Lepomis gibbosus) from the Adirondack region of New York. In some lakes, pumpkinseed forms that use shallow- and open-water environments coexist. We tested four predictions relating to an hypothesis of character release: (1) Pump- kinseed divergence is favored in the absence of the closely related bluegill sunfish (L. macrochirus), a known competitor elsewhere, (2) divergence is favored when the combined effects of other planktivorous fish taxa that may impose diffuse competition is reduced, (3) divergence is positively related to lake size because larger lakes provide more distinct and diverse environments, and (4) divergence is favored in lakes dominated by open-water habitat. In two lakes with introduced bluegill (sympatry), bluegill appear to limit pump- kinseed divergence, as they do elsewhere. Pumpkinseeds were neither found in the open- water habitat, nor did they eat significant amounts of open-water prey. In nine allopatric lakes selected for their similarity to the sympatric lakes, pumpkinseeds were ecologically and morphologically divergent, consistent with a character release hypothesis. Morpho- logical divergence followed functional expectations. Open-water pumpkinseeds generally had longer caudal peduncles, shorter heads, thinner bodies, and smaller paired fins compared to shallow-water pumpkinseeds (heritable variation in body shape has been demonstrated elsewhere). Among 26 allopatric lakes, other planktivores had a negative effect on intra- population divergence as if they collectively imposed diffuse competition. Surprisingly, lake size was unrelated to divergence, and the amount of open-water habitat had an un- expected negative effect. Mean body size was also negatively related to divergence. Our findings illustrate that a variety of ecological factors, in addition to competition, may influence the divergence of pumpkinseeds, and possibly other fishes in postglacial north temperate lakes.