ARE PARALLEL MORPHOLOGIES OF CAVE ORGANISMS THE RESULT OF SIMILAR SELECTION PRESSURES?

The amphipod Gammarus minus is present in both caves and springs, with cave populations showing elaborated (size and antennae) and reduced (eye) characters relative to spring populations. Earlier studies have shown that cave populations resulted from independent invasions of hydrologically isolated subterranean drainages and that there is genetic variation for both elaborated and reduced characters. In this study we tested the hypothesis that a similar pattern of selection on isolated cave populations is responsible for the parallel evolution of cave morphologies. We used variation in mating success and fecundity to test for the presence of directional selection on eye, antennal, and body size characters in a set of cave and spring populations during a series of seasonal cross-sectional samplings. We found significant directional selection for smaller eyes in caves and larger eyes in springs, which supports the hypothesis that selection is responsible for reduced eye size in cave populations. We also found selection for larger body and antennal size in cave populations, which is consistent with the hypothesis that parallel patterns of selection in caves are responsible for the elaboration of body and antennal size. However, we found selection for larger body and antennal size in spring populations that is not consistent with the observed divergence of spring and cave populations. We suggest that unmeasured components of viability selection could be more important in springs than in caves and may act against the selection for larger size found in spring populations.     

The evolution of eye size in response to increased fish predation in Daphnia

Variation in eye size is ubiquitous across taxa. Increased eye size is correlated with improved vision and increased fitness via shifts in behavior. Tests of the drivers of eye size evolution have focused on macroevolutionary studies evaluating the importance of light availability. Predator‐induced mortality has recently been identified as a potential driver of eye size variation. Here, we tested the influence of increased predation by the fish predator, the alewife (Alosa pseudoharengus) on eye size evolution in waterfleas (Daphnia ambigua) from lakes in Connecticut. We quantified the relative eye size of Daphnia from lakes with and without alewife using wild‐caught and third‐generation laboratory reared specimens. This includes comparisons between lakes where alewife are present seasonally (anadromous) or permanently (landlocked). Wild‐caught specimens did not differ in eye size across all lakes. However, third‐generation lab reared Daphnia from lakes with alewife, irrespective of the form of alewife predation, exhibited significantly larger eyes than Daphnia from lakes without alewife. This genetically based increase in eye size may enhance the ability of Daphnia to detect predators. Alternatively, such shifts in eye size may be an indirect response to Daphnia aggregating at the bottom of lakes. To test these mechanisms, we collected Daphnia as a function of depth and found that eye size differed in Daphnia found at the surface versus the bottom of the water column between anadromous alewife and no alewife lakes. However, we found no evidence of Daphnia aggregating at the bottom of lakes. Such results indicate that the evolution of a larger eye may be explained by a connection between eyes and enhanced survival. We discuss the cause of the lack of concordance in eye size variation between our phenotypic and genetic specimens and the ultimate drivers of eye size.     

Abundance, body composition and reproductive output of Gammarus minus (Crustacea: Amphipoda) in ten cold springs differing in pH and ionic content

1. A survey of thirty-two rheocrene springs in central Pennsylvania revealed that, Like Gammarus in lakes and streams, Gammarus minus is absent from springs with pH <6.0 and conductivity <25μS cm^?1 (total range in pH = 4.6–7.7, and in conductivity = 14–411 μS cm^?1). 2. In ten springs G. minus density was positively correlated with Ca²⁺ and Mg²⁺ hardness, but not with pH, unless three springs with either an exceptionally high velocity or extremely high densities of the potentially competing snail Fontigens nickliniana were omitted. 3. Adults were larger in the springs with few or no large predators than in those with more predators. In all ten springs, adult dry mass was unrelated to spring pH and ionic content, but brooding female dry mass covaried positively with Ca²⁺ and Mg²⁺ hardness in the five predator-poor springs. 4. Body water, Na and Ca contents and body mass/length ratios varied independently of spring pH and ionic content. Water content was inversely correlated with fat content, but even when expressed as a percentage of fat-free wet mass, it was unrelated to water chemistry. 5. In juveniles, males and non-brooding females, fat content varied independently of spring pH and ionic content, but in brooding females it was correlated with alkalinity and Ca²⁺ and Mg²⁺ hardness. The cost of reproduction in brooding females may have been a factor here; they had significantly lower per cent fat than did non-brooding females. Juvenile fat content did not differ significantly among spring populations, whereas adult fat content did. The per cent fat of brooding females covaried positively with body size among springs, and this was marginally true for non-brooding females, as well. The residuals of brooding female per cent fat against dry mass were not related to water chemistry. 6. Brood size (number of embryos in a brood) and brood mass varied significantly among populations, but independently of spring pH and ionic content. Both covaried positively with maternal body size among springs. The residuals of these relationships were unrelated to water chemistry, as was the percentage of females brooding. 7. G. minus from a pH 6 spring survived better and lost less body mass in acidic soft water than did those from a pH 7.6 spring. However, although G. minus has apparently been able to adapt (or acclimate) to pH 6 water it has failed to adapt to more acidic waters. A physiological or structural constraint may be involved because this species has probably had ample opportunity to evolve resistance to dilute acidic water. This hypothesis is consistent with the threshold effect observed: above pH 6 G. minus shows very little evidence of osmotic or metabolic stress, but beiow pH 6 viable populations apparently cannot survive at all. However, the gradual linear decrease in population density of G. minus with decreasing alkalinity and Ca²⁺ and Mg²⁺ hardness suggests that other factors may also be involved (e.g. a decrease in food quality).     

Resource limitation and intraspecific patterns of weight x length variation among spring detritivores

This study examined the intraspecific variability of a phenotypic trait, the body weight/body length ratio, and its adequacy to provide unbiased information about patterns of resource availability among conspecific individuals. Individual body weight and length were measured for the amphipod Gammarus minus Say (and other detritivores) in samples from freshwater springs differing in expected resource availability, and from sites in which detritus abundance had been manipulated. Mean individual weight per length was lower: (a) in populations from low-richness than from high-richness detritivore guilds; (b) in populations with size-abundance distributions similar to that of the entire guild, rather than statistically different; (c) in population samples from outside than from inside the areas of detritus addition. Small-sized individuals (< 3 mm) showed the largest variation among both populations and population samples. Similar differences were qualitatively observed for the other common detritivores co-occurring with G. minus in some springs. These observed patterns were in agreement with the variation of resource availability expected among field conditions, supporting the relevance of weight per length as a measure of food limitation.     

Ecological constraints on sensory systems: compound eye size in Daphnia is reduced by resource limitation

Eye size is an indicator of visual capability, and macroevolutionary patterns reveal that taxa inhabiting dim environments have larger eyes than taxa from bright environments. This suggests that the light environment is a key driver of variation in eye size. Yet other factors not directly linked with visual tasks (i.e., non-sensory factors) may influence eye size. We sought to jointly investigate the roles of sensory (light) and non-sensory factors (food) in determining eye size and ask whether non-sensory factors could constrain visual capabilities. We tested environmental influences on eye size in four species of the freshwater crustacean Daphnia, crossing bright and dim light levels with high and low resource levels. We measured absolute eye size and eye size relative to body size in early and late adulthood. In general, Daphnia reared on low resources had smaller eyes, both absolutely and relatively. In contrast to the dominant macroevolutionary pattern, phenotypic plasticity in response to light was rarely significant. These patterns of phenotypic plasticity were true for overall diameter of the eye and the diameter of individual facets. We conclude that non-sensory environmental factors can influence sensory systems, and in particular, that resource availability may be an important constraint on visual capability.     

Predation on freshwater snails in Miocene Lake Steinheim: a trigger for intralacustrine evolution?

Shells of the freshwater gastropods Gyraulus trochiformis (Planorbidae or ramshorn snails) and Bania pseudoglobula (formerly Pseudamnicola, Hydrobiidae) from Miocene Steinheim Basin in SW Germany contain small holes with a mean diameter of 0.8 mm. Analyses of comparable holes are so far unknown from fossil or extant assemblages of freshwater shells. This analysis of the perforated shells suggests that the holes were not formed by post‐depositional or pure taphonomic processes. Instead, they were most likely produced by predators. This analysis widens the means for identification of predation on freshwater snail shells that can be used in other palaeolimnological studies. The co‐occurrence of fish teeth and perforated shells in the studied samples as well as the perforation features suggest that the predator was either barbel or tench fish. The correlation between shell sizes and hole diameters suggests a size relationship between predator and prey that may generally be related to gape‐limited fish predators. The co‐occurrence of perforated shells in these lake sediments with a dominance of large and armoured shells suggest that these larger shells with protuberances and knobs could not be crushed by the gape‐limited fish predators. This analysis is the basis for a hypothesis that the endemic evolution of Gyraulus in Lake Steinheim, with some varied forms of shell thickness and morphology, was triggered by a predator–prey relationship based on adaptations to avoid shell‐breaking predators.     

Changes in morphological characteristics of an introduced population of the threespine stickleback <Emphasis Type="Italic">Gasterosteus aculeatus</Emphasis> in Lake Towada,northern Japan

A freshwater population of the threespine stickleback Gasterosteus aculeatus occurs at an elevation of 400m in Lake Towada in the mountains of northern Japan. Morphological characteristics of each sex of this population were studied between 1985 and 1992. The 1985 fishes were larger in standard length than even anadromous populations, as well as all other freshwater populations, so the large body size of this population was unusual for the species. The population was also characterized by large ratios of head length, eye diameter, and gill raker length, and a higher ovary volume in 1992 than in 1985, but it was not readily characterized by meristic characters. The population may have been introduced within the last 20 years (probably in the early 1980s). The large body size of the population may not reflect a defense against predators because coexisting fishes were not important predators on threespine stickleback adults. It is supposed that the change of body size was induced by environmental changes in prey abundance in the lake and/or by the longevity of fishes.     

Predation pressure shapes brain anatomy in the wild

There is remarkable diversity in brain anatomy among vertebrates and evidence is accumulating that predatory interactions are crucially important for this diversity. To test this hypothesis, we collected female guppies (Poecilia reticulata) from 16 wild populations and related their brain anatomy to several aspects of predation pressure in this ecosystem, such as the biomass of the four major predators of guppies (one prawn and three fish species), and predator diversity (number of predatory fish species in each site). We found that populations from localities with higher prawn biomass had relatively larger telencephalon size as well as larger brains. Optic tectum size was positively associated with one of the fish predator’s biomass and with overall predator diversity. However, both olfactory bulb and hypothalamus size were negatively associated with the biomass of another of the fish predators. Hence, while fish predator occurrence is associated with variation in brain anatomy, prawn occurrence is associated with variation in brain size. Our results suggest that cognitive challenges posed by local differences in predator communities may lead to changes in prey brain anatomy in the wild.     

Standing genetic variation as a potential mechanism of novel cave phenotype evolution in the freshwater isopod,Asellus aquaticus

Novel phenotypes can come about through a variety of mechanisms including standing genetic variation from a founding population. Cave animals are an excellent system in which to study the evolution of novel phenotypes such as loss of pigmentation and eyes. Asellus aquaticus is a freshwater isopod crustacean found in Europe and has both a surface and a cave ecomorph which vary in multiple phenotypic traits. An orange eye phenotype was previously revealed by F₂ crosses and backcrosses to the cave parent within two examined Slovenian cave populations. Complete loss of pigmentation, both in eye and body, is epistatic to the orange eye phenotype and therefore the orange eye phenotype is hidden within the cave populations. Our goal was to investigate the origin of the orange eye alleles within the Slovenian cave populations by examining A. aquaticus individuals from Slovenian and Romanian surface populations and Asellus aquaticus infernus individuals from a Romanian cave population. We found orange eye individuals present in lab raised surface populations of A. aquaticus from both Slovenia and Romania. Using a mapping approach with crosses between individuals of two surface populations, we found that the region known to be responsible for the orange eye phenotype within the two previously examined Slovenian cave populations was also responsible within both the Slovenian and the Romanian surface populations. Complementation crosses between orange eye Slovenian and orange eye Romanian surface individuals suggest that the same gene is responsible for the orange eye phenotype in both surface populations. Additionally, we observed a low frequency phenotype of eye loss in crosses generated between the two surface populations and also in the Romanian surface population. Finally, in a cave population from Romania, A. aquaticus infernus, we found that the same region is also responsible for the orange eye phenotype as the Slovenian cave populations and the Slovenian and Romanian surface populations. Therefore, we present evidence that variation present in the cave populations could originate from standing variation present in the surface populations and/or transgressive hybridization of different surface phylogenetic lineages rather than de novo mutations.     

Divergence in reproductive ecology of the three-spined stickleback,Gasterosteus aculeatus

An anadromous population (trachurus form) and three freshwater populations (leiurus form) of the three-spined stickleback,Gasterosteus aculeatus, in central Japan were compared with one another in the reproductive traits: body length at maturity, egg size, clutch size, the swell of abdomen and the number of eggs per nest. The anadromous fish which become larger in body size at maturity spawn eggs smaller in size and greater in number than the freshwater fish. The abdomen swell of gravid females expressed by the proportion of abdomen width to body length was greater in the freshwater fish. The anadromous male fish collected a mean of 2,638 eggs with a range of 1,119 to 4,052 eggs from about 6–7 females. In the three freshwater (the Yamayoke, the Tsuya and the Jizo) populations, males must have mated with about 9–22, 7–18 and 4–7 females respectively. It seems that theleiurus form increases its reproductive success by its much more mating opportunities and the parental efforts of nesting males as well as by spawning large eggs. Furthermore, among the freshwater populations, the Jizo one inhabiting the upper stream was clearly larger in body size, in egg size and in clutch size than the Yamayoke and the Tsuya ones which inhabit stable waterbodies with springs. It is possible that the Jizo population adopted the strategy of spawning a few large eggs as an adaptation to its habitat. The causal and functional explanations in reproductive characteristics among the four populations are discussed in regard to differences in the environmental conditions.     

Is fatter fitter? Body storage and reproduction in ten populations of the freshwater amphipod Gammarus minus

Relationships between body storage (estimated as fat content and residuals of body mass regressed against body length) and offspring investment [brood mass, brood size (number of embryos per brood) and embryo mass] were examined within and among populations of the amphipod Gammarus minus in ten cold springs in central Pennsylvania, USA. Two major hypotheses and six corollary hypotheses were tested. Total reproductive investment (brood mass and brood size) was usually strongly positively correlated with maternal body length and body storage both within and among populations. These positive associations between reproductive and somatic investments are expected if individual variation in resource acquisition exceeds that of resource allocation. That is, individuals or populations that are able to acquire more resources should also be able to allocate more resources to both reproduction and somatic reserves than those acquiring fewer resources. This hypothesis is consistent with evidence showing that individual differences in body storage in G. minus and other amphipods are related to differences in resource acquisition. Positive associations between reproductive and somatic investments do not mean that energy costs of reproduction do not exist in G. minus. Evidence for reproductive energy costs included the lower body-fat contents of brooding versus nonbrooding females and the relatively low body mass per length of females who had just deposited eggs in their brood pouch. Unlike brood mass and brood size, individual embryo mass was usually unrelated to maternal body length and body storage. This pattern is largely consistent with optimal offspring investment theory, which predicts that offspring size should be insensitive to variation in parental resource status. However, in contrast to theory, embryo mass increased in winter when brooding females were significantly ”fatter”, presumably due to the availability of autumn-shed leaf food. This seasonal change in offspring size may be a maternally mediated effect of increased resource availability, though other explanations are possible. Overall, this study suggests that ”fatter” female amphipods are fitter than ”thinner” ones, though both the costs and benefits of increased body storage and brood size require investigation to substantiate this claim. This study also suggests that effects of individual variation in resource acquisition on life-history patterns deserve more theoretical and empirical attention by ecologists than they have received. It should be recognized that positive and/or nonsignificant correlations between life-history traits are just as interesting and important as are the negative correlations predicted by many theoretical models. Received: 20 January 1999 / Accepted: 26 September 1999     

Coordinated evolution of brain size,structure, and eye size in Trinidadian killifish

Brain size, brain architecture, and eye size vary extensively in vertebrates. However, the extent to which the evolution of these components is intricately connected remains unclear. Trinidadian killifish, Anablepsoides hartii, are found in sites that differ in the presence and absence of large predatory fish. Decreased rates of predation are associated with evolutionary shifts in brain size; males from sites without predators have evolved a relatively larger brain and eye size than males from sites with predators. Here, we evaluated the extent to which the evolution of brain size, brain structure, and eye size covary in male killifish. We utilized wild‐caught and common garden‐reared specimens to determine whether specific components of the brain have evolved in response to differences in predation and to determine if there is covariation between the evolution of brain size, brain structure, and eye size. We observed consistent shifts in brain architecture in second generation common garden reared, but not wild caught preserved fish. Male killifish from sites that lack predators exhibited a significantly larger telencephalon, optic tectum, cerebellum, and dorsal medulla when compared with fish from sites with predators. We also found positive connections between the evolution of brain structure and eye size but not between overall brain size and eye size. These results provide evidence for evolutionary covariation between the components of the brain and eye size. Such results suggest that selection, directly or indirectly, acts upon specific regions of the brain, rather than overall brain size, to enhance visual capabilities.     

Molecular genetic variation and population structure in morphologically differentiated cave and surface populations of the freshwater amphipod Gammarus minus

Gammarus minus is an important component of surface spring and cave ecosystems throughout Appalachia, and is a useful indicator of the hydrology and gene flow in freshwater communities. Gammarus minus populations occupying large cave passages (> 2 km) are usually troglomorphic, having reduced eyes, fewer ommatidia, larger body size, longer antennae, and reduced pigmentation relative to surface populations. We surveyed five cave stream and 10 surface spring populations for DNA sequence variation in the cytochrome c oxidase I (COI) and internal transcribed spacer 1 (ITS-1) genes with an aim towards characterizing phylogeographical structure and comparing the nature of genetic variation in cave vs. surface populations. Although standing variation at both loci was rather low within populations, a significant degree of divergence and spatial structuring of populations was observed. Levels of genetic variation within cave and spring populations differed substantially, with caves harbouring significantly less variation at the COI locus than surface springs. Codon usage bias was significantly lower in caves, indicating that cave streams harbour smaller and/or more recently colonized populations. Overall these data indicate limited gene flow among populations and suggest that the cave populations sampled in this study are prone to bottlenecks, possibly due to larger temperature fluctuations and more frequent incidence of drought conditions associated with these particular cave habitats.     

Transplant experiments demonstrate that larger brains are favoured in high-competition environments in Trinidadian killifish

The extent to which the evolution of a larger brain is adaptive remains controversial. Trinidadian killifish (Anablepsoides hartii) are found in sites that differ in predation intensity; fish that experience decreased predation and increased intraspecific competition exhibit larger brains. We evaluated the connection between brain size and fitness (survival and growth) when killifish are found in their native habitats and when fish are transplanted from sites with predators to high-competition sites that lack predators. Selection for a larger brain was absent within locally adapted populations. Conversely, there was a strong positive relationship between brain size and growth in transplanted but not resident fish in high-competition environments. We also observed significantly larger brain sizes in the transplanted fish that were recaptured at the end of the experiment versus those that were not. Our results provide experimental support that larger brains increase fitness and are favoured in high-competition environments.     

Ontogenetic and spatial variation in size‐selective mortality of a marine fish

Although body size can affect individual fitness, ontogenetic and spatial variation in the ecology of an organism may determine the relative advantages of size and growth. During an 8‐year field study in the Bahamas, we examined selective mortality on size and growth throughout the entire reef‐associated life phase of a common coral‐reef fish, Stegastes partitus (the bicolour damselfish). On average, faster‐growing juveniles experienced greater mortality, though as adults, larger individuals had higher survival. Comparing patterns of selection observed at four separate populations revealed that greater population density was associated with stronger selection for larger adult size. Large adults may be favoured because they are superior competitors and less susceptible to gape‐limited predators. Laboratory experiments suggested that selective mortality of fast‐growing juveniles was likely because of risk‐prone foraging behaviour. These patterns suggest that variation in ecological interactions may lead to complex patterns of lifetime selection on body size.     

Development of a technique for quarantining Great Artesian Basin springs from colonisation by the invasive fish Eastern Gambusia (Gambusia holbrooki)

A species recovery project for the endangered fish Red‐finned Blue‐eye (Scaturiginichthys vermeilipinnis) commenced in 2009 at Edgbaston Reserve in central western Queensland. To date, the project has included relocations and control of alien Eastern Gambusia (henceforth Gambusia, Gambusia holbrooki), using the piscicide rotenone. Preventing Gambusia colonising springs where Red‐finned Blue‐eye occur or have been relocated is crucial to the success of the project. As such, this study tested the use of porous silt fences that were installed around five springs from 2012. Results indicate that silt fencing is a suitable barrier material and that provided installed silt fences are serviced regularly, the material is an effective method of quarantining populations of endangered species in isolated aquatic environments. Four of the five installed fences were effective, and maintenance consisted of small repairs at regular intervals (quarterly) followed by replacement after 2 years. As such, silt fencing may have wider application in the conservation of endangered fish and other organisms, particularly in areas where isolated waterbodies occur and where their size renders exclusion fencing a practical recovery action.     

Life-history phenotypes in populations of Brachyrhaphis episcopi (Poeciliidae) with different predator communities

Variation among populations in extrinsic mortality schedules selects for different patterns of investment in key life-history traits. We compared life-history phenotypes among 12 populations of the live-bearing fish Brachyrhaphis episcopi. Five populations co-occurred with predatory fish large enough to prey upon adults, while the other seven populations lacked these predators. At sites with large predatory fish, both sexes reached maturity at a smaller size. Females of small to average length that co-occurred with predators had higher fecundity and greater reproductive allotment than those from populations that lacked predators, but the fecundity and reproductive allotment of females one standard deviation larger than mean body length did not differ among sites. In populations with large predatory fish, offspring mass was significantly reduced. In each population, fecundity, offspring size and reproductive allotment increased with female body size. When controlling for maternal size, offspring mass and number were significantly negatively correlated, indicating a phenotypic trade-off. This trade-off was non-linear, however, because reproductive allotment still increased with brood size after controlling for maternal size. Similar differences in life-history phenotypes among populations with and without large aquatic predators have been reported for Brachyrhaphis rhabdophora in Costa Rica and Poecilia reticulata (a guppy) in Trinidad. This may represent a convergent adaptation in life-history strategies attributable to predator-mediated effects or environmental correlates of predator presence.     

Evolutionary interplay between ecology,morphology and reproductive behavior in threespine stickleback,Gasterosteus aculeatus

Synopsis Throughout its range, freshwater populations in the Gasterosteus aculeatus species complex display remarkable differentiation of morphology and behavior, much of which reflects differences in ecological conditions among habitats. We first describe the ecological conditions that have led to morphological and behavioral divergence in two common lake types in British Columbia, Canada. Deep, oligotrophic lakes have favored the evolution of slender fish well adapted for feeding on plankton (limnetic, sensu McPhail 1984), whereas shallow, more eutrophic lakes with extensive littoral zones favor fish that are deeper-bodied and well adapted for feeding on benthic invertebrates. The latter forage in large groups that attack nests guarded by males and cannibalize the young within. Courtship in these lakes is relatively inconspicuous, a feature that apparently enhances nest survivorship. In limnetic populations, this form of cannibalism is usually absent and courtship is conspicuous. Because benthic populations tend to have larger bodies and hence, larger gapes than do limnetic fish we suggest that cannibalism may be facilitated by large body size or a correlated trait. We test this by comparing the morphology of populations exhibiting both group cannibalism and a second kind of cannibalism in which solitary females court males, gain access to nests as a consequence, and then cannibalize eggs without spawning. Our results suggest that differences in body size cannot explain variation among populations in cannibalistic tendencies but that body size may affect the effectiveness of cannibalism by females within populations.     

Ecosystem size predicts eco‐morphological variability in a postglacial diversification

Identifying the processes by which new phenotypes and species emerge has been a long‐standing effort in evolutionary biology. Young adaptive radiations provide a model to study patterns of morphological and ecological diversification in environmental context. Here, we use the recent radiation (ca. 12k years old) of the freshwater fish Arctic charr (Salvelinus alpinus) to identify abiotic and biotic environmental factors associated with adaptive morphological variation. Arctic charr are exceptionally diverse, and in postglacial lakes there is strong evidence of repeated parallel evolution of similar morphologies associated with foraging. We measured head depth (a trait reflecting general eco‐morphology and foraging ecology) of 1,091 individuals across 30 lake populations to test whether fish morphological variation was associated with lake bathymetry and/or ecological parameters. Across populations, we found a significant relationship between the variation in head depth of the charr and abiotic environmental characteristics: positively with ecosystem size (i.e., lake volume, surface area, depth) and negatively with the amount of littoral zone. In addition, extremely robust‐headed phenotypes tended to be associated with larger and deeper lakes. We identified no influence of co‐existing biotic community on Arctic charr trophic morphology. This study evidences the role of the extrinsic environment as a facilitator of rapid eco‐morphological diversification.     

Rapid evolution of thermal plasticity in mountain lake Daphnia populations

Populations at risk of extinction due to climate change may be rescued by adaptive evolution or plasticity. Selective agents, such as introduced predators, may enhance or constrain plastic or adaptive responses to temperature. We tested responses of Daphnia to temperature by collecting populations from lakes across an elevational gradient in the presence and absence of fish predators (long‐term selection). We subsequently grew these populations at two elevations in field mesocosms over two years (short‐term selection), followed by a common‐garden experiment at two temperatures in the lab to measure life‐history traits. Both long‐term and short‐term selection affected traits, suggesting that genetic variation of plasticity within populations enabled individuals to rapidly evolve plasticity in response to high temperature. We found that short‐term selection by high temperature increased plasticity for growth rate in all populations. Fecundity was higher in populations from fishless lakes and body size showed greater plasticity in populations from warm lakes (long‐term selection). Neither body size nor fecundity were affected by short‐term thermal selection. These results demonstrate that plasticity is an important component of the life‐history response of Daphnia, and that genetic variation within populations enabled rapid evolution of plasticity in response to selection by temperature.