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Abstract The ability of organisms to change color in response to a change in environmental conditions is widespread across taxa. Predation represents the longstanding hypothesis for the evolution of such coloration plasticity. Yet, tests of the evolutionary drivers of coloration plasticity remain rare. Here, we examine how predation shapes both baseline coloration and coloration plasticity in the Trinidadian killifish (Anablepsoides hartii). This species inhabits streams that vary in fish predator presence, creating a replicated natural experiment across three rivers. We hypothesized that fish from high-predation sites would exhibit lighter baseline coloration due to associations with open canopy and increased light, and that predators would select for stronger plasticity in background-induced color change. Our results did reveal hypothesized shifts in baseline coloration with high-predation fish generally lighter. Anablepsoides hartii also displayed strong plasticity, darkening on black backgrounds and lightening on white. However, the effect of predation on baseline color and coloration plasticity was inconsistent across rivers, suggesting that additional ecological factors also contribute to these responses. Our study provides empirical evidence that predators are not the sole driver of variation in coloration plasticity and that local ecological factors that covary with predators may also exert selection on body color. 

Abstract Behavioural plasticity is expected to be favoured in risky environments, such as when prey species coexist with predators because prey must alternate between fitness-related foraging/mating behaviours and antipredator behaviours that enhance survival. We compared behavioural plasticity in Trinidadian killifish that are found in sites with and without predators. We quantified aggressive and antipredator behaviours via a mirror assay in second-generation lab-reared and wild-caught killifish before and after exposure to predator cues. We compared 2 types of aggression including: overt aggression (ramming, biting, lunging, and tail-slapping) and display aggression (spine arching, bending into an s-shape, and opercular flaring). We additionally compared the amount of time the fish spent frozen as a proxy for antipredator behaviour. We show clear differences in plasticity between populations with and without predators. Killifish from sites with predators decreased overt aggression in response to exposure to predator chemical cues. Plastic responses to the predator cue were lower in killifish from sites that lack predators. Interestingly, wild fish from sites without predators did respond to the predator cue by decreasing overt aggression and increasing time spent frozen, though to a lesser degree compared to the fish from sites with predators. Our results support the expectation that development in a risky environment favours evolutionary changes in predator-mediated behavioural plasticity. 

Abstract Theory asserts larger brains facilitate behaviours that enhance fitness. Research has demonstrated that increased brain size improves cognition and survival. However, the majority of research has focused on cross‐species comparisons. Experiments that manipulate selection to investigate the connection between brain size, behaviour and fitness are needed.Trinidadian killifish (Anablepsoides hartii) live in communities with (high predation: HP) and without (killifish only: KO) predators. Predator absence is associated with high population densities, increased intraspecific competition and evolved larger brain sizes.We tested for evolutionary shifts in behaviour by subjecting second‐generation lab‐reared killifish to a mirror aggression assay. We also quantify selection on brain size and behaviour by transplanting wild HP killifish to KO sites and tracking individual fitness (growth rates) with a mark‐recapture design.Lab‐reared killifish from KO sites—specifically males—exhibited higher levels of aggression than HP killifish. In the transplant experiment, HP killifish exhibited strong increases in aggression following the introduction to KO sites. Increased brain size was correlated with increased growth in transplanted HP killifish, yet there was no association between brain size, aggression and growth.Our results indicate that declines in predation and increased competition favour increases in aggression but further research is needed to determine if and how brain size and behaviour are linked through natural selection. Read the freePlain Language Summaryfor this article on the Journal blog. 

Externally laid eggs are often responsive to environmental cues; however, it is unclear how such plasticity evolves. In Trinidad, the killifish (Anablepsoides hartii) is found in communities with and without predators. Here, killifish inhabit shallower, ephemeral habitats in sites with predators. Such shifts may increase the exposure of eggs to air and lead to possible desiccation. We compared egg-hatching plasticity between communities by rearing eggs terrestrially on peat moss or in water. The timing of hatching did not differ between communities when eggs were reared in water. Eggs from sites with predators responded to terrestrial incubation by hatching significantly earlier compared with water-reared eggs. These responses were weaker in sites with no predators. Such divergent trends show that the presence of predators is associated with evolutionary shifts in hatching plasticity. Our results provide evidence for local adaptation in embryonic plasticity at the population scale. 

Abstract Intraspecific variation in vertebrate eye size is well known. Ecological factors such as light availability are often correlated with shifts in relative eye size. However, experimental tests of selection on eye size are lacking. Trinidadian killifish (Anablepsoides hartii) are found in sites that differ in predation intensity. Sites that lack predators are characterized by lower light, high killifish densities, low resource availability, and intense competition for food. We previously found that killifish in sites that lack predators have evolved a larger “relative” eye size (eye size corrected for body size) than fish from sites with predators. Here, we used transplant experiments to test how selection operates on eye size when fish that are adapted to sites with predators are translocated into sites where predators are absent. We observed a significant “population × relative eye size” interaction; the relationship between relative eye size and a proxy for fitness (rates of individual growth) was positive in the transplanted fish. The trend was the opposite for resident fish. Such results provide experimental support that larger eyes enhance fitness and are favoured in environments characterized by low light and high competition. 

ABSTRACT It has long been recognized that the environment experienced by parents can influence the traits of offspring (i.e. ‘parental effects’). Much research has explored whether mothers respond to predictable shifts in environmental signals by modifying offspring phenotypes to best match future conditions. Many organisms experience conditions that theory predicts should favor the evolution of such ‘anticipatory parental effects’, but such predictions have received limited empirical support. ‘Condition transfer effects’ are an alternative to anticipatory effects that occur when the environment experienced by parents during development influences offspring fitness. Condition transfer effects occur when parents that experience high-quality conditions produce offspring that exhibit higher fitness irrespective of the environmental conditions in the offspring generation. Condition transfer effects are not driven by external signals but are instead a byproduct of past environmental quality. They are also likely adaptive but have received far less attention than anticipatory effects. Here, we review the generality of condition transfer effects and show that they are much more widespread than is currently appreciated. Condition transfer effects are observed across taxa and are commonly associated with experimental manipulations of resource conditions experienced by parents. Our Review calls for increased research into condition transfer effects when considering the role of parental effects in ecology and evolution. 

Abstract There exists extensive variation in eye size. Much work has provided a connection between light availability and differences in eye size across taxa. Experimental tests of the role of the light environment on the evolution of eye size are lacking. Here, we performed a selection experiment that examined the influence of light availability on shifts in eye size and the connection between eye size and phototactic (anti‐predator) behaviour inDaphnia. We set‐up replicate experimental populations ofDaphnia, repeatedly evaluated phenotypic shifts in eye size during the ~50‐day experiment, and performed a common garden experiment at the end of the experiment to test for evolutionary shifts in eye size and behaviour. Our phenotypic analyses showed that eye size rapidly diverged between the light treatments; relative eye size was consistently larger in the low versus high light treatments. Selection on eye size was also modified by variation in density as increases inDaphniadensity favoured a larger eye. However, we did not observe differences in eye size between the light treatments following two generations of common garden rearing at the end of the experiment. We instead observed strong shifts in anti‐predator behaviour.Daphniafrom the low light treatment exhibited decreased phototactic responses to light. Our results show that decreased light relaxes selection on anti‐predator behaviour. Such trends provide new insights into selection on eye size and behaviour. 

Abstract Eco‐evolutionary experiments are typically conducted in semi‐unnatural controlled settings, such as mesocosms; yet inferences about how evolution and ecology interact in the real world would surely benefit from experiments in natural uncontrolled settings. Opportunities for such experiments are rare but do arise in the context of restoration ecology—where different “types” of a given species can be introduced into different “replicate” locations. Designing such experiments requires wrestling with consequential questions. (Q1) Which specific “types” of a focal species should be introduced to the restoration location? (Q2) How many sources of each type should be used—and should they be mixed together? (Q3) Whichspecificsource populations should be used? (Q4) Which type(s) or population(s) should be introduced into which restoration sites? We recently grappled with these questions when designing an eco‐evolutionary experiment with threespine stickleback (Gasterosteus aculeatus) introduced into nine small lakes and ponds on the Kenai Peninsula in Alaska that required restoration. After considering the options at length, we decided to use benthic versus limnetic ecotypes (Q1) to create a mixed group of colonists from four source populations of each ecotype (Q2), where ecotypes were identified based on trophic morphology (Q3), and were then introduced into nine restoration lakes scaled by lake size (Q4). We hope that outlining the alternatives and resulting choices will make the rationales clear for future studies leveraging our experiment, while also proving useful for investigators considering similar experiments in the future. 

It is well established that environmental signals can induce phenotypic responses that persist for multiple generations. The induction of such ‘transgenerational plasticity’ (TGP) depends upon the ability of organisms to accurately receive and process information from environmental signals. Thus, sensory systems are likely intertwined with TGP. Here we tested the link between an environmental stressor and transgenerational responses in a component of the sensory system (eye size) that is linked to enhanced vision and ecologically relevant behaviours. We reared 45 clones of Daphnia pulicaria in the presence and absence of a low-quality resource (cyanobacteria) and evaluated shifts in relative eye size in offspring. Our results revealed divergent shifts in relative eye size within- and across-generations. Parental Daphnia that were fed cyanobacteria produced a smaller eye than Daphnia fed high-quality algae. Such differences were then reversed in the offspring generation; Daphnia whose mothers were fed cyanobacteria produced larger eyes than Daphnia that were continually fed green algae. We discuss the extent to which this maternal effect on eye size is an adaptive response linked to improved foraging. 

The role of phenotypic plasticity in adaptive evolution has been debated for decades. This is because the strength of natural selection is dependent on the direction and magnitude of phenotypic responses to environmental signals. Therefore, the connection between plasticity and adaptation will depend on the patterns of plasticity harbored by ancestral populations before a change in the environment. Yet few studies have directly assessed ancestral variation in plasticity and tracked phenotypic changes over time. Here we resurrected historic propagules of Daphnia spanning multiple species and lakes in Wisconsin following the invasion and proliferation of a novel predator (spiny waterflea, Bythotrephes longimanus ). This approach revealed extensive genetic variation in predator-induced plasticity in ancestral populations of Daphnia . It is unlikely that the standing patterns of plasticity shielded Daphnia from selection to permit long-term coexistence with a novel predator. Instead, this variation in plasticity provided the raw materials for Bythotrephes -mediated selection to drive rapid shifts in Daphnia behavior and life history. Surprisingly, there was little evidence for the evolution of trait plasticity as genetic variation in plasticity was maintained in the face of a novel predator. Such results provide insight into the link between plasticity and adaptation and highlight the importance of quantifying genetic variation in plasticity when evaluating the drivers of evolutionary change in the wild.