Disentangling the selective factors that act on male colour in wild guppies

The colour pattern of male guppies ( Poecilia reticulata ) is thought to evolve as a compromise between sexual selection (favouring conspicuousness) and natural selection (favouring crypsis). Underpinning this classic explanation is the observation that guppies living with dangerous fish predators are less colourful than guppies living without these predators. However, high fish-predation sites are generally farther downstream than low fish-predation sites, and so may also differ in physical habitat features related to stream size, as well as in the abundance of predatory prawns ( Macrobrachium crenulatum ). The goal of our study was to disentangle the effects of fish predation on colour evolution from the potential effects of physical habitat features and predation by prawns. We collected 20 male guppies from each of 29 sites in two Trinidadian rivers. We then quantified the colour pattern of these fish; each spot was measured for size and assigned to a colour category. For each site, we determined the fish predation regime and quantified stream size, water colour, canopy openness, and prawn abundance. We then used regressions to assess the relative importance of these factors in explaining variation in guppy colour. Supporting previous work, the presence of predatory fishes was the most important explanatory variable for many components of colour pattern. Physical habitat features explained some of the remaining variation, but in inconsistent ways between the two rivers. The abundance of predatory prawns also explained variation in male colour. Our results suggest that predatory fishes impose the strongest selection on the colour pattern of male guppies but that other factors are also important.     

Predator exposure alters stress physiology in guppies across timescales

In vertebrates, glucocorticoids mediate a wide-range of responses to stressors. For this reason, they are implicated in adaptation to changes in predation pressure. Trinidadian guppies (Poecilia reticulata) from high-predation environments have repeatedly and independently colonized and adapted to low-predation environments, resulting in parallel changes in life history, morphology, and behavior. We validated methods for non-invasive waterborne hormone sample collection in this species, and used this technique to examine genetic and environmental effects of predation on basal glucocorticoid (cortisol) levels. To examine genetic differences, we compared waterborne cortisol levels in high- and low-predation fish from two distinct population pairs. We found that fish from high-predation localities had lower cortisol levels than their low-predation counterparts. To isolate environmental influences, we compared waterborne cortisol levels in genetically similar fish reared with and without exposure to predator chemical cues. We found that fish reared with predator chemical cues had lower waterborne cortisol levels than those reared without. Comparisons of waterborne and whole-body cortisol levels demonstrated that populations differed in overall cortisol levels in the body, whereas rearing conditions altered the release of cortisol from the body into the water. Thus, evolutionary history with predators and lifetime exposure to predator cues were both associated with lower cortisol release, but depended on distinct physiological mechanisms.     

Evolution of juvenile growth rates in female guppies (Poecilia reticulata): predator regime or resource level?

Recent theoretical and empirical work argues that growth rate can evolve and be optimized, rather than always being maximized. Chronically low resource availability is predicted to favour the evolution of slow growth, whereas attaining a size-refuge from mortality risk is predicted to favour the evolution of rapid growth. Guppies (Poecilia reticulata) evolve differences in behaviour, morphology and life-history traits in response to predation, thus demonstrating that predators are potent agents of selection. Predators in low-predation environments prey preferentially on small guppies, but those in high-predation environments appear to be non-selective. Because guppies can outgrow their main predator in low- but not high-predation localities, we predict that predation will select for higher growth rates in the low-predation environments.However, low-predation localities also tend to have lower productivity than high-predation localities, yield-ing the prediction that guppies from these sites should have slower growth rates. Here we compare the growth rates of the second laboratory-born generation of guppies from paired high- and low-predation localities from four different drainages. In two out of four comparisons, guppies from high-predation sites grew significantly faster than their low-predation counterparts. We also compare laboratory born descendants from a field introduction experiment and show that guppies introduced to a low-predation environment evolved slower growth rates after 13 years, although this was evident only at the high food level. The weight of the evidence suggests that resource availability plays a more important role than predation in shaping the evolution of growth rates.     

Ornamental evolution in Trinidadian guppies (Poecilia reticulata): insights from sensory processing‐based analyses of entire colour patterns

The evolution of exaggerated sexual ornamentation is classically thought to proceed as a compromise between opposing vectors of sexual and natural selection. In colour‐based ornamentation, as exhibited by guppies (Poecilia reticulata), heightened trait expression may be beneficial in promoting attractiveness, but costly in terms of predation. Opportunities to reconcile this compromise will exist if there are differences between conspecifics and predators in their sensory systems; in such situations guppies should evolve to exploit the ways in which their ornamentation would appear maximally conspicuous to conspecifics. In the present study, we addressed this hypothesis via a study of geographic variation employing the most sophisticated colour analysis yet attempted for Trinidadian guppies. We made two paired contrasts, one between two Aripo populations that vary in the presence of the potential predator Aequidens pulcher, and another between Quare and Marianne populations that vary in exposure to a predatory prawn, Macrobrachium crenulatum. We predicted that, if ornamentation is constrained by the presence of either predator, then guppy conspicuousness should change most markedly across each of the two paired populations as viewed by that predator. Although disparity analysis of entire colour patterns indicated significant differences in both contrasts, this prediction was most clearly supported for the Marianne/Quare contrast. Marianne fish, which co‐exist with prawns, exhibited larger black spots coupled with less extensive, less bright flank iridescence. The brightness reductions are notable because, as the only potential guppy predator with a dedicated ultraviolet (UV) photoreceptor, prawns may detect passing male guppies via their UV‐bright blues, violets and ‘UV/oranges’. We discuss our findings in light of the additional insights that might be obtained by combining spectral assessments and visual modeling with more traditional methods of colour pattern appraisal. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 95 , 734–747.     

Back to school: can antipredator behaviour in guppies be enhanced through social learning?

The ability to recognize and respond to predators often has a learned component, but few studies have examined the role of social learning in the development of antipredator behaviour. We investigated whether wild-caught juvenile guppies, Poecilia reticulata, from a low-predation river in Trinidad increase their response towards a novel predator through association with conspecifics from a high-predation river. We assigned fish to one of three treatment groups: (1) repeated exposure to a model accompanied by high-predation conspecifics; (2) repeated exposure to a model with low-predation conspecifics; (3) a control group in which focal fish interacted with high-predation fish in the absence of the model. Guppies trained with high-predation, but not low-predation, ‘demonstrators’ significantly improved their antipredator behaviour (spent more time schooling and inspected the model from further away). The guppies assigned to the control group showed no significant improvement in antipredator behaviour after the training period, suggesting that association with experienced conspecifics in the absence of the model is not sufficient to enhance the antipredator behaviour of na?ve fish. We conclude that guppies can improve their antipredator behaviour through association with more experienced conspecifics in the presence of visual cues simulating high predation risk. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.        

Life-history evolution in guppies. VII. The comparative ecology of high- and low-predation environments

Prior research has demonstrated a strong association between the species of predators that co-occur with guppies and the evolution of guppy life histories. The evolution of these differences in life histories has been attributed to the higher mortality rates experienced by guppies in high-predation environments. Here, we evaluate whether there might be indirect effects of predation on the evolution of life-history patterns and whether there are environmental differences that are correlated with predation. To do so, we quantified features of the physical and chemical environment and the population biology of guppies from seven high- and low-predation localities. We found that high-predation environments tend to be larger streams with higher light levels and higher primary productivity, which should enhance food availability for guppies. We also found that guppy populations from high-predation environments have many more small individuals and fewer large individuals than those from low-predation environments, which is caused by their higher birth rates and death rates. Because of these differences in size distribution, guppies from high-predation environments have only one-fourth of the biomass per unit area, which should also enhance food availability for guppies in these localities. Guppies from high-predation sites allocate more resources to reproduction, grow faster, and attain larger asymptotic sizes, all of which are consistent with higher levels of resource availability. We conclude that guppies from high-predation environments experience higher levels of resource availability in part because of correlated differences in the environment (light levels, primary productivity) and in part as an indirect consequence of predation (death rates and biomass density). These differences in resource availability can, in turn, augment the effect of predator-induced mortality as factors that shape the evolution of guppy life-history patterns. We found no differences in the invertebrate communities from high- and low-predation localities, so we conclude that there do not appear to be multitrophic, indirect effects associated with these differences in predation.     

Disturbance cues as a source of risk assessment information under natural conditions

1. Disturbance cues are released by stressed or disturbed prey prior to a predator attack and convey useful risk assessment information regarding local threats. While studies have shown that disturbance cues may be important early on within the predation sequence (prior to an attack), their role in predator–prey interactions remains relatively overlooked by ecologists. Critically, experimental studies examining disturbance cues, especially among prey fishes, have been conducted primarily under laboratory or semi-natural conditions.
2. Here, we tested the prediction that disturbance cues function as sources of risk assessment information in situ. We exposed Trinidadian guppies, in two natural populations differing in predation risk, to a model predator paired with stream water or the disturbance cue collected from guppies from either a high- or low-predation risk population.
3. We found that the predator inspection response of guppies to disturbance cues depends on the level of risk of both the focal and the cue source population. Guppies from both populations exhibited increased latencies to inspect, lower inspection rates and reduced inspecting group sizes towards the model paired with conspecific disturbance cues versus a stream water control. Interestingly, guppies of both populations showed evidence of higher perceived predation risk towards the disturbance cues collected from high-predation risk donors compared to low-predation risk donors.
4. Our results support the hypothesis that disturbance cues function as a source of information used by prey fish in the assessment of predation risk and provide the first evidence of disturbance cue function under fully natural conditions.

     

Predation-associated differences in sex linkage of wild guppy coloration

Evolutionary theory predicts that the sex linkage of sexually selected traits can influence the direction and rate of evolutionary change, and also itself be subject to selection. Theory abounds on how sex-specific selection, mate choice, or other phenomena should favor different types of sex-linked inheritance, yet evidence in nature remains limited. Here, we use hormone assays in Trinidadian guppies to explore the extent to which linkage of male coloration differs among populations adapted to varying predation regimes. Results show there is consistently higher degree of X- and autosomal linkage in body coloration among populations adapted to low-predation environments. More strikingly, analyses of an introduced population of guppies from a high- to a low-predation environment suggest that this difference can change in 50 years or less.     

Ultraviolet dermal reflexion and mate choice in the guppy, Poecilia reticulata

Some fish, including the guppy, have the ability to perceive ultraviolet (UV) wavelengths. Female guppies prefer to associate with males that are viewed under light conditions that include UV-A, in preference to conditions lacking these wavelengths. We used reflexion spectrophotometry to show that male guppies reflect UV light from both their structural (purple, green and white) and pigment (orange) colour patches and that males differ in the levels of UV light reflected. Varying components of UV may affect both the brightness and hue of particular colour patches. This may produce nonspectral colours that are visible to the guppy but that are outside human perception. We used video analysis to quantify male reflexion in the UV and visible wavebands. Male guppies with high and low UV reflexion, but similar human-visible coloration and area of coloration, were paired for use in mate choice experiments. Female guppies shown pairs of males differing in their levels of UV reflexion had no preference for either high or low UV reflexion. This suggests that UV reflexion does not provide particularly significant information relating to male quality or influence female preference in this population of guppies. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour      

Rapid evolution of escape ability in Trinidadian guppies (Poecilia reticulata)

Predators are widely assumed to create selection that shapes the evolution of prey escape abilities. However, this assumption is difficult to test directly due to the challenge of recording both predation and its evolutionary consequences in the wild. We examined these events by studying natural and experimental populations of Trinidadian guppies, Poecilia reticulata, which occur in distinct high-predation and low-predation environments within streams. Importantly, in the last two decades several populations of guppies have been experimentally introduced from one type of predatory environment into the other, allowing measurements of the consequences of change. We used this system to test two hypotheses: First, that changes in predatory environments create phenotypic selection favoring changes in escape ability of guppies, and second, that this selection can result in rapid evolution. For the first test we compared escape ability of wild caught guppies from high- versus low-predation environments by measuring survival rates during staged encounters with a major predator, the pike cichlid Crenicichla alta. We used guppies from three streams, comparing two within-stream pairs of natural populations and three within-stream pairs of an introduced population versus its natural source population. In every comparison, guppies from the high-predation population showed higher survival. These multiple, parallel divergences in guppy survival phenotype suggest that predatory environment does create selection of escape ability. We tested our second hypothesis by rearing guppies in common garden conditions in the laboratory, then repeating the earlier experiments using the F2 generation. As before, each comparison resulted in higher survival of guppies descended from the high-predation populations, demonstrating that population differences in escape ability have a genetic basis. These results also show that escape ability can evolve very rapidly in nature, that is, within 26-36 generations in the introduced populations. Interestingly, we found rapid evolutionary loss of escape ability in populations introduced into low-predation environments, suggesting that steep fitness trade-offs may influence the evolution of escape traits.     

The role of learning by a predator,Rivulus hartii,in the rare‐morph survival advantage in guppies

Negative frequency‐dependent selection (NFDS), where rare types are favoured by selection, can maintain diversity. However, the ecological processes that mediate NFDS are often not known. Male guppies (Poecilia reticulata) exhibit extreme diversity of colour patterning and, in a previous field experiment, rare morphs had a survival advantage. Here, we test the hypothesis that predators impose NFDS because they are efficient at capturing familiar prey morphs, but are less efficient at capturing unfamiliar morphs. Over a series of trials, we presented Rivulus hartii, a natural predator of guppies, with male guppies with the same colour patterning (A trials); then, for a second series of trials, we presented the rivulus with guppies with a new colour pattern (B trials). The success of rivulus at capturing guppies on the first attack increased over successive A trials. First attack success decreased significantly for the early B trials, and then increased during successive B trials, eventually reaching the same level as in the best A trials. This experiment demonstrates that learning, perhaps through long‐term search image formation, plays a role in predation success on familiar vs. unfamiliar prey morphs. These results support the hypothesis that predator learning contributes to the maintenance of the extreme male guppy polymorphism seen in nature.     

This is not déjà vu all over again: male guppy colour in a new experimental introduction

We use an experimental introduction in nature to examine factors that influence parallel evolution. In 1996, 200 high-predation guppies (Poecilia reticulata) from the Yarra River were introduced into the Damier River, which previously lacked guppies. Eight years later, we quantified the colour of wild-caught guppies ('phenotypic' divergence) and lab-reared guppies ('genetic' divergence) from low- and high-predation environments in both rivers. Phenotypic and genetic divergence between predation environments within the Yarra was evident for black and for orange. Phenotypic divergence within the Damier was parallel to the Yarra for black but not for orange. Genetic divergence was absent between predation environments within the Damier, but was evident when comparing both Damier populations to their Yarra ancestors. The evolution of male colour thus depends on factors other than the simple contrast between 'high' and 'low' predation. We suggest that the parallel evolution of male signalling traits may sometimes first require the parallel evolution of female preferences.     

Predator-induced plasticity in guppy (<Emphasis Type="Italic">Poecilia reticulata</Emphasis>) life history traits

A number of invertebrates show predator-induced plasticity in life-history and morphological traits that are considered adaptive. Evidence is accumulating that vertebrates may also adjust their life-history traits in response to predators; however, some of the patterns of plasticity, which appear to be an adaptive response specifically to the risk of size-selective predation, may instead result from reduced foraging in response to predator presence. Here, we describe a study of predator-induced plasticity in guppies (Poecilia reticulata). We have predicted that the plastic response to cues from a small, gape-limited, natural predator of guppies, the killlifish (Rivulus hartii), would be the opposite of that caused by reduced food intake. We have found that male guppies increased their size at maturity, both length and mass, in response to the non-lethal presence of this predator. This pattern of plasticity is the opposite of that observed in response to reduced food intake, where male guppies reduce size at maturity. The increase in size at maturity that we observed would likely reduce predation on adult male guppies by this native predator because it is gape-limited and can only eat juvenile and small adult guppies. This size advantage would be important especially because male guppies grow very little after maturity. Therefore, the pattern of plasticity that we observed is likely adaptive. In contrast, female guppies showed no significant response in size at first parturition to the experimental manipulation; however, we did find evidence suggesting that females may produce more, smaller offspring in response to cues from this predator.     

Metabolic stoichiometry and the ecology of fear in Trinidadian guppies: consequences for life histories and stream ecosystems

Consumer-driven nutrient recycling, the release of chemicals as byproducts and excesses of consumer physiology, can alter ecosystems by changing the availability of limiting nutrients at the base of the food web. The mere presence of predators can alter consumer physiology by restricting food intake and inducing stress. Predation risk, then, can influence ecosystem function by modifying the role of prey as nutrient recyclers, yet there are few empirical tests of how predation risk alters nutrient recycling by prey. Here, we present the results of a test for the effects of predation risk on the C and N budgets of Trinidadian guppies (Poecilia reticulata). We reared female guppies for 7 weeks on diets of varying quality, and we compared control individuals to those exposed continuously to chemical cues emitted by a guppy predator, Crenicichla alta. We measured food consumption, growth rate, tissue elemental stoichiometry and N excretion by guppies on all treatments. Guppies strongly reduced food intake in the presence of predator cues; however, cue-exposed guppies assimilated nutrients more efficiently than controls. Specifically, cue-exposed guppies strongly increased N retention efficiency while only moderately increasing C efficiency. Consequently, guppies reared with predator cues excreted 39 % less N than control guppies. We suggest that reduced foraging, enhanced nutrient efficiency, and decreased N excretion are adaptive responses to the extrinsic mortality threat posed by guppy predators. The resulting substantial reduction in N excretion by guppies may influence ecosystem function in natural streams by reducing the supply of a limiting nutrient.     

The Relationship Among Swimming Performance, Courtship Behavior, and Carotenoid Pigmentation of Guppies in Four Rivers of Trinidad

In the laboratory, courtship behavior and carotenoid pigmentation of male guppies are condition-dependent traits, since their expression is affected by physical vigor and environmental factors such as water velocity and diet. Whether these relationships exist in guppies under field conditions is yet to be determined. We compared the swimming performance, courtship behavior, and carotenoid pigmentation of guppies from headwater and downstream localities in four rivers of Trinidad. Swimming performance and courtship behavior of males differed among rivers and between headwater and downstream sites. Guppies from headwater sites swam significantly faster and had higher display rates than those from downstream sites. Mean swimming performance across sites was positively correlated with mean water velocity, but was correlated with the number of orange color spots (carotenoid pigment) in only one river. These results indicate that the courtship behavior of Trinidadian guppies is condition-dependent because the amount of display behavior is positively correlated to swimming performance, a measure of physical endurance. The proximal cause for this condition-dependence may be predator induced variation in microhabitat use by guppies in headwater and downstream locations.     

Comparative studies of senescence in natural populations of guppies

Investigators have rarely sought evidence for senescence in natural populations because it is assumed that relatively few individuals will survive long enough in the wild to exhibit the intrinsic increase in mortality with age expected from senescent individuals. Nevertheless, senescence has been documented in some natural populations, mostly in birds and mammals. Here we report on a comparative study of senescence in two natural populations of guppies (Poecilia reticulata). We document senescence as an age-specific increase in mortality rate, with use of mark-recapture studies and implementation of program MARK for analysis of such observations. Extrinsic mortality was controlled for by choosing populations that experience low rates of predation because they coexist with only a single piscine predator (Rivulus hartii). These populations differ in their evolutionary history because one was native to such a site whereas the other was introduced to a site that previously contained no guppies. The source of the introduced guppies was a high-predation population downstream below a barrier waterfall. Theory predicts that the guppies derived from a high-predation locality should experience senescence at an earlier age than the native low-predation population; however, the historical differences among these populations are also confounded with everything else that differs among the two localities. We found that females from a natural low-predation population have delayed senescence compared with the recently established population and hence that the differences among localities in senescence conform to theoretical predictions. The males from natural low-predation environments also had lower overall mortality rates, but contrary to predictions, the pattern of senescence for males did not differ between populations. The difference between the sexes is potentially attributable to two factors that lower the statistical power for distinguishing differences in the age-specific acceleration of mortality in males. One factor is that males have higher mortality rates, so fewer survive to advanced ages. A second is that we had a greater ability to discriminate among older age classes in females. We also found that the introduced population sustained a higher rate of disease than the native low-predation population. Such disease may represent a confounding factor in our comparison, but it may also reflect one of the trade-offs inherent in the life-history differences of these populations.     

Competition as a selective mechanism for larger offspring size in guppies

Highly competitive environments are predicted to select for larger offspring. Guppies Poecilia reticulata from low-predation populations have evolved to make fewer, larger offspring than their counterparts from high-predation populations. As predation co-varies with the strength of competition in natural guppy populations, here I present two laboratory experiments that evaluate the role of competition in selecting for larger offspring size. In the first experiment, paired groups of large and small newborns from either a high- or a low-predation population were reared in mesocosms under a high- or a low-competition treatment. While large newborns retained their size advantage over small newborns in both treatments, newborn size increased growth only in the high-competition treatment. Moreover, the increase in growth with size was greater in guppies derived from the low-predation population. In the second experiment, pairs of large and small newborns were reared in a highly competitive environment until reproductive maturity. Small size at birth delayed maturation and the effect of birth size on male age of maturity was greater in the low-predation population. These results support the importance of competition as a selective mechanism in offspring size evolution.     

Influence of prey foraging posture on flight behavior and predation risk: predators take advantage of unwary prey

Foraging in animals is often associated with characteristicbody postures, such as the head-down posture. When foragingconflicts with the ability to detect predators or to flee, individualsmay incur a greater risk of mortality to predation than otherwise.Here we investigate the influence of different foraging postures(horizontal versus nose-down body posture) on the ability ofindividuals to respond to approaching predators and on the riskof mortality to predation in the guppy (Poecilia reticulata).Individuals engaged in nose-down foraging were assumed to beable to visually scan a smaller area for predators and to escapeless effectively due to their body posture, and thus are morevulnerable to stalking predators than horizontally foragingones. In a first experiment, we separately exposed nonforaging,horizontally foraging, and nose-down foraging guppies to anapproaching cichlid fish predator model. Nonforaging guppiesreacted sooner to and initiated flight further away from theapproaching model than did foraging fish collectively, and horizontallyforaging individuals responded sooner to the model than nose-downforaging ones. Comparing all test guppies, nose-down foragingindividuals were the most likely not to exhibit any responseto the predator model. When presented with a simultaneous choiceof two guppies behind a one-way mirror, individual blue acaracichlid (Aequidens pulcher), a natural predator of the guppy,preferred to attack foraging guppies over nonforaging ones andnose-down foraging guppies over horizontally foraging individuals.In a final experiment with free-swimming cichlids and guppies,we demonstrated that individual risk of predation for guppiesforaging nose down was greater than for guppies foraging horizontally,and both were at greater risk than nonforaging guppies. Thislatter result is consistent with the above differences in theguppy's responsiveness to approaching predators depending ontheir foraging behavior, and with the finding that cichlid predatorspreferred fish that were less likely to show any response tothem. Our results therefore indicate that the ability to respondto approaching predators and the risk of mortality to predationin the guppy is strongly influenced by their foraging activity,and in particular their foraging posture, and that cichlid predatorspreferentially select less wary and more vulnerable guppies.[BehavEcol 7: 264–271 (1996)]     

On the genetic architecture of rapidly adapting and convergent life history traits in guppies

The genetic basis of traits shapes and constrains how adaptation proceeds in nature; rapid adaptation can proceed using stores of polygenic standing genetic variation or hard selective sweeps, and increasing polygenicity fuels genetic redundancy, reducing gene re-use (genetic convergence). Guppy life history traits evolve rapidly and convergently among natural high- and low-predation environments in northern Trinidad. This system has been studied extensively at the phenotypic level, but little is known about the underlying genetic architecture. Here, we use four independent F2 QTL crosses to examine the genetic basis of seven (five female, two male) guppy life history phenotypes and discuss how these genetic architectures may facilitate or constrain rapid adaptation and convergence. We use RAD-sequencing data (16,539 SNPs) from 370 male and 267 female F2 individuals. We perform linkage mapping, estimates of genome-wide and per-chromosome heritability (multi-locus associations), and QTL mapping (single-locus associations). Our results are consistent with architectures of many loci of small-effect for male age and size at maturity and female interbrood period. Male trait associations are clustered on specific chromosomes, but female interbrood period exhibits a weak genome-wide signal suggesting a potentially highly polygenic component. Offspring weight and female size at maturity are also associated with a single significant QTL each. These results suggest rapid, repeatable phenotypic evolution of guppies may be facilitated by polygenic trait architectures, but subsequent genetic redundancy may limit gene re-use across populations, in agreement with an absence of strong signatures of genetic convergence from recent analyses of wild guppies.Subject terms: Evolutionary genetics, Quantitative trait     

Predicting the direction of ornament evolution in Trinidadian guppies (Poecilia reticulata)

Sexual selection is thought to be opposed by natural selection such that ornamental traits express a balance between these two antagonistic influences. Phenotypic variation among populations may indicate local shifts in this balance, or that different stable ‘solutions’ are possible, but testing these alternatives presents a major challenge. In the guppy (Poecilia reticulata), a small freshwater fish with male-limited ornamental coloration, these issues can be addressed by transplanting fish among sites of varying predation pressure, thus effectively manipulating the strength and nature of natural selection. Here, we contrast the evolutionary outcome of two such introductions conducted in the Trinidadian El Cedro and Aripo Rivers. We use sophisticated colour appraisal methods that account for full spectrum colour variation and which incorporate the very latest visual sensitivity data for guppies and their predators. Our data indicate that ornamentation evolved along different trajectories: whereas Aripo males evolved more numerous and/or larger orange, black and iridescent markings, El Cedro males only evolved more extensive and brighter iridescence. Examination of the El Cedro experiment also revealed little or no ornamental evolution at the control site over 29 years, which contrasts markedly with the rapid (approx. 2–3 years) changes reported for introduction populations. Finally, whole colour-pattern analysis suggested that the greatest visual difference between El Cedro introduction and control fish would be perceived by the two most salient viewers: guppies and the putatively dangerous predator Crenicichla alta. We discuss whether and how these evolutionary trajectories may result from founder effects, population-specific mate preferences and/or sensory drive.