Evolution Of Colour Polymorphism Research Articles

Exploring polymorphism in a palatable prey: predation risk and frequency dependence in relation to distinct levels of conspicuousness

Abstract Camouflage and warning signals are different antipredator strategies, which offer an excellent opportunity to study the evolutionary forces acting on prey appearance. Edible prey often escape detection via camouflage, which usually leads to apostatic selection favoring rare morphs. By contrast, defended prey often display conspicuous coloration acting as warning signals to predators, which usually leads to positive frequency dependence and signal uniformity. However, when two morphs of the same species vary greatly in conspicuousness, the maintenance of both cryptic and conspicuous forms in profitable prey populations remains enigmatic. Using the white and melanic morphs of the invasive box tree moth (Cydalima perspectalis) presented at three different frequencies, we investigate (a) the palatability of caterpillars and adult moths to birds, (b) predation rates on the less conspicuous melanic morph, and (c) the role of frequency dependence in balancing morph frequencies. Our results show that caterpillars are distasteful for birds but not adult moths that are fully palatable. We found that the less conspicuous, melanic morph, benefits from reduced predation due to its lower detectability. The more conspicuous, white morph, instead, is more predated and is best off when common, suggesting positive frequency dependence. These results offer new insights into the evolution of color polymorphism and prey defenses in a polymorphic moth species. Further investigation is required to understand the role of different predation regimes on the maintenance of the polymorphism in this species and test whether additional selection pressures operate in natural populations.

Climate mediates color morph turnover in a species exhibiting alternative reproductive strategies

Sexual selection is considered the primary driver of morph turnover in many color polymorphic taxa, yet the potential for other factors (like climate) to contribute to polymorphism maintenance and evolution remains unclear. Appreciation for a role of environmental conditions in the maintenance and evolution of color polymorphisms has grown in recent years, generating evidence suggesting that color morphs linked to sexual selection may also diverge in climate sensitivity. Focusing on the three color components contributing to the male tree lizard (Urosaurus ornatus) color morphs, I reveal a marked concordance between patterns of turnover over space and time, with a general affinity of orange- and yellow-colored males to hotter, more variable conditions, and blue colored males to wetter, cooler conditions. An assessment of long-term turnover in the blue color component in response to recent climate change over the past 60 years reinforces these findings. Overall, behavioral asymmetries attributed to sexual selection likely expose competing morphs to divergent environmental conditions in heterogeneous habitats, creating opportunity for natural selection to shape climate sensitivities that also drive turnover in morph color composition. Ultimately, these processes may favor stark asymmetries in morph persistence over the coming decades.

Pterin-pigmented nanospheres create the colours of the polymorphic damselfly Ischnura elegans.

Animal colours commonly act as signals for mates or predators. In many damselfly species, both sexes go through a developmental colour change as adults, and females often show colour polymorphism, which may have a function in mate choice, avoidance of mating harassment and camouflage. In the blue-tailed damselfly, Ischnura elegans, young males are bright green and turn blue as they reach maturity. Females are red ( rufescens) or violet ( violacea) as immatures and, when mature, either mimic the blue colour of the males ( androchrome), or acquire an inconspicuous olive-green ( infuscans) or olive-brown ( obsoleta). The genetic basis of these differences is still unknown. Here, we quantify the colour development of all morphs of I. elegans and investigate colour formation by combining anatomical data and reflectance spectra with optical finite-difference time-domain simulations. While the coloration primarily arises from a disordered assembly of nanospheres in the epidermis, morph-dependent changes result from adjustments in the composition of pterin pigments within the nanospheres, and from associated shifts in optical density. Other pigments fine-tune hue and brilliance by absorbing stray light. These mechanisms produce an impressive palette of colours and offer guidance for genetic studies on the evolution of colour polymorphism and visual communication.

The evolution of colour polymorphism in British winter-active Lepidoptera in response to search image use by avian predators.

Phenotypic polymorphism in cryptic species is widespread. This may evolve in response to search image use by predators exerting negative frequency-dependent selection on intraspecific colour morphs, 'apostatic selection'. Evidence exists to indicate search image formation by predators and apostatic selection operating on wild prey populations, though not to demonstrate search image use directly resulting in apostatic selection. The present study attempted to address this deficiency, using British Lepidoptera active in winter as a model system. It has been proposed that the typically polymorphic wing colouration of these species represents an anti-search image adaptation against birds. To test (a) for search image-driven apostatic selection, dimorphic populations of artificial moth-like models were established in woodland at varying relative morph frequencies and exposed to predation by natural populations of birds. In addition, to test (b) whether abundance and degree of polymorphism are correlated across British winter-active moths, as predicted where search image use drives apostatic selection, a series of phylogenetic comparative analyses were conducted. There was a positive relationship between artificial morph frequency and probability of predation, consistent with birds utilizing search images and exerting apostatic selection. Abundance and degree of polymorphism were found to be positively correlated across British Lepidoptera active in winter, though not across all taxonomic groups analysed. This evidence is consistent with polymorphism in this group having evolved in response to search image-driven apostatic selection and supports the viability of this mechanism as a means by which phenotypic and genetic variation may be maintained in natural populations.

Metabolism, oxidative stress and territorial behaviour in a female colour polymorphic cichlid fish

Intrasexual selection on body coloration is thought to play an important role in the evolution of colour polymorphism, but its physiological underpinnings have received limited attention. In the colour polymorphic cichlid Neochromis omnicaeruleus, three fully sympatric female colour morphs—a plain morph (P) and two conspicuously coloured blotched morphs, black-and-white blotched (WB) and orange blotched (OB)—differ in agonistic behaviour. We compared routine metabolic rate (when females were housed in social isolation), short-term energetic costs of interacting with a same-colour rival housed in an adjacent transparent chamber and oxidative stress between the three female colour morphs. WB females had a lower routine metabolic rate compared with the other colour morphs. WB females also had a lower active metabolic rate during inter-female interactions than OB females, while OB females used more oxygen per unit aggressive act than the other two colour morphs. However, there were no consistent differences in oxidative stress between the three morphs. Concerted divergence in colour, behaviour and metabolism might contribute to the evolution of these polymorphisms in sympatry.

Is melanism in pygmy grasshoppers induced by crowding?

Color polymorphisms in animals may result from plasticity of the developmental system in response to genetic cues in the form of allelic variation at polymorphic loci, environmental cues, or a combination of genetic and environmental cues. An increased understanding of the evolution of color polymorphisms requires better knowledge of when we should expect genetic and environmental cues respectively to influence phenotype determination. Theory posits that the developmental systems of organisms should evolve sensitivity to such cues that most accurately predict coming selective conditions. Pygmy grasshoppers (Orthoptera, Tetrigidae) vary in color pattern within and among populations and show fire melanism, i.e., an increased frequency of black and dark colored phenotypes in high density populations inhabiting fire-ravaged areas. We examined if the population density experienced by individuals during development influenced the phenotypic expression of color pattern in Tetrix subulata. Individuals were experimentally reared either in solitude, at intermediate density or under crowded conditions. We found that color patterns of experimental individuals were independent of rearing density but strongly influenced by maternal color pattern. High population density and crowding may not constitute reliable predictors of the selective regime that characterizes post-fire environments.

MC1R variants correlate with thinhorn sheep colour cline but not individual colour

The melanocortin 1 receptor (MC1R) gene has provided a window on the evolution of colour polymorphisms. Recent studies have demonstrated associations between MC1R and melanism in a number of wildlife species. Thinhorn sheep ( Ovis dalli Nelson, 1884) are a possible species to test for association between MC1R polymorphisms and melanism. Across their range the pelage colour of thinhorn sheep intergrades between all white phenotypes to very dark phenotypes. Most thinhorn sheep are white; however, there is a 1200 km cline from light to dark sheep that occurs over three genetically distinct polymorphic populations. We sequenced the entire MC1R gene from 40 individuals across the range of thinhorn sheep. We found a single nucleotide polymorphism in MC1R at base pair position 921 that coincided with the geographic cline. However, at the individual level we found no relationship between individual melanism and genotype.

The Evolution of Color Polymorphism: Crypticity, Searching Images, and Apostatic Selection

The development and maintenance of color polymorphism in cryptic prey species is a source of enduring fascination, in part because it appears to result from selective processes operating across multiple levels of analysis, ranging from cognitive psychology to population ecology. Since the 1960s, prey species with diverse phenotypes have been viewed as the evolved reflection of the perceptual and cognitive characteristics of their predators. Because it is harder to search simultaneously for two or more cryptic prey types than to search for only one, visual predators should tend to focus on the most abundant forms and effectively overlook the others. The result should be frequency-dependent, apostatic selection, which will tend to stabilize the prey polymorphism. Validating this elegant hypothesis has been difficult, and many details have been established only relatively recently. This review clarifies the argument for a perceptual selective mechanism and examines the relevant experimental evidence.

Spatial heterogeneity, predator cognition, and the evolution of color polymorphism in virtual prey.

Cryptically colored prey species are often polymorphic, occurring in multiple distinctive pattern variants. Visual predators promote such phenotypic variation through apostatic selection, in which they attack more abundant prey types disproportionately often. In heterogeneous environments, disruptive selection to match the coloration of disparate habitat patches could also produce polymorphism, but how apostatic and disruptive selection interact in these circumstances is unknown. Here we report the first controlled selection experiment on the evolution of prey coloration on heterogeneous backgrounds, in which blue jays (Cyanocitta cristata) searched for digital moths on mixtures of dark and light patches at three different scales of heterogeneity. As predicted by ecological theory, coarse-grained backgrounds produced a functional dimorphism of specialists on the two patch types; fine-grained backgrounds produced generalists. The searching strategies of the jays also varied with the habitat configuration, however. Complex backgrounds with many moth-like features elicited a slow, serial search that depended heavily on selective attention. The result was increased apostatic selection, producing a broad range of moth phenotypes. Backgrounds with larger, more uniform patches allowed the birds to focus on the currently most rewarding patch type and to search entire patches rapidly in parallel. The result was less apostatic selection and lower phenotypic variability. The evolution of polymorphism in camouflaged prey depends on a complex interaction between habitat structure and predator cognition.

Predator-prey relationships and the evolution of colour polymorphism: a comparative analysis in diurnal raptors

Genetically based variation in coloration occurs in populations of many organisms belonging to various taxa, including birds, mammals, frogs, molluscs, insects and plants. Colour polymorphism has evolved in raptors more often than in any other group of birds, suggesting that predator–prey relationships was a driving evolutionary force. Individuals displaying a new invading colour morph may enjoy an initial foraging advantage because prey have difficulties in learning the colour of a rare morph (apostatic selection), or because morphs provide alternative foraging benefits allowing differently coloured individuals to exploit distinct food niches (disruptive selection). Plumage polymorphism should therefore have evolved in species that prey upon animals having the physiological ability to distinguish between differently coloured predators but also to flee once a predator has been detected. From this assumption, we can predict that closely related polymorphic and monomorphic species prey upon different animals. They may also differ in morphology, because foraging upon different prey may require different foraging modes, and in turn different morphological structures. We tested these two predictions in a comparative study of raptors. As expected, polymorphic and monomorphic species had a different diet, and there was a difference in wing length between polymorphic and monomorphic species within two genera (Buteo and Accipiter). Across all raptors for which phylogenetic relationships are known, polymorphic species preyed more often upon mammals than did monomorphic ones. These two types of raptor did not differ in the frequency of birds, insects and reptiles in their diets. We discuss these results in the light of the hypothesis that predator–prey relationships played a role in the evolution of colour polymorphism.