Multivariate phenotypes and the potential for alternative phenotypic optima in wall lizard (Podarcis muralis) ventral colour morphs

A major goal in evolutionary biology is to determine how phenotypic variation arises and is maintained in natural populations. Recent studies examining the morphological, physiological and behavioural differences among discrete colour morphotypes (morphs) have revealed several mechanisms that maintain discrete variation within populations, including frequency‐dependence, density‐dependence and correlational selection. For example, trade‐offs over resource allocation to morphological, physiological and behavioural traits can drive correlational selection for morph‐specific phenotypic optima. Here, we describe a ventral colour polymorphism in the wall lizard (Podarcis muralis) and test the hypothesis that morphs differ along multivariate axes defined by trade‐offs in morphological, physiological, and immunological traits. We show that ventral colour is a discrete trait and that morphs differ in body size, prevalence of infection by parasites and infection intensity. We also find that morphs differ along multivariate phenotypic axes and experience different multivariate selection pressures. Our results suggest that multivariate selection pressures may favour alternative optimal morph‐specific phenotypes in P. muralis.     

Differential predation on sexes affects colour polymorphism of the isopod Idotea baltica (Pallas)

Variable selection, including spatio-temporal variation, frequency-dependent selection and differential selection due to habitat choice, may maintain polymorphism in heterogeneous environments. We studied predation as a selective agent on colour polymorphism of the aquatic isopod I baltica. Variable predation on this species can arise from at least three sources. First, apostatic selection was studied by testing the formation of preferences on colour morphs in the perch, a common predator of I baltica. Such acquired preferences should induce apostatic selection. While our results indicate some acquired preferences, there was significant heterogeneity in the behaviour of predator individuals. Second, temporal variation in selection can arise due to habitat shift from the green algae juvenile habitat to the bladderwrack adult habitat, and the consequent change in the crypsis of the morphs. Different crypsis between sexes probably promoted high predation mortality among females in the juvenile habitat. The high rate of male mortality during the breeding period, on the other hand, was presumably due to their high mate-searching activity. Third, the sex-dependent habitat choice of I baltica leads to sexual differences in the susceptibility of morphs to predation. Predators preferred the white-spotted morph over the uniform one in males but not in females, supporting the 'dimorphic niche' hypothesis as an explanation of sexual differences in morph frequencies. Finally, no evidence was found that the colouration patterns were under sexual selection. We therefore conclude diat variable predation is the most promising explanation for the maintenance of polymorphism in I. baltica.     

Adaptive significance of permanent female mimicry in a bird of prey

Permanent female mimicry, in which adult males express a female phenotype, is known only from two bird species. A likely benefit of female mimicry is reduced intrasexual competition, allowing female-like males to access breeding resources while avoiding costly fights with typical territorial males. We tested this hypothesis in a population of marsh harriers Circus aeruginosus in which approximately 40 per cent of sexually mature males exhibit a permanent, i.e. lifelong, female plumage phenotype. Using simulated territorial intrusions, we measured aggressive responses of breeding males towards conspecific decoys of females, female-like males and typical males. We show that aggressive responses varied with both the type of decoys and the type of defending male. Typical males were aggressive towards typical male decoys more than they were towards female-like male decoys; female-like male decoys were attacked at a rate similar to that of female decoys. By contrast, female-like males tolerated male decoys (both typical and female-like) and directed their aggression towards female decoys. Thus, agonistic responses were intrasexual in typical males but intersexual in female-like males, indicating that the latter not only look like females but also behave like them when defending breeding resources. When intrasexual aggression is high, permanent female mimicry is arguably adaptive and could be seen as a permanent 'non-aggression pact' with other males.     

鸟类羽色多态现象：概念及其进化机制假说

多态现象对了解物种的遗传、变异和进化有着重要意义,鸟类羽色多态现象的进化机制,是多态现象研究中非常重要的内容。鸟类羽色多态现象的进化机制假说主要有：分化选择假说、异类选择假说和非随机性交配假说等。本文对鸟类羽色多态现象的概念和以上进化机制假说进行了综述,并针对鸟类羽色多态现象进化机制研究中存在争议的问题及研究的不足提出了自己的看法。     

Selection by wild birds on artificial dimorphic prey on varied backgrounds

Our aim was to test the effects of prey frequency and background composition on selection by free-ranging birds. We did three series of experiments with populations of grey and orange pastry prey scattered among coloured stones that made the prey either conspicuous or inconspicuous. Series 1 tested whether the predicted equilibrium frequency of the two prey types was influenced by the frequency of matching grey and orange stones. Birds at a single site were given a random sequence of combinations of prey frequency and stone frequency. Selection was dependent on background and the effect of prey frequency also varied with background. In series 2, we explored the frequency-independent effect of background: birds at five sites were given equal numbers of the two prey in three frequencies of matching stones and two of non-matching. There was a higher risk of predation for prey that matched rarer stones. In series 3 we attempted to measure, at a single site, the actual equilibrium prey frequencies in three different backgrounds: two extreme stone frequencies and one intermediate. Each experiment started with a population of equal numbers of grey and orange prey. After half the prey had been eaten we calculated the frequencies of the survivors and presented a new population of the original size but with the new prey frequencies; each experiment ran for 25 such 'generations'. The results suggested that at equilibrium the commoner 'morph' was the one that resembled the commoner colour of stone. Overall, our findings support the idea that visual selection can result in morph frequencies becoming related to the proportions of their matching background components and that this equilibrium will 'track' temporal or spatial changes in the background.     

Colour-associated mating success in a polymorphic Ladybird Beetle, Harmonia axyridis

1. Body size is often an important character in mating success, but has been only infrequently mentioned in regard to colour polymorphism. In this study, mating success was investigated in a colour polymorphic Ladybird Beetle, Harmonia axyridis , with reference both to colour morph and to body size.
2. In the non-melanic males the mating individuals were significantly larger than solitary individuals, while in melanic males there was no significant difference.
3. The mating pattern was close to random mating with respect to colour morph and there was no significant deviation.
4. The results suggest both body size and colour morph affect the male mating success and males of different body size obtain mating advantage according to the colour morph. Colour polymorphism in this species is controlled by alleles on a single locus. Thus, the alleles on that locus significantly influence the effect of selection on the quantitative character.     

The evolution, maintenance and adaptive function of genetic colour polymorphism in birds

The hypothesis that ornaments can honestly signal quality only if their expression is condition-dependent has dominated the study of the evolution and function of colour traits. Much less interest has been devoted to the adaptive function of colour traits for which the expression is not, or is to a low extent, sensitive to body condition and the environment in which individuals live. The aim of the present paper is to review the current theoretical and empirical knowledge of the evolution, maintenance and adaptive function of colour plumage traits for which the expression is mainly under genetic control. The finding that in many bird species the inheritance of colour morphs follows the laws of Mendel indicates that genetic colour polymorphism is frequent. Polymorphism may have evolved or be maintained because each colour morph facilitates the exploitation of alternative ecological niches as suggested by the observation that individuals are not randomly distributed among habitats with respect to coloration. Consistent with the hypothesis that different colour morphs are linked to alternative strategies is the finding that in a majority of species polymorphism is associated with reproductive parameters, and behavioural, life-history and physiological traits. Experimental studies showed that such covariations can have a genetic basis. These observations suggest that colour polymorphism has an adaptive function. Aviary and field experiments demonstrated that colour polymorphism is used as a criterion in mate-choice decisions and dominance interactions confirming the claim that conspecifics assess each other's colour morphs. The factors favouring the evolution and maintenance of genetic variation in coloration are reviewed, but empirical data are virtually lacking to assess their importance. Although current theory predicts that only condition-dependent traits can signal quality, the present review shows that genetically inherited morphs can reveal the same qualities. The study of genetic colour polymorphism will provide important and original insights on the adaptive function of conspicuous traits.     

Colour variation and alternative reproductive strategies in females of the common lizard Lacerta vivipara

Within-sex colour variation is a widespread phenomenon in animals that often plays a role in social selection. In males, colour variation is typically associated with the existence of alternative reproductive strategies. Despite ecological conditions theoretically favourable to the emergence of such alternative strategies in females, the social significance of colour variation in females has less commonly been addressed, relative to the attention given to male strategies. In a population of the common lizard, females display three classes of ventral colouration: pale yellow, orange and mixed. These ventral colours are stable through individual's life and maternally heritable. Females of different ventral colourations displayed different responses of clutch size, clutch hatching success and clutch sex-ratio to several individual and environmental parameters. Such reaction patterns might reflect alternative reproductive strategies in females. Spatial heterogeneity and presence of density- and frequency-dependent feedbacks in the environment could allow for the emergence of such alternative strategies in this population and the maintenance of colour variation in females.     

Cast adrift on an island: introduced populations experience an altered balance between selection and drift

A long-standing question in evolutionary biology is what becomes of adaptive traits when a species expands its range into novel environments. Here, we report the results of a study on an adaptive colour pattern polymorphism (stripes) of the coqui frog, Eleutherodactylus coqui, following its introduction to Hawaii from Puerto Rico. We compared population differentiation ( ) for the stripes locus-which underlies this colour pattern polymorphism-with neutral microsatellite loci to test for a signature of selection among native and introduced populations. Among native populations, for stripes were lower than expected under the neutral model, suggesting uniform balancing selection. Alternatively, among introduced populations, for stripes did not differ from the neutral model. These results suggest that the evolutionary dynamics of this previously adaptive trait have become dominated by random genetic drift following the range expansion.     

A molecular genetic examination of the mating system of pumpkinseed sunfish reveals high pay-offs for specialized sneakers

Intrasexual variation in reproductive behaviour and morphology are common in nature. Often, such variation appears to result from conditional strategies in which some individuals (e.g. younger males or those in poor condition) adopt a low pay-off phenotype as a 'best of a bad job'. Alternatively, reproductive polymorphisms can be maintained by balancing selection, with male phenotypes having equal fitnesses at equilibrium, but examples from nature are rare. Many species of sunfish (genus Lepomis) are thought to have alternative male reproductive behaviours, but most empirical work has focused on the bluegill sunfish and the mating systems of other sunfish remain poorly understood. We studied a population of pumpkinseed sunfish (Lepomis gibbosus) in upstate New York. Field observations confirm the existence of two male reproductive strategies: 'parentals' were relatively old and large males that maintained nests, and 'sneakers' were relatively young and small males that fertilize eggs by darting into nests of parentals during spawning. The sneaker and parental male strategies appear to be distinct life-history trajectories. Sneaker males represented 39% of the males observed spawning, and sneakers intruded on 43% of all mating attempts. Microsatellite analyses revealed that sneaker males fertilized an average of 15% of the eggs within a nest. This level of paternity by sneaker males appears to be higher than seen in most other fishes, and preliminary analyses suggest that the two male reproductive strategies are maintained as a balanced polymorphism.