Patterns of differentiation in a colour polymorphism and in neutral markers reveal rapid genetic changes in natural damselfly populations

The existence and mode of selection operating on heritable adaptive traits can be inferred by comparing population differentiation in neutral genetic variation between populations (often using F(ST) values) with the corresponding estimates for adaptive traits. Such comparisons indicate if selection acts in a diversifying way between populations, in which case differentiation in selected traits is expected to exceed differentiation in neutral markers [F(ST )(selected) > F(ST )(neutral)], or if negative frequency-dependent selection maintains genetic polymorphisms and pulls populations towards a common stable equilibrium [F(ST) (selected) < F(ST) (neutral)]. Here, we compared F(ST) values for putatively neutral data (obtained using amplified fragment length polymorphism) with estimates of differentiation in morph frequencies in the colour-polymorphic damselfly Ischnura elegans. We found that in the first year (2000), population differentiation in morph frequencies was significantly greater than differentiation in neutral loci, while in 2002 (only 2 years and 2 generations later), population differentiation in morph frequencies had decreased to a level significantly lower than differentiation in neutral loci. Genetic drift as an explanation for population differentiation in morph frequencies could thus be rejected in both years. These results indicate that the type and/or strength of selection on morph frequencies in this system can change substantially between years. We suggest that an approach to a common equilibrium morph frequency across all populations, driven by negative frequency-dependent selection, is the cause of these temporal changes. We conclude that inferences about selection obtained by comparing F(ST) values from neutral and adaptive genetic variation are most useful when spatial and temporal data are available from several populations and time points and when such information is combined with other ecological sources of data.     

An experimental test of the role of predators in the maintenance of a genetically based polymorphism

Polymorphisms provide one of the most useful tools for understanding the maintenance of genetic and phenotypic variation in nature. We have previously described a genetically based polymorphism in dorsal patterning that is expressed by female brown anole lizards, Anolis sagrei, which occur in Bar, Diamond and intermediate Diamond-Bar morphs. Previous studies of island populations in The Bahamas support a role for selection in maintaining the polymorphism, but the agents responsible for this selection remain unclear. We tested two main hypotheses regarding the importance of predation as a selective agent that maintains the polymorphism within populations. First, we tested whether correlational selection favours different combinations of morph, locomotor performance and escape behaviour by measuring morph-specific natural selection on sprint speed, running endurance and the propensity of females to either 'freeze' or 'run' in response to attempted capture. Morphs did not differ in any of these traits, nor did correlational selection consistently favour any particular combinations of morph and antipredator behaviour. Second, we experimentally excluded bird and snake predators from two entire island populations, allowed these predators access to two additional islands and then measured subsequent differences in natural selection on morphs in each population. Predators reduced the survival of Bar and Diamond females, but not of genetically intermediate Diamond-Bar females. These results provide limited evidence that predation may play a role in maintaining this polymorphism, although the functional traits that could account for differential susceptibility to predation remain unclear.     

COLOR PATTERN POLYMORPHISM IN THE LAKE ERIE WATER SNAKE,NERODIA SIPEDON INSULARUM

Populations of the water snake, Nerodia sipedon, on islands in western Lake Erie are polymorphic for color pattern. These populations include banded, intermediate, and unbanded morphs while surrounding mainland populations consist solely of the banded morph. The hypothesis that this polymorphism is maintained by strong selection and migration pressures is widely accepted. Unbanded morphs are apparently more cryptic along island shorelines while banded morphs are more cryptic on the mainland. Migration of banded morphs from the mainland explains their persistence in island populations. Data collected in a capture-mark-recapture program on six islands provide no evidence of differential selection among morphs; morph frequencies do not differ among age classes, between once-captured and multiply-captured snakes, or between scarred and unscarred snakes. Furthermore, herring gulls, the most common snake predators in the island area, appear to detect banded and unbanded model snakes with equal ease. High site fidelity of water snakes and the distribution of morphs among islands suggest that migration from the mainland is not common. However, islands close to each other are similar in morph frequency, and water snakes have colonized islands elsewhere in the Great Lakes, indicating that some migration does occur. Recently, the frequency of banded morphs has increased in island populations while adult population sizes have declined. This increase in banded morphs is interpreted as reflecting an increased impact of migration from the mainland into these reduced populations. One scenario for the evolution and maintenance of this polymorphism is that selection was important in establishing unbanded morphs in island populations as they became isolated from the mainland. As populations declined to their present size, the impact of migration from the mainland increased and is now swamping the effect of selection. Further declines in island population size may result in fixation of the banded morph.     

Opposing fitness consequences of colour pattern in male and female snakes

Local populations of the adder, Vipera berus, are polymorphic for dorsal colour pattern, containing both melanistic (black) and zig-zag patterned individuals. Colour patterns in snakes influence crypsis and thermoregulatory capacity and therefore may be subjected to natural selection. To find an explanation for the maintenance of this polymorphism I examined temporal and spatial variation in morph frequency, and tested for differential selection among morphs using data from a six year capture-mark-recapture study. The data derive from six groups of islands in the Baltic Sea off the Swedish east coast, two mainland localities near the coast, and one inland locality. Morph frequency did not change over time within a population but varied among populations: melanistic individuals were not found at the inland locality, but comprised from 17 to 62% of the coastal and island populations. Adders frequently moved between islands within a group, but the tendency to disperse was independent of morph. These results suggest that the polymorphism is stable and maintained by a deterministic process. Scar frequency was twice as high among melanistic as among zig-zag snakes, and melanistic individuals were easier to capture, indicating that predation may be higher on the melanistic morph. Colour morphs did not differ in body size, but analysis of recapture data shows evidence for differential survival among morphs. Zig-zag males survived better than melanistic males, but the relative survival rates of morphs were reversed in females. This difference was consistent through time and may be due to sexual differences in behaviour, with melanism increasing predation intensity when associated with male but not with female behaviour. Opposing fitness consequences of colour pattern in the two sexes may help maintain colour polymorphism within populations of Vipera berus.     

Genetic drift within a protected polymorphism: enigmatic variation in color-morph frequencies in the candy-stripe spider, Enoplognatha ovata

The candy-stripe spider, Enoplognatha ovata, exhibits a striking color polymorphism comprising three morphs. A number of lines of evidence strongly suggest that this polymorphism is maintained by natural selection: its presence in a sister species, E. latimana; the physical nature of the variation; the virtual lack of monomorphic populations; the highly consistent rank-order of morphs within populations; and the presence of large-scale clines associated with climatic variables. However, the absence of selection is equally strongly suggested by very local surveys of morph frequencies over space and time, perturbation experiments, and a variance in morph frequency between populations that is virtually independent of spatial scale. In addition, local spatial patterns in one study site (Nidderdale, Yorkshire, England) have been explained in terms of intermittent drift over half a century ago, a hypothesis supported here by the distributions of four other genetic markers (two allozyme and two visible polymorphisms). A heuristic model is suggested that reconciles these apparently contradictory messages regarding the importance of drift and selection in this system. It is proposed that when allele frequencies of the color morph redimita lie between approximately 0.05 and 0.3, the deltaq on q plot is very shallow, so that within this region, where the majority of populations lie, selection is weak and drift is the major force determining local morph frequencies. However, outside this range of frequencies, powerful selection acts to protect the polymorphism. This model may apply to polymorphisms in other species and explain why evidence of selection in natural populations is often elusive.     

Spatial analyses of two color polymorphisms in an alpine grasshopper reveal a role of small‐scale heterogeneity

Discrete color polymorphisms represent a fascinating aspect of intraspecific diversity. Color morph ratios often vary clinally, but in some cases, there are no marked clines and mixes of different morphs occur at appreciable frequencies in most populations. This poses the questions of how polymorphisms are maintained. We here study the spatial and temporal distribution of a very conspicuous color polymorphism in the club‐legged grasshopper Gomphocerus sibiricus. The species occurs in a green and a nongreen (predominately brown) morph, a green–brown polymorphism that is common among Orthopteran insects. We sampled color morph ratios at 42 sites across the alpine range of the species and related color morph ratios to local habitat parameters and climatic conditions. Green morphs occurred in both sexes, and their morph ratios were highly correlated among sites, suggesting shared control of the polymorphism in females and males. We found that in at least 40 of 42 sites green and brown morphs co‐occurred with proportions of green ranging from 0% to 70% with significant spatial heterogeneity. The proportion of green individuals tended to increase with decreasing summer and winter precipitations. Nongreen individuals can be further distinguished into brown and pied individuals, and again, this polymorphism is shared with other grasshopper species. We found pied individuals at all sites with proportions ranging from 3% to 75%, with slight, but significant variation between years. Pied morphs show a clinal increase in frequency from east to west and decreased with altitude and lower temperatures and were more common on grazed sites. The results suggest that both small‐scale and large‐scale spatial heterogeneity affects color morph ratios. The almost universal co‐occurrence of all three color morphs argues against strong effects of genetic drift. Instead, the data suggest that small‐scale migration–selection balance and/or local balancing selection maintain populations polymorphic.     

The adaptive significance of colour pattern polymorphism in the Australian scincid lizard Lampropholis delicata

Females of Lampropholis delicata are dimorphic for colour pattern, the difference between morphs being the presence or absence of a distinct white mid-lateral stripe. A less distinct striped morph occurs also in males. We evaluated alternative hypotheses for the maintenance of this polymorphism by examining temporal and spatial variation in morph frequency, testing for differential selection among morphs using data on body size and reproductive traits from preserved specimens, and experimentally manipulating colour pattern in free-ranging lizards of both sexes, to assess the influence of the lateral stripe on survival rates. We found that the relative frequency of striped individuals varied among populations and decreased from north to south in both sexes, coincident with an increasing incidence of regenerated tails. Morph frequencies did not change through time within a population. Striped gravid females appeared to survive better and produced larger clutches than did non-striped females. In our experimental study, the relationship between survival and colour morph differed between the two sexes; males painted with a white lateral stripe had lower survival than control (brown stripe) males, but survival did not differ between striped and control females. The different response in the two sexes may be due partly to differences in temperature and microhabitat selection. We propose that the white lateral stripe decreases susceptibility to predators in gravid females but increases risk of predation in males, especially in combination with low temperatures. The polymorphism might be maintained by: (1) opposing fitness consequences of the stripe in males and females; (2) sex-specific habitat selection; and (3) gene flow in combination with spatial variation in relative fitness of the two morphs.     

Thirty-four years of climatic selection in the land snail Theba pisana

Few continuous, long-term studies have measured the intensity and variability of natural selection within a framework of clear adaptive hypotheses. In the snail Theba pisana, the proportion of effectively unbanded shells is higher in exposed habitats than in adjacent acacia thickets, which has been explained by microclimatic selection. Comparisons across an ecotone for 34 consecutive years determined the combined effects on morph frequencies of habitat and changes in weather conditions during summer. The long-term average (±s.e.) frequency of effectively unbanded shells was 0.577±0.011 in the open habitat when compared with 0.353±0.005 in the acacia. The persistent association of shell banding with habitat accounted for 34% of the variation in morph frequencies. Differences among years were also large, representing 23% of the variation. Higher proportions of effectively unbanded snails were associated with hotter, sunnier summers. Thus, temporal variation supports the hypothesis of microclimatic selection, consistent with the spatial association with habitat. Based on observed rates of change, the mean annual selection on this polymorphism was about 0.13, but with a large variance: s was as high as 0.5, but ⩽0.05 in about 40% of the years. The large variance and frequent reversals in direction of selection indicate a potential for rapid genetic change, but with little net change in morph frequencies over three decades, highlighting the value of long-term continuous studies of populations facing natural environmental variation.     

An experimental test for alternative reproductive strategies underlying a female-limited polymorphism

Polymorphism often corresponds to alternative mating tactics in males, but much less is known about this relationship in females. However, recent work suggests that selection for alternative reproductive strategies in females can maintain genetic variation in important life-history traits. Brown anole lizards (Anolis sagrei) exhibit a genetically based polymorphism in dorsal pattern that is expressed only by females, which occur in bar (B), diamond (D) and intermediate diamond-bar (DB) morphs. Here, we use a combination of natural history data, captive breeding studies and phenotypic manipulations of reproductive investment to test the hypothesis that this polymorphism corresponds to morph-specific patterns of reproductive investment. Three years of data from wild females and two generations of captive breeding revealed no differences among morphs in the frequency of egg production or in the number, frequency, size or sex ratio of offspring. Manipulations of reproductive investment via surgical ovariectomy revealed significant costs of reproduction with respect to survival, growth, immune function and haematocrit, but the magnitudes of these costs did not differ among morphs. Collectively, our results refute the hypothesis that this sex-limited polymorphism is maintained by selection for alternative reproductive strategies. We compare this finding to other systems in which polymorphic females exhibit alternative reproductive tactics and discuss other selective factors that could maintain polymorphism in anoles.     

性选择、配偶外父亲身份确认程度、遗传变异性和保护

岛屿动物中的性选择强度不高,其原因可能是由于岛屿种群的遗传变异性水平较低。本文作者检验了鸟类岛屿种群是否具有较低的遗传变异性、性选择强度大的种群是否具有较高的突变输入率（rate of mutational input),在鸟类岛屿种群中是否具有较低的性选择强度（可以根据配偶外父亲身份的频率来估计）。小卫星共有谱带系数（minisatellite band sharing coefficient)可确定无亲源关系个体之间的遗传变异性,对与遗传变异性有关的雄性个体的父亲（paternity)进行了成对比较以检验如下假说：在具有较多遗传变异的种群中,雌性个体更经常地进行配偶外交配。在小卫星谱带系数较低的鸟类种群中,配偶外父亲的频次较高。对岛屿和大陆鸟类进行的第二个比较分析表明：岛屿种群中的配偶外父亲频次较低,遗传变异性也较低,其部分原因在于突变输入（mutational input)减少。上述发现表明：（1）父亲确认程度（parternity)随遗传变异性的数量而增加;（2）在遗传变异性较大的种群中,突变率较高,性选择的程度更激烈;（3）岛屿种群中性选择的强度一般比大陆种群弱。这对于理解遗传变异性的空间变异、理解岛屿种群和其它隔离种群的保护问题有重要启示。     

SELECTION ON THE COLOR POLYMORPHISM IN HAWAIIAN HAPPY-FACE SPIDERS: EVIDENCE FROM GENETIC STRUCTURE AND TEMPORAL FLUCTUATIONS

Throughout this century genetic polymorphisms for color have been widely used as a research tool to allow insights into key evolutionary processes. Although color variants can often be diverse within populations, frequencies of different morphs may be similar across populations, either as a result of balancing selection or gene flow. Under these circumstances selection can be extremely difficult to demonstrate. Here we test for balancing selection on the naturally occurring color forms of the Hawaiian happy-face spider, Theridion grallator with two approaches. First, allozyme loci are used to generate a null model against which to test selection. Frequencies of alleles involved in the color polymorphism of T. grallator are used to generate another estimate for comparison. The results suggest that statistically similar frequencies of color morphs among populations of T. grallator may be maintained by some form of balancing selection. Second, we make use of an unusual event in which the normally stable frequencies of unpatterned and patterned morphs within a population were found to have shifted toward an excess of unpatterned morphs. We scored offspring of all fertilized, unpatterned (bottom-recessive) females found during this period of skewed morph frequencies and also in a year when morph frequencies were normal to deduce paternal color phenotypes. Mating was found to be random in the normal year, but in the perturbed year females had mated with rare (patterned) males twice as frequently as expected on the basis of the frequency of this morph type in the population. Both of these results are consistent with selection operating on the color polymorphism, and we speculate that apostatic selection, perhaps mediated by bird predators, may provide the mechanism.     

THE INTERPLAY BETWEEN LOCAL ECOLOGY,DIVERGENT SELECTION,AND GENETIC DRIFT IN POPULATION DIVERGENCE OF A SEXUALLY ANTAGONISTIC FEMALE TRAIT

Genetically polymorphic species offer the possibility to study maintenance of genetic variation and the potential role for genetic drift in population divergence. Indirect inference of the selection regimes operating on polymorphic traits can be achieved by comparing population divergence in neutral genetic markers with population divergence in trait frequencies. Such an approach could further be combined with ecological data to better understand agents of selection. Here, we infer the selective regimes acting on a polymorphic mating trait in an insect group; the dorsal structures (either rough or smooth) of female diving beetles. Our recent work suggests that the rough structures have a sexually antagonistic function in reducing male mating attempts. For two species (Dytiscus lapponicus and Graphoderus zonatus), we could not reject genetic drift as an explanation for population divergence in morph frequencies, whereas for the third (Hygrotus impressopunctatus) we found that divergent selection pulls morph frequencies apart across populations. Furthermore, population morph frequencies in H. impressopunctatus were significantly related to local bioclimatic factors, providing an additional line of evidence for local adaptation in this species. These data, therefore, suggest that local ecological factors and sexual conflict interact over larger spatial scales to shape population divergence in the polymorphism.     

Spatial sorting may explain evolutionary dynamics of wing polymorphism in pygmy grasshoppers

Wing polymorphism in insects provides a good model system for investigating evolutionary dynamics and population divergence in dispersal‐enhancing traits. This study investigates the contribution of divergent selection, trade‐offs, behaviour and spatial sorting to the evolutionary dynamics of wing polymorphism in the pygmy grasshopper Tetrix subulata (Tetrigidae: Orthoptera). We use data for > 2800 wild‐caught individuals from 13 populations and demonstrate that the incidence of the long‐winged (macropterous) morph is higher and changes faster between years in disturbed habitats characterized by succession than in stable habitats. Common garden and mother‐offspring resemblance studies indicate that variation among populations and families is genetically determined and not influenced to any important degree by developmental plasticity in response to maternal condition, rearing density or individual growth rate. Performance trials show that only the macropterous morph is capable of flight and that propensity to fly differs according to environment. Mark–recapture data reveal no difference in the distance moved between free‐ranging long‐ and short‐winged individuals. There is no consistent difference across populations and years in number of hatchlings produced by long‐ and shorter‐winged females. Our findings suggest that the variable frequency of the long‐winged morph among and within pygmy grasshopper populations may reflect evolutionary modifications driven by spatial sorting due to phenotype‐ and habitat type–dependent emigration and immigration.     

Biogeographical survey identifies consistent alternative physiological optima and a minor role for environmental drivers in maintaining a polymorphism

The contribution of adaptive mechanisms in maintaining genetic polymorphisms is still debated in many systems. To understand the contribution of selective factors in maintaining polymorphism, we investigated large-scale (>1000 km) geographic variation in morph frequencies and fitness-related physiological traits in the damselfly Nehalennia irene. As fitness-related physiological traits, we investigated investment in immune function (phenoloxidase activity), energy storage and fecundity (abdomen protein and lipid content), and flight muscles (thorax protein content). In the first part of the study, our aim was to identify selective agents maintaining the large-scale spatial variation in morph frequencies. Morph frequencies varied considerably among populations, but, in contrast to expectation, in a geographically unstructured way. Furthermore, frequencies co-varied only weakly with the numerous investigated ecological parameters. This suggests that spatial frequency patterns are driven by stochastic processes, or alternatively, are consequence of highly variable and currently unidentified ecological conditions. In line with this, the investigated ecological parameters did not affect the fitness-related physiological traits differently in both morphs. In the second part of the study, we aimed at identifying trade-offs between fitness-related physiological traits that may contribute to the local maintenance of both colour morphs by defining alternative phenotypic optima, and test the spatial consistency of such trade-off patterns. The female morph with higher levels of phenoloxidase activity had a lower thorax protein content, and vice versa, suggesting a trade-off between investments in immune function and in flight muscles. This physiological trade-off was consistent across the geographical scale studied and supports widespread correlational selection, possibly driven by male harassment, favouring alternative trait combinations in both female morphs.     

Spatial variation in colour morph, spotting and allozyme frequencies in the candy-stripe spider, Enoplognatha ovata (Theridiidae) on two Swedish archipelagos

The selective significance, if any, of many invertebrate visible polymorphisms is still not fully understood. Here we examine colour- and black spotting-morph frequencies in the spider Enoplognatha ovata in populations on two Swedish archipelagos with respect to different spatial scales and, in one archipelago, against the background of variation at four putative neutral allozyme marker loci. Every population studied was polymorphic for colour and 28 out of 30 contained all three colour morphs – lineata, redimita and ovata. We found no evidence for a breakdown in the traditional colour morph designation previously suggested for other northern European populations of this species. For colour there is no significant heterogeneity at spatial scales greater than between local populations within islands. Black spotting frequencies show a similar lack of pattern over larger spatial scales except that there are significant differences between the Stockholm and Göteborg archipelagos. Measures of population differentiation (θ) within the Stockholm islands for the two visible systems show them to be significantly more differentiated than the neutral markers, suggesting local selection acting on them in a population-specific manner. On the basis of previous observations and the distribution of spotting phenotypes on a European scale, it is argued that thermal selection might operate on black spotting during the juvenile stages favouring more spots in continental climates. It is not clear what selective forces act on colour.     

Sexual Antagonism,Temporally Fluctuating Selection,and Variable Dominance Affect a Regulatory Polymorphism in Drosophila melanogaster

Understanding how genetic variation is maintained within species is a major goal of evolutionary genetics that can shed light on the preservation of biodiversity. Here, we examined the maintenance of a regulatory single-nucleotide polymorphism (SNP) of the X-linked Drosophila melanogaster gene fezzik. The derived variant at this site is at intermediate frequency in many worldwide populations but absent in populations from the ancestral species range in sub-Saharan Africa. We collected and genotyped wild-caught individuals from a single European population biannually over a period of 5 years, which revealed an overall difference in allele frequency between the sexes and a consistent change in allele frequency across seasons in females but not in males. Modeling based on the observed allele and genotype frequencies suggested that both sexually antagonistic and temporally fluctuating selection may help maintain variation at this site. The derived variant is predicted to be female-beneficial and mostly recessive; however, there was uncertainty surrounding our dominance estimates and long-term modeling projections suggest that it is more likely to be dominant. By examining gene expression phenotypes, we found that phenotypic dominance was variable and dependent upon developmental stage and genetic background, suggesting that dominance may be variable at this locus. We further determined that fezzik expression and genotype are associated with starvation resistance in a sex-dependent manner, suggesting a potential phenotypic target of selection. By characterizing the mechanisms of selection acting on this SNP, our results improve our understanding of how selection maintains genetic and phenotypic variation in natural populations.     

Genome-wide population genetic analysis identifies evolutionary forces establishing continuous population divergence

Elucidating the mechanism shaping the spatial variations of traits has long been a central concern of evolutionary biologists. Geographic clines of allele/morph frequencies along environmental gradients are suggested to be established and maintained by the balancing of two opposing evolutionary forces, namely selection that generates spatial differentiation in morph frequencies, and selection and/or stochastic factors that lead to the coexistence of multiple morphs within a population. Thus, testing for both selection and stochastic factors is necessary for a comprehensive understanding of the mechanism underlying clinal variation in morph/allele frequency in natural populations. Here, I identified the evolutionary forces responsible for clinal variation of color morph frequency in Ischnura senegalensis by comparing the population divergence of putatively neutral loci generated by high-throughput next-generation sequencing (F _STn) with that of the putative color locus (F _STc). No strong correlation was observed between F _STn and F _STc, suggesting that stochastic factors contribute less to color-locus population divergence. F _STc was less than F _STn between populations exposed to similar environmental conditions, but greater than F _STn between populations exposed to different environmental conditions, suggesting that both balancing selection and divergent selection act on the color locus. Therefore, two antagonistic selection factors rather than stochastic and historical factors contribute to establishing the clinal variation of morph frequency in I. senegalensis.     

COLOR-PATTERN VARIATION IN LAKE ERIE WATER SNAKES: THE ROLE OF GENE FLOW

In an effort to clarify the evolutionary processes influencing color-pattern variation in Lake Erie island water snake (Nerodia sipedon) populations, rates of gene flow among island and mainland populations were estimated from patterns of allozymic variation detected using electrophoresis. Rates of gene flow were high with Nm, the number of migrants per generation, averaging 25.5 among island sites, 9.2 between the Ontario mainland and the islands, and 3.6 between the Ohio mainland and the islands. Based on estimates of current population size from mark-recapture work and of past population size extrapolated from the extent of shoreline habitat, values of m between island and mainland populations ranged from 0.0008–0.01. Synthesis of estimates of the rate of gene flow with information on inheritance of color pattern, the strength of natural selection, and population history supports the hypothesis that color-pattern variation in island populations results from a balance between gene flow and natural selection. However, depending on the mode of inheritance of color pattern, stochastic processes such as drift may have been important in the initial stages of differentiation between island and mainland populations.     

Fruit colour polymorphism in Acacia ligulata: seed and seedling performance, clinal patterns, and chemical variation

Fruit colour polymorphisms are widespread in nature, but their ecological and evolutionary dynamics remain poorly understood. Here we examine Acacia ligulata, a shrub of the Australian arid zone which exhibits a red/orange/yellow aril colour polymorphism. We asked whether the polymorphism had a genetic basis; whether selection acted differentially on morphs during the seed and seedling stages; whether geographic variation in morph frequencies was correlated with environmental factors; and whether morphs differed in physical or chemical characteristics that might influence selection on them. When grown to maturity in a common greenhouse environment, maternal families of seeds showed phenotypic patterns consistent with biparental genetic control of the polymorphism. In contrast to other fruit-colour polymorphic species, progeny of A. ligulata morphs did not vary in rates of seedling emergence or survival in a common garden. Sampling along a 580 km transect revealed clinal variation in morph frequencies. Frequencies of the yellow morph decreased, and frequencies of the red morph increased, across a gradient of decreasing temperature and increasing rainfall. Morphs did not differ in seed mass, aril mass, or in profiles of fatty acids and flavonoids in either arils or seeds. However, morphs showed consistent differences in carotenoid profiles' and elemental content of arils, suggesting that selection by avian and insect seed dispersers, seed predators and herbivores should be investigated. These patterns indicate that both abiotic and biotic factors may contribute to selection on the A. ligulata polymorphism. This revised version was published online in August 2006 with corrections to the Cover Date.     

Selective pressures on MHC class II genes in the guppy (Poecilia reticulata) as inferred by hierarchical analysis of population structure

Genes of the major histocompatibility complex, which are the most polymorphic of all vertebrate genes, are a pre‐eminent system for the study of selective pressures that arise from host–pathogen interactions. Balancing selection capable of maintaining high polymorphism should lead to the homogenization of MHC allele frequencies among populations, but there is some evidence to suggest that diversifying selection also operates on the MHC. However, the pattern of population structure observed at MHC loci is likely to depend on the spatial and/or temporal scale examined. Here, we investigated selection acting on MHC genes at different geographic scales using Venezuelan guppy populations inhabiting four regions. We found a significant correlation between MHC and microsatellite allelic richness across populations, which suggests the role of genetic drift in shaping MHC diversity. However, compared to microsatellites, more MHC variation was explained by differences between populations within larger geographic regions and less by the differences between the regions. Furthermore, among proximate populations, variation in MHC allele frequencies was significantly higher compared to microsatellites, indicating that selection acting on MHC may increase population structure at small spatial scales. However, in populations that have significantly diverged at neutral markers, the population‐genetic signature of diversifying selection may be eradicated in the long term by that of balancing selection, which acts to preserve rare alleles and thus maintain a common pool of MHC alleles.