Melanic Phenotypes Research Articles

A Reversible Color Polyphenism in American Peppered Moth (Biston betularia cognataria) Caterpillars

Insect body color polyphenisms enhance survival by producing crypsis in diverse backgrounds. While color polyphenisms are often indirectly induced by temperature, rearing density, or diet, insects can benefit from immediate crypsis if they evolve polyphenisms directly induced by exposure to the background color, hence immediately deriving protection from predation. Here, we examine such a directly induced color polyphenism in caterpillars of the geometrid peppered moth (Biston betularia). This larval color polyphenism is unrelated to the genetic polymorphism for melanic phenotypes in adult moths. B. betularia caterpillars are generalist feeders and develop body colors that closely match the brown or green twigs of their host plant. We expand on previous studies examining the proximal cues that stimulate color development. Under controlled rearing conditions, we manipulated diets and background reflectance, using both natural and artificial twigs, and show that visual experience has a much stronger effect than does diet in promoting precise color matching. Their induced body color was not a simple response to reflectance or light intensity but instead specifically matched the wavelength of light to which they were exposed. We also show that the potential to change color is retained until the final (sixth) larval instar. Given their broad host range, this directly induced color polyphenism likely provides the caterpillars with strong protection from bird predation.

Testing whether macroevolution follows microevolution: Are colour differences among swans (Cygnus) attributable to variation at the MC1R locus?

BackgroundThe MC1R (melanocortin-1 receptor) locus underlies intraspecific variation in melanin-based dark plumage coloration in several unrelated birds with plumage polymorphisms. There is far less evidence for functional variants of MC1R being involved in interspecific variation, in which spurious genotype-phenotype associations arising through population history are a far greater problem than in intraspecific studies. We investigated the relationship between MC1R variation and plumage coloration in swans (Cygnus), which show extreme variation in melanic plumage phenotypes among species (white to black).ResultsThe two species with melanic plumage, C. atratus and C. melanocoryphus (black and black-necked swans respectively), both have amino acid changes at important functional sites in MC1R that are consistent with increased MC1R activity and melanism. Reconstruction of MC1R evolution over a newly generated independent molecular phylogeny of Cygnus and related genera shows that these putative melanizing mutations were independently derived in the two melanic lineages. However, interpretation is complicated by the fact that one of the outgroup genera, Coscoroba, also has a putative melanizing mutation at MC1R that has arisen independently but has nearly pure white plumage. Epistasis at other loci seems the most likely explanation for this discrepancy. Unexpectedly, the phylogeny shows that the genus Cygnus may not be monophyletic, with C. melanocoryphus placed as a sister group to true geese (Anser), but further data will be needed to confirm this.ConclusionOur study highlights the difficulty of extrapolating from intraspecific studies to understand the genetic basis of interspecific adaptive phenotypic evolution, even with a gene whose structure-function relationships are as well understood as MC1R as confounding variation make clear genotype/phenotype associations difficult at the macroevolutionary scale. However, the identification of substitutions in the black and black-necked swan that are known to be associated with melanic phenotypes, suggests Cygnus may be another example where there appears to be convergent evolution at MC1R. This study therefore provides a novel example where previously described intraspecific genotype/phenotype associations occur at the macroevolutionary level.

The Genetic Basis of the Plumage Polymorphism in Red-Footed Boobies (Sula sula): a Melanocortin-1 Receptor (MC1R) Analysis

The red-footed booby (Sula sula) is considered one of the most polymorphic seabirds, with 3 recognized major adult plumage types: 1) white, 2) white-tailed brown, and 3) brown and several degrees of intermediates. Here we show that the white/melanic polymorphism observed in this species is perfectly associated with 2 point substitutions, Val85Met and His207Arg, at the melanocortin-1 receptor (MC1R) gene. Among the melanic plumage variants, we also found a strong association between the degree of melanism and the number of copies of variant MC1R alleles. Furthermore, the Val85Met point substitution has been previously shown to be associated with melanic phenotypes in the lesser snow goose (Anser c. caerulescens), suggesting parallel evolution of the melanic allele, and hence, melanism, between these 2 distantly related species. We also compared the MC1R locus in red-footed boobies with a nonpolymorphic congener, the Nazca booby (Sula granti), in which all adults are white. We found that Nazca boobies present the same genotype at sites 85 and 207 as white morph red-footed boobies.

A NarrowHeliconius cydno(Nymphalidae; Heliconiini) Hybrid Zone With Differences in Morph Sex Ratios1

ABSTRACTIn the western cordillera of the Cauca valley, Colombia, there is a narrow hybrid zone between two races ofHeliconius cydno, one of which is polymorphic. Three large broods show that most of the phenotypic variation observed can be explained by four loci of major effect, namedSb,Yb,L,andK. Two further loci,GandWo, were identified that control more minor phenotypic variation.Sb, Yb,andWoare linked and the latter is differentially expressed between the sexes. The transition betweenH. c. cydnidesin the north andH. c. weymerito the south occurs over approximately 15 km. Collections from a single site near the center of the hybrid zone show that gene frequencies have been stable over the 10 yr from 1991 to 2001 and that color‐pattern genes are in Hardy–Weinberg equilibrium, with little evidence for linkage disequilibrium. This suggests that mating is random between color‐pattern forms. Nonetheless, there is evidence for variation in the sex ratio, with parental phenotypes and the most melanic hybrid phenotypes showing a fairly even sex ratio compared to a strong male bias observed in the remaining hybrids. We hypothesize that this might be explained by differential selection between sexes and phenotypes, phenomena that could have important implications for hybrid zone analysis and the evolution of Müllerian mimicry.

Local adaptation in the rock pocket mouse (Chaetodipus intermedius): natural selection and phylogenetic history of populations

Elucidating the causes of population divergence is a central goal of evolutionary biology. Rock pocket mice, Chaeotdipus intermedius, are an ideal system in which to study intraspecific phenotypic divergence because of the extensive color variation observed within this species. Here, we investigate whether phenotypic variation in color is correlated with local environmental conditions or with phylogenetic history. First, we quantified variation in pelage color (n=107 mice) and habitat color (n=51 rocks) using a spectrophotometer, and showed that there was a correlation between pelage color and habitat color across 14 sampled populations (R2=0.43). Analyses of mtDNA sequences from these same individuals revealed strong population structure in this species across its range, where most variation (63%) was partitioned between five geographic regions. Using Mantel tests, we show that there is no correlation between color variation and mtDNA phylogeny, suggesting that pelage coloration has evolved rapidly. At a finer geographical scale, high levels of gene flow between neighboring melanic and light populations suggest the selection acting on color must be quite strong to maintain habitat-specific phenotypic distributions. Finally, we raise the possibility that, in some cases, migration between populations of pocket mice inhabiting different lava flows may be responsible for similar melanic phenotypes in different populations. Together, the results suggest that color variation can evolve very rapidly over small geographic scales and that gene flow can both hinder and promote local adaptation.

Allelic Melanism in American and British Peppered Moths

Parallel evolutionary changes in the incidence of melanism are well documented in widely geographically separated subspecies of the peppered moth (Biston betularia). The British melanic phenotype (f. carbonaria) and the American melanic phenotype (f. swettaria) are indistinguishable in appearance, and previous genetic analysis has established that both are inherited as autosomal dominants. This report demonstrates through hybridizations of the subspecies and Mendelian testcrosses of melanic progeny that carbonaria and swettaria are phenotypes produced by alleles (isoalleles) at a single locus. The possibility of close linkage at two loci remains, but the simpler one-locus model cannot be rejected in the absence of contrary evidence.

Genetic analysis in a mainland melanic lizard (Podarcis sicula) population from Cilento and Vallo di Diano national park (Campania, Italy)

The results are presented of a microsatellite study performed on melanic and normal specimens of Podarcis sicula found sympatric in a very small area in the Cilento mountain region (Roscigno, southern Italy). The melanic sub‐population showed low variability and departure from HW equilibrium in the considered loci (three out of five). We also provide evidence that genetic exchanges between normal and melanic phenotypes are minimal. This genetic evidence is discussed and an eco‐biogeographic based hypothesis is advanced.

Polymorphism of Philaenus spumarius L. (Hemiptera: Cercopidae) in the Samsun (Mid-Black Sea Region) populations of Turkey

Variation in the frequency of colour/pattern phenotypes of Philaenus spumarius (L.) is studied in the three local populations of Samsun (Mid-Black Sea Region), Turkey. The data, obtained from the 91 samples including 10,427 adult spittlebugs, 5,340 females and 5,087 males were analysed according to phenotype distributions in the study sites. The adult spittlebugs were collected during eleven months except with April. The mean sample size was 114.6±19.9 (Mean±S.E.) for 91 samplings. Eight different phenotypes of P spumarius were detected in the Samsun populations. Of these, four were non-melanic and four were melanic phenotypes with the lack of the “O” group. In females, non-melanic frequency was over 95%, which TYP (typicus) was the predominant phenotype, but POP (populi) in males. However the phenotype frequency distributions were not different between the sites studied. Although, frequency of females reached up to 85% in some samples, the overall sex ratio did not differ from the 1:1.

The rise and fall of the Carbonaria form of the peppered moth.

The evidence for change in frequency of the melanic carbonaria morph in the peppered moth Biston betularia (L.) (Lepidoptera: Geometridae) in England and Wales is reviewed. At mid-20th century a steep cline of melanic phenotype frequency running from the north of Wales to the southern coast of England separated a region of 5% or less to west from 90% or more to northeast. By the 1980s the plateau of 90% frequency had contracted to northern England. The frequency has since continued to drop so that the maximum is now less than 50% and in most places below 10%. There have been similar declines in Europe and North America. Evidence from surveys and from two-point records shows the change to require 5% to 20% selection against the melanic. The melanic is more disadvantageous in regions where its frequency was initially high than in regions where it was low. Experiments to investigate predation by birds show a net advantage to carbonaria morphs in regions where typical frequencies were low at the time of the experiment, and a disadvantage where typical frequencies were high. This would be expected if environment and frequency were associated, and selective predation played a part in generating the association. The cryptic advantage of carbonaria was large in areas of heavy pollution where typical frequencies were 20% or less. The moth usually has a low density but is relatively highly mobile. The ability of present information to explain the patterns has been tested in simulations. They indicate a system under strong selection that has always been in a dynamic state without equilibria.

Melanic body colour and aggressive mating behaviour are correlated traits in male mosquitofish (Gambusia holbrooki).

Correlated traits are important from an evolutionary perspective as natural selection acting on one trait may indirectly affect other traits. Further, the response to selection can be constrained or hastened as a result of correlations. Because mating behaviour and body colour can dramatically affect fitness, a correlation between them can have important fitness ramifications. In this work, melanic (black) male mosquitofishes (Gambusia holbrooki) with temperature-sensitive body-colour expression are bred in captivity. Half of the sons of each melanic sire are reared at 19 degrees C (and express a black body colour) and half are reared at 31 degrees C (and express a silver body colour). The two colour morphs are placed in the same social setting and monitored for behavioural differences. Mating behaviour and colour are correlated traits. Mating behaviour differs markedly between the two phenotypes, despite high genetic relatedness. Melanic (black) phenotypes are more aggressive towards females, chasing them and attempting more matings than their silver siblings. Females avoid melanic-male mating attempts more than silver-male mating attempts. When males with temperature-sensitive colour expression are melanic and aggressive, they probably experience a very different selective regime in nature from when they are silver and less aggressive. Under some conditions (e.g. predation), melanic coloration and/or aggression is advantageous compared with silver coloration and/or less aggressive behaviour. However, under different conditions (e.g. high-frequency melanism), melanism and/or aggression appears to be disadvantageous and melanic males have reduced survival and reproduction. Selective advantages to each morph under different conditions may enable the long-term persistence of this temperature-sensitive genotype.

Recent history of melanism in American peppered moths.

Industrial melanism in peppered moths has been studied most intensively in Britain. The first melanic phenotype (effectively solid black) was recorded near Manchester in 1848. By 1895 about 98% of the specimens near Manchester were melanic, and this once rare phenotype had spread across regions of the country blackened by industrial soot. In rural, unpolluted regions, well away from industrial centers, the pale phenotype (peppered with white and black scales) remained the predominant form. During the latter half of the 20th century, following legislation designed to improve air quality, melanics began to decline in frequency and are now rare where once they had been common. Similar evolutionary changes have occurred elsewhere, but records from outside Britain are fragmentary. We have extended previous surveys of American peppered moth populations and present a composite picture of the recent decline in melanism in northern industrial states-Michigan and Pennsylvania-where melanic phenotypes decreased from more than 90% in 1959 to 6% by 2001. We contrast these changes to the near absence of melanism in a southern state-Virginia-during that same period. As in Britain, the decline in melanism in American peppered moths followed clean air legislation.

Geographic and temporal variation in the incidence of melanism in peppered moth populations in America and Britain

Industrial melanism in the peppered moth (Biston betularia is the classic example of observable evolution by natural selection. The phenomenon has been most thoroughly documented in Britain, where once rare melanic phenotypes increased in frequency in concert with increased levels of atmospheric pollution generated by industrial development. Populations of the American subspecies of the peppered moth (B. betularia cognataria) also experienced increases in melanisms, which are especially well documented in southern Michigan. Following a succession of governmental regulations designed to improve air quality, melanic phenotypes have been on the decline in moth populations in the United Kingdom and the United States. In both countries, the declines in melanism appear to be correlated primarily with reductions in atmospheric sulfur dioxide. Detailed studies of the evolutionary parallels have focused mainly on two locations: Caldy Common (near Liverpool) in England, and the George Reserve (near Detroit) in the United States. We have expanded the geographic range of American samples to include northeastern Pennsylvania and southwestern Virginia, and we have conducted a national survey in Britain to produce a current melanic frequency distribution map. The results clearly show that the decline in melanism is widespread in both countries, and the changes are generally consistent with decreases in levels of atmospheric SO2.

Lead concentration in the bones of the feral pigeons (Columba livia): sources of variation relating to body condition and death.

This paper reports on lead concentrations in the tarsometatarsi of 84 individuals of adult feral pigeons Columba livia found dead or experimentally captured in Bratislava, Slovakia. The interrelationships between lead concentrations and time of death, place of death, body measurements, sex, condition, and plumage phenotype were investigated. Size and shape of pigeons was not correlated with bone lead contamination. Sex and plumage color and pattern were slightly associated with variation in bone lead levels, females and melanic (urban) phenotypes tending to have higher bone lead concentrations than males and blue-bar (wild) individuals. Birds with antibodies to chlamydiae did not contain significantly higher lead concentrations in the tarsometatarsi than birds without antibodies. Concentration of lead in tarsi was significantly higher in birds dying in winter, compared to birds dying at the end of summer. Chronic lead poisoning probably causes mortality in pigeons in winter. The natural stressor, cold weather, has the capability of exacerbating the effects of lead poisoning, and the mortality is due to lead exposure coupled with cold stress.

Polymorphism, mating preferences and sexual selection in the Arctic skua

AbstractThe intensive study of the Arctic skua (Stercorarius parasiticus) population on Fair Isle has provided an example of the maintenance of a stable polymorphism by sexual selection in conjunction with assortative mating (O'Donald, 1983). We tested several of O'Donald's hypotheses using data collected from the large colony of Arctic skuas on Foula. There was some evidence for assortative mating of the phenotypes but this did not confer any short‐term fitness advantage. In contrast to the situation on Fair Isle, we found little indication that sexual selection took place for dark males since no differences in timing of breeding or productivity were evident between the two male phenotypes in new partnerships. Nor could we detect variation in adult body size, body condition, or any life‐history characteristic between melanic and pale birds of either sex. Many of the selection pressures documented on Fair Isle appear to be absent at the neighbouring Foula, despite a fair degree of gene flow between these colonies. The frequency of pale adults on Foula and in Shetland as a whole has declined since the 1970s, indicating that currently directional selection may be favouring the melanic phenotypes at the southern end of the breeding range.

Parallel Rise and Fall of Melanic Peppered Moths in America and Britain

The moth (Biston betularia) is well known for the rapid rise in the frequency of alleles producing melanic phenotypes correlated with a general blackening of the environment following the nineteenth-century industrial revolution. In recent years the frequency of melanics has been dropping steadily in Britain in apparent response to improved air quality. Some regional American populations of this same species also experienced significant increases in melanics, by 1959 exceeding 90% in southeastern Michigan, but Michigan populations were not reexamined for over 30 years. In 1994 and 1995 we trapped moths in southeastern Michigan and here report that a parallel decline in melanism has occurred in American moths. Furthermore, we document that changes in Michigan's air quality as measured by atmospheric sulfur dioxide (SO2) and suspended particulates also parallel the changes recorded in Britain. The traditional interpretation is that pale phenotypes of moths at rest by day on lichen-encrus ted trees are camouflaged from bird predators; industrial fallout kills lichens and darkens resting surfaces, favoring darker colored moths. However, the changes in allele frequencies in the moth populations we sampled have occurred in the absence of perceptible changes in the local lichen floras. We suggest that the role of lichens has been inappropriately emphasized in chronicles about the evolution of melanism in moths. Industrial melanism in the Lepidoptera ranks among the best examples of observable evolution by natural selection. The most thoroughly documented case is that of the geometrid moth Biston betularia, with the fundamental details of its recent evolutionary history described in virtually every biology textbook. Its common name, peppered moth, describes the appearance of the so-called form or phenotype which is covered with white scales peppered with black over the wings and body. Several other pigmentation phenotypes are known. A melanic form, named carbonaria, is nearly solid black. Intermediates between the pale and fully melanic phenotypes are called insularia. The range in variation from lightest to darkest is continuous, but the phenotypic differences result from multiple alleles at a single locus which exhibit an approximate dominance hierarchy: the carbonaria allele shows complete dominance over the insularia and typical alleles, and

N- β-Alanyldopamine metabolism for puparial tanning in wild-type and mutant niger strains of the mediterranean fruit fly, Ceratitis capitata

N- β-Alanyldopamine (NBAD) metabolism was investigated in wild-type and melanic mutant niger strains of the Mediterranean fruit fly, Ceratitis capitata, during the process of pupariation. When radioactive β-alanine (BALA) was injected at the onset of pupariation, the wild-type puparium incorporated 70% of the label, whereas less than 5% was detected in the niger puparium. BALA was catabolized and eliminated as carbon dioxide substantially more in the mutant than the wild-type strain. Significant differences between the two strains were found in the composition of catecholamines extracted from fully sclerotized puparial cuticle. NBAD levels in niger were five times lower than those in the wild type, whereas dopamine (DA) levels were 20 times higher. The concentration of NBAD in hemolymph also was greatly reduced in niger, whereas DA and N-acetyldopamine levels remained high during pupariation. The synthesis of NBAD was assayed both in vivo and in vitro, revealing that the conversion of BALA into NBAD was several fold higher in the wild-type than in the niger strain. Because NBAD hydrolase activity was found to be similar in both strains, an impairment in NBAD synthesis appears to be responsible for the melanic phenotype. We also found that NBAD synthesis in the wild type was restricted to a narrow developmental window, which overlaps the time period in which puparial tanning occurs. Because we were able to induce niger-wild-type cuticle mosaics in heterozygous embryos, NBAD synthetase appears to be expressed by the epidermis. We conclude that the niger mutant is defective in NBAD synthesis and that this defect prevents normal pigmentation.

The colour/pattern polymorphism of Philaenus spumarius (L.) (Homoptera: Cercopidae) in England and Wales

The spittlebug Philaenus spumarius is a common and widespread Palaearctic insect exhibiting a striking dorsal colour/pattern polymorphism, which is conveniently divisible into eight ‘melanic’ and five ‘non-melanic’ phenotypes. The polymorphism is controlled by seven alleles at a single autosomal locus with complete or partial female-limitation and variable penetrance in males of certain melanic phenotypes. It is a universal polymorphism but, despite much investigation, little is known of the factors which maintain it or influence morph frequency. Patterns of geographic variation are useful in both these contexts. This study presents an extensive survey of morph frequency variation in 548 populations of P. spumarius across England and Wales. Frequencies of individual phenotypes within the melanic category vary heterogeneously; frequency variation for a ‘core’ group of five phenotypes is broadly in parallel, whereas the other three vary independently. Furthermore, the degree of penetrance in males varies between the melanic phenotypes, which in turn varies between populations; one consequence is that higher melanic frequencies are associated with enhanced expression of certain melanic phenotypes in males. Comparisons are made with previously reported populations around a point source of serious gaseous and particulate pollution in the Cynon Valley, South Wales and with intensively studied populations in southern Finland. The data indicate a pronounced ‘urban effect’ on morph frequencies, with populations in major conurbations exhibiting higher mean melanic frequencies ( x̄ = 19.1%) than rural ones ( x̄ = 7.3%). Populations around three (of the eight investigated) point sources of atmospheric pollution show elevated melanic frequencies, although all are substantially below those recorded from the Cynon Valley Although Finnish populations exhibit substantially higher melanic frequencies than analogous rural populations in Britain the strict female-limitation of certain melanic phenotypes in Finland is relaxed in British populations, where male melanics are approximately 20-fold more frequent. The universality and stability of this polymorphism strongly suggest the overriding influence of natural selection in its maintenance. The predominance of the phenotypes GIB and LCE (mostly black, but with pale heads) over the completely black form (LOP) in populations with the highest overall melanic frequencies demonstrates that the selective agent(s) favouring melanics in intensely polluted areas cannot be operating simply for melanism per se . The near-equality of melanic frequencies between the sexes in the majority of populations reported here suggests that the model of genetic control established for populations in the Cynon Valley also applies to most British populations and remains distinct from the model applying to Finnish populations.

Non-random mating in the two-spot ladybird (Adalia bipunctata): I. A reassessment of the evidence

There is contradictory evidence concerning non-random mating among melanic and non-melanic phenotypes of Adalia bipunctata. Although most studies have found a mating advantage in favour of melanic individuals, one found random mating. Furthermore, in some samples the melanic mating advantage was frequency-dependent though not in others. In one population males alone gained a frequency-dependent mating advantage, and it has been argued that this is a result of a female sexual preference. There is no direct evidence for mate choice from any other population. In any case, melanic individuals of both sexes often gain a mating advantage so if mate choice is relevant, then these populations must contain “choosy” males as well as “choosy” females. Some of these apparent contradictions are explained by insufficient sampling and/or unsatisfactory statistical analysis. Nevertheless, populations are clearly different from one another and this is important when considering the nature of the melanic mating advantage.

Genetic control of colour/pattern polymorphism in British populations of the spittlebug Philaenus spumarius (L.) (Homoptera: Aphrophoridae)

The results of breeding experiments designed to determine the genetic control of the colour/pattern polymorphism in Philaenus spumarius in certain British populations are described and contrasted to previously published results for material taken from populations in southern Finland. The 11 principal phenotypes are controlled by seven alleles at a single autosomal locus, with complex patterns of dominance and co-dominance. In Finland, and most of the rest of the species' extensive range, the eight melanic morphs are wholly or substantially limited to the females, in which they are dominant to the typical form. Previously, this sex limitation has been explained in terms of reduced penetrance of melanic alleles in males and/or dominance reversal between the sexes, such that the eight melanic phenotypes (controlled by five alleles which are adjacent in the dominance heirarchy) are dominant to typical in the females but recessive in the males. Published data on morph frequencies in S Finland and on breeding experiments using material collected from these populations are re-examined and it is shown that neither theory accounts adequately for all the evidence. By contrast, in most British populations there is greatly increased penetrance in males of the melanic morphs which are strictly female-limited elsewhere. Breeding experiments, using P. spumarius taken from selected British populations, provide clear evidence that melanics are dominant to typical in both sexes. This difference in genetic control is entirely consistent with the balance of morph frequencies between the sexes in the populations from which the breeding material for the two separate studies was taken. Experimental material for this study was taken from populations in the Cynon Valley in south Wales, which have the highest melanic frequencies in the species' range and show particularly clear expression of melanic phenotypes in males. Both of these features are strongly correlated with severe aerial pollution from a local smokeless fuel factory. Selection for melanism in these populations is clearly strong, which probably accounts for the high level of penetrance and the dominance of melanics to typical in both sexes. Although further experiments are needed, it is suggested that regulatory loci, controlling the penetrance and expression of melanic alleles in males, could account for the observed differences in genetic control between populations in Britain and Finland.

Localized industrial melanism in the spittlebug Philaenus spumarius (L.) (Homoptera: Aphrophoridae) in Cardiff docks, south Wales

Twenty-four large adult samples of the meadow spittlebug, Philaenus spumarius, were collected from within a 4 times 3 km area containing Cardiff docks, south Wales. All samples are highly polymorphic for colour and pattern and show marked geographic variation in the combined frequency of the eight melanic morphs (from 75%). Maximum melanic phenotype frequencies in both sexes coincide with dock wharfs currently containing large accumulations of pulverized coal and these samples are heavily contaminated with small coal particles. However, interpretation of this association is complicated by the existence, until 1976, of a smokeless fuel factory causing local air pollution immediately adjacent to the area of highest melanic frequency. It is suggested that the association of high levels of melanism in P. spumarius with areas heavily contaminated with particulate pollutants is due to direct selective effects of small particles favouring melanies.