Tut-tut Tutt, not so fast. Did Kettlewell really test Tutt's explanation of industrial melanism?

Over the past fifty years a legend has developed surrounding H.B.D. Kettlewell's pioneering research on the phenomenon of industrial melanism as illustrated in the peppered moth, Biston betularia. This legend is particularly pernicious in that it unfairly diminishes Kettlewell's intellectual involvement in the work for which he is most famous (Rudge 2006). One of the most persistent allegations made is that Kettlewell's published work neglects or ignores his debt to J.W. Tutt, identified as the first to propose the explanation of industrial melanism at issue in Kettlewell's original investigations (Owen 1997). The following essay first examines whether J.W. Tutt was indeed the first to explain the phenomenon of industrial melanism in terms of differential bird predation and the long term effects of air pollution, before considering further whether J.W. Tutt's explanation was, in point of fact, the explanation at issue in Kettlewell's investigations.     

Taking the Peppered Moth with a Grain of Salt

H. B. D. Kettlewell's (1955, 1956) classic field experiments on industrial melanism in polluted and unpolluted settings using the peppered moth, Biston betularia, are routinely cited as establishing that the melanic (dark) form of the moth rose in frequency downwind of industrial centers because of the cryptic advantage dark coloration provides against visual predators in soot-darkened environments. This paper critiques three common myths surrounding these investigations: (1) that Kettlewell used a model that identified crypsis as the only selective force responsible for the spread of the melanic gene, (2) that Kettlewell's field experiments alone established that selection for crypsis was the most important factor in the spread of melanic forms, and (3) that Kettlewell's investigations in an unpolluted wood near Dorset constituted a control for his earlier Birmingham studies (contra Hagen 1993, 1996). This analysis further identifies two features that distinguish manipulative experiments in evolutionary biology from experiments in other contexts. First, experiments in evolutionary biology rest on a wealth of information provided by strictly observational ecological studies; in the absence of such information experiments in evolutionary biology make no sense. Second, there is a trade-off between how much control investigators have over the conditions being studied and how informative the results of the experiment will be with regard to natural populations.     

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.     

Variation in the evolutionary integration of melanism with behavioral and physiological traits in <Emphasis Type="Italic">Xiphophorus variatus</Emphasis>

Vertebrate pigmentation is emerging as a powerful system for studying the evolution of adaptive traits and the maintenance of genetic and phenotypic variation in natural populations. Though melanism has been linked to physiological and behavioral traits in a variety of taxa, the generality of these associations for many taxa such as fishes remains unclear. Here I tested whether variation in melanism in a livebearing fish was correlated with a variety of traits often tested in other taxa: locomotor stress coping style during confinement, boldness in a novel environment, and metabolic rate. There were significant negative associations between an individual’s amount of melanistic pigmentation and both activity in confinement and boldness in a novel environment. In contrast with evidence from many prior studies, there was no relationship between melanism and metabolic rate. Overall, the data provide some support for documented relationships between melanism and behavioral traits, but did not support the generally reported relationship between melanism and metabolic rate. Links between melanism and behavioral coping strategies related to environmental stressors may have important implications for the evolution and maintenance of behavioral and morphological variation in natural populations. Nonetheless, these results also suggest variation among taxa in the extent to which pleiotropy has evolved between melanism and diverse organismal traits.     

Changing views on melanic moths

The rapid rise in frequency of melanic morphs in several moth species, especially the peppered moth Biston betukria , in industrial regions during the 19th century, and the subsequent rapid decline, indicate the action of strong selection. There has recently been a tendency to criticise and question all aspects of research on industrial melanism, including the experiments which suggest that selective predation plays an important part in the changes. These experiments are reexamined, together with evidence for changes in appearance of tree surfaces and for relation of initial melanic frequency to subsequent rate of decline. It is suggested that intense pollution may have been required to drive the carbonaria morph to a high frequency, with frequency patterns over a mosaic environment smoothed by migration. Improvements in these extreme locations then triggered the decline, with litde indication of the environmental changes in areas of moderate pollution. Reasons for criticism of past work are discussed. Industrial melanism continues to provide an exceptional opportunity to analyse a pattern of selection and change in gene frequency.     

昆虫黑化现象

昆虫黑化是自然界中普遍存在的一种多型现象,也成为揭示物种在自然界中进化与适应的经典范例。昆虫黑化形成的原因与类别多种多样,黑化的遗传调控和分子基础也各不相同。本文根据国外黑化昆虫研究取得的主要进展,结合我国东方粘虫Mythimna separata种群中发生的黑化现象,对昆虫黑化的形成原因、类别、遗传调控、生物学变异以及黑化的分子基础等方面研究现状进行了概述,并对该领域的未来研究热点进行探讨。     

Immune defence,a possible nonvisual selective factor behind the industrial melanism of moths (Lepidoptera)

It is generally believed that industrial melanism in Lepidoptera is mainly caused by differential predation by birds. In polluted areas, melanic individuals are favoured by natural selection because they are better camouflaged than pale moths on lichen‐free and sooty tree trunks. In this article, we show that, in the black arches moth (Lymantria monacha), melanic morphs have a stronger encapsulation response than pale morphs against nylon monofilament implants. This indicates that the melanic and pale morphs differ in the strength of their immune defence. The same chemical precursors and their end product, melanin pigment, are involved in the encapsulation response and in the external coloration. Thus, it seems that there may be two possible, not mutually exclusive, explanations for the frequency changes observed in the industrial melanism of moths. The dominant gene causes an increase in the amount of melanin pigment and its precursors. This increase causes two changes: an intensified immune defence as a form of improved encapsulation ability of foreign objects, and the well‐known protective dark coloration (a case of relational pleiotropy). It seems possible that industrial melanism is a by‐product of selection on the strength of immunity. In the field, these pleiotropic aspects are exceedingly difficult to distinguish from each other, and the factors may even be compensatory. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 99 , 831–838.     

Genetic basis of stage-specific melanism: a putative role for a cysteine sulfinic acid decarboxylase in insect pigmentation

Melanism, the overall darkening of the body, is a widespread form of animal adaptation to particular environments, and includes bookcase examples of evolution by natural selection, such as industrial melanism in the peppered moth. The major components of the melanin biosynthesis pathway have been characterized in model insects, but little is known about the genetic basis of life-stage specific melanism such as cases described in some lepidopteran species. Here, we investigate two melanic mutations of Bicyclus anynana butterflies, called Chocolate and melanine, that exclusively affect pigmentation of the larval and adult stages, respectively. Our analysis of Mendelian segregation patterns reveals that the larval and adult melanic phenotypes are due to alleles at different, independently segregating loci. Our linkage mapping analysis excludes the pigmentation candidate gene black as the melanine locus, and implicates a gene encoding a putative pyridoxal phosphate-dependant cysteine sulfinic acid decarboxylase as the Chocolate locus. We show variation in coding sequence and in expression levels for this candidate larval melanism locus. This is the first study that suggests a biological function for this gene in insects. Our findings open up exciting opportunities to study the role of this locus in the evolution of adaptive variation in pigmentation, and the uncoupling of regulation of pigment biosynthesis across developmental stages with different ecologies and pressures on body coloration.     

How the Leopard Hides Its Spots: ASIP Mutations and Melanism in Wild Cats

The occurrence of melanism (darkening of the background coloration) is documented in 13 felid species, in some cases reaching high frequencies at the population level. Recent analyses have indicated that it arose multiple times in the Felidae, with three different species exhibiting unique mutations associated with this trait. The causative mutations in the remaining species have so far not been identified, precluding a broader assessment of the evolutionary dynamics of melanism in the Felidae. Among these, the leopard (Panthera pardus) is a particularly important target for research, given the iconic status of the ‘black panther’ and the extremely high frequency of melanism observed in some Asian populations. Another felid species from the same region, the Asian golden cat (Pardofelis temminckii), also exhibits frequent records of melanism in some areas. We have sequenced the coding region of the Agouti Signaling Protein (ASIP) gene in multiple leopard and Asian golden cat individuals, and identified distinct mutations strongly associated with melanism in each of them. The single nucleotide polymorphism (SNP) detected among the P. pardus individuals was caused by a nonsense mutation predicted to completely ablate ASIP function. A different SNP was identified in P. temminckii, causing a predicted amino acid change that should also induce loss of function. Our results reveal two additional cases of species-specific mutations implicated in melanism in the Felidae, and indicate that ASIP mutations may play an important role in naturally-occurring coloration polymorphism.     

The adaptive significance of alpine melanism in the butterfly Parnassius phoebus F. (Lepidoptera: Papilionidae)

Summary The adaptive significance of alpine melanism, the tendancy for insects to become darker with increased elevation and latitude, was investigated using the butterfly Parnassius phoebus. The effects on temperature dependent activity of five components of overall wing melanism, as well as size, were examined. The components of wing melanism examined were the transparency of the basal hindwing and distal fore-wing areas, the width of the black patch in the basal hind-wing area and the proportion of black to white scales in that area, and the proportion of the distal fore-wing covered by predominantly black scaling.The body temperature of dead specimens was correlated with air temperature, solar radiation, the width of the black patch at the base of the wings, and the proportion of black to white scales at the base of the wings. The minimum air temperatures and solar radiation levels required for initiation of flight did not vary with wing melanism of P. phoebus, in contrast to the results found for Colias butterflies by Roland (1982). However, under environmental conditions suitable for flight initiation, males with a higher proportion of black to white scales in the basal area of the hind-wing, and wider basal black patches, spent a greater proportion of time in flight at low air temperatures and low insolation. Increased basal wing melanism was also associated with increased movement of males within a population. In contrast, melanism in the distal area of the wings has no effect on activities which are dependant on body temperature. The amount of time spent feeding did not vary with differences in wing melanism. I suggest that in dorsal basking, slow-flying butterflies (Parnassius) basal wing color affects body temperature primarily during flight (rather than while basking), such that butterflies with darker wing bases cool down less rapidly because they absorb more solar radiation during flight.     

Industrial Melanism in the Peppered Moth, Biston betularia: An Excellent Teaching Example of Darwinian Evolution in Action

The case of industrial melanism in the peppered moth has been used as a teaching example of Darwinian natural selection in action for half a century. However, over the last decade, this case has come under attack from those who oppose Darwinian evolution. Here, the main elements of the case are outlined and the reasons that the peppered moth case became the most cited example of Darwinian evolution in action are described. Four categories of criticism of the case are then evaluated. Criticisms of experimental work in the 1950s that centered on lack of knowledge of the behavior and ecology of the moth, poor experimental procedure, or artificiality in experiments have been addressed in subsequent work. Some criticisms of the work are shown to be the result of lack of understanding of evolutionary genetics and ecological entomology on the part of the critics. Accusations of data fudging and scientific fraud in the case are found to be vacuous. The conclusion from this analysis of criticisms of the case is that industrial melanism in the peppered moth is still one of the clearest and most easily understood examples of Darwinian evolution in action and that it should be taught as such in biology classes.     

Melanism in guinea fowl (Numida meleagris) is associated with a deletion of Phenylalanine‐256 in the MC1R gene

We have characterized a deletion in the MC1R gene causing the loss of one amino acid (p.Phe256del), which is perfectly associated with melanism in guinea fowl (Numida meleagris). Co‐segregation of the p.Phe256del with melanism was confirmed in 25 offspring born from a cross of two heterozygote birds; therefore we suggest that this mutation is responsible for the black phenotype. Interestingly, this is the first case of recessive melanism linked to MC1R.     

Melanism in a larval Lepidoptera: repeatability and heritability of a dynamic trait

Abstract.  1. Although it is well established that the deposition of melanin pigment in the cuticle of larval Lepidoptera is influenced by both environmental and genetic factors, few studies have examined intra-individual regional variation in the degree of melanism or the ontogenetic dynamics of this trait. Here, heritable and density-dependent effects on within-individual and stage-specific variation in melanism were examined in caterpillars of the Egyptian cotton leafworm, Spodoptera littoralis (Boisduval).
2. Using quantitative spectrometric methods, it is shown that cuticular melanism changes dramatically within larval stadia, showing the highest and lowest levels of melanism early (first day) and late (final day) in each larval stadium respectively. However, solitary-reared caterpillars were significantly paler than those reared gregariously at all stages of development and maintained greater levels of variation in melanism. This variation in melanism was repeatable and exhibited a significant heritable component (narrow sense heritability based on offspring–parent regressions: h ² = 0.18–0.30).
3. The degree of melanism was correlated negatively with larval body weight in solitary caterpillars, but not gregarious ones. Melanism also varied spatially, with the lateral longitudinal band being consistently darker than the dorsal or dorso-lateral bands. Crowd-rearing increased melanism in all regions of larval cuticle, but the extent of crowding-induced melanism was more pronounced in the dorsal and dorso-lateral bands than in the lateral one.
4. These results indicate that although cuticular melanism is a highly dynamic trait, ontogenetic changes in relative cuticular melanism are both predictable and repeatable within individuals and genotypes. This has implications for our understanding of the evolution of melanism and for applying artificial selection on the basis of colour.     

Selective bird predation on the peppered moth: the last experiment of Michael Majerus

Colour variation in the peppered moth Biston betularia was long accepted to be under strong natural selection. Melanics were believed to be fitter than pale morphs because of lower predation at daytime resting sites on dark, sooty bark. Melanics became common during the industrial revolution, but since 1970 there has been a rapid reversal, assumed to have been caused by predators selecting against melanics resting on today's less sooty bark. Recently, these classical explanations of melanism were attacked, and there has been general scepticism about birds as selective agents. Experiments and observations were accordingly carried out by Michael Majerus to address perceived weaknesses of earlier work. Unfortunately, he did not live to publish the results, which are analysed and presented here by the authors. Majerus released 4864 moths in his six-year experiment, the largest ever attempted for any similar study. There was strong differential bird predation against melanic peppered moths. Daily selection against melanics (s ≈ 0.1) was sufficient in magnitude and direction to explain the recent rapid decline of melanism in post-industrial Britain. These data provide the most direct evidence yet to implicate camouflage and bird predation as the overriding explanation for the rise and fall of melanism in moths.     

Melanin-based coloration is a nondirectionally selected sex-specific signal of offspring development in the alpine swift

Two mutually exclusive hypotheses have been put forward to explain the evolution and adaptive function of melanin-based color traits. According to sexual selection theory melanism is a directionally selected signal of individual quality, whereas theory on the maintenance of genetic polymorphism proposes that alternative melanin-based variants achieve equal fitness. Alpine swift (Apus melba) males and females have a conspicuous patch of white feathers on the breast with their rachis varying continuously from white to black, and hence the breast varies from white to striated. If this trait is a sexually selected signal of quality, its expression should be condition dependent and the degree of melanism directionally selected. If variation in melanism is a polymorphism, its expression should be genetically determined and fitness of melanin-based variants equal. We experimentally tested these predictions by exchanging eggs or hatchlings between randomly chosen nests and by estimating survival and reproduction in relation to melanism. We found that breast melanism is heritable and that the environment and body condition do not significantly influence its expression. Between 5 and 50 days of age nestlings were heavier and their wings longer when breast feathers of their biological father were blacker, and they also fledged at a younger age. This shows that aspects of offspring quality covary positively with the degree of melanism. However, this did not result in directional selection because nestling survival and recruitment in the local breeding population were not associated with father breast melanism. Furthermore, adult survival, age at first reproduction and probability of skipping reproduction did not covary with the degree of melanism. Genetic variation in breast melanism is therefore maintained either because nonmelanic males achieve fitness similar to melanic males via a different route than producing fast-growing offspring, or because the advantage of producing fast-growing offspring is not sufficiently pronounced to result in directional selection.     

Insect melanism: the molecules matter

Insect melanism, especially in the peppered moth Biston betularia, has long been a textbook case of evolution in action. Hypotheses of the role of natural selection in maintaining melanic polymorphisms have implicated a wide range of explanations in various species, but to understand fully the ecology of melanism, we need to understand its molecular and developmental genetic basis. Because developmental genes often affect more than one trait, identifying the genes responsible for melanism is crucial for a thorough understanding of the fitnesses and selective responses of melanic alleles in nature. Molecular genetic information is also vital for elucidating the evolutionary history and possible mechanistic diversity of melanism among species. Recent studies of the developmental genetics of melanin pigmentation in Drosophila, and of the genetics of pigmentation differences among other insect species, have provided valuable insights into the underpinnings of this important source of polymorphism throughout the Insecta.     

Why cordylid lizards are black at the south-western tip of Africa

There is a high incidence of melanism among the cordylid lizards of Africa and it was suggested that melanism has a thermoregulatory function in these species. One prediction of the thermal melanism hypothesis would be that melanistic cordylid species would be restricted to cool environments. The aim of our study was to determine the climatic conditions with which melanistic cordylids are currently associated. Distributional data for the eight melanistic cordylid taxa occurring at the south-western tip of Africa were obtained from the CapeNature State of Biodiversity database. Using GIS and principal component analyses, the climatic variables best describing the geographical distribution of melanistic cordylids were investigated. We found that several melanistic cordylid populations show a distinct association with a high incidence of fog and cloud cover, underscoring the thermal melanism hypothesis. Several other populations, however, do not show such an association. We conclude that some species, such as Pseudocordylus capensis , have a morphology that provides them with great flexibility in habitat use, allowing them to overcome the constraints of melanism in warm environments. The available information suggests that melanism in cordylids evolved during the Miocene in response to the development of the cold Benguela Current along the west coast of Africa. We also conclude that viviparity, a sit-and-wait foraging strategy and a rock-dwelling lifestyle would have preadapted cordylids for survival in cool conditions brought about by the development of the cold Benguela Current. These conditions would have been unfavourable for oviparous, active-foraging lizards. The co-occurrence of melanistic lizards and melanistic molluscs at the Landdroskop locality casts some doubt on improved heating rates as the only explanation for melanism in cordylids, because molluscs are not heliothermic baskers.     

对家蚕第18连锁群隐性基因elp、ch-2和mln测交系的分子定位分析

家蚕长形卵(elp)、第二隐性赤蚁(ch-2)、暗化型(mln)均为第18染色体上的隐性突变, 在经典连锁图谱上的顺序和遗传距离已经排定。文章采用正常卵、正常黑蚁及正常白蛾品种P50与包含此3个隐性突变的三隐性测交系W18组配正反交群体, F1回交W18后获得回交群体(P50×W18)♀×W18♂ 和W18♀×(P50×W18)♂, 分别记作BC1F和BC1M, 利用已构建的家蚕SSR分子连锁图谱和根据家蚕基因组精细图设计的STS标记, 对这3个突变基因elp、ch-2、mln进行了分子定位研究, 并根据家蚕基因组精细图, 将第18连锁群的经典遗传图、分子连锁图和基因组物理图进行了对应。整合后的图谱遗传距离为94.2 cM, 突变基因和分子标记的排列顺序分别与形态标记连锁图和基因组精细图相一致, 研究结果对家蚕第18 染色体上其他突变的定位与克隆有重要的借鉴作用。     

A proposed selective mechanism based on metal chelation in industrial melanic moths

Industrial melanism, a phenomenon observed in some moths and especially in the case of the peppered moth (Biston betularia), has received much attention as an example of Darwinian evolution in action. The rapid rise in the proportion of the darker melanic form of the adult moth coincided with the advent of atmospheric pollution resulting from industrialization, and was ascribed to the improved camouflage of the melanotic insects against a background blackened by soot, which conferred a selective advantage in the avoidance of predation by birds. The topic of the increase in melanization during the initial period of industrial expansion and the reversal of the process after the introduction of the Clean Air Act has received much attention. Although there is sound experimental evidence to support selective avian predation as a major mechanism to account for the changes in the relative frequency of melanics, it is not clear that this is the only selective factor involved in industrial melanism. It is possible that other processes may have made a contribution to the preponderance of melanic variants. In the present study, the hypothesis is advanced that melanization may have conferred a selective advantage by protecting the insects from the toxic effects of metals by virtue of the strong metal chelating action of melanin. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 109 , 298–301.     

Selection for cuticular melanism reveals immune function and life-history trade-offs in Spodoptera littoralis

Several insect species show an increase in cuticular melanism in response to high densities. In some species, there is evidence that this melanism is correlated with an up-regulation of certain immune system components, particularly phenoloxidase (PO) activity, and with the down-regulation of lysozyme activity, suggesting a trade-off between the two traits. As melanism has a genetic component, we selected both melanic and nonmelanic lines of the phase-polyphenic lepidopteran, Spodoptera littoralis, in order to test for a causative genetic link between melanism, PO activity and lysozyme activity, and to establish if there are any life-history costs associated with the melanic response. We found that, in fact, melanic lines had lower PO activity and higher lysozyme activity than nonmelanic lines, confirming a genetic trade-off between the two immune responses, but also indicating a genetic trade-off between melanism and PO activity. In addition, we found that lines with high PO activity had slower development rates suggesting that investment in PO, rather than in melanism, is costly.