Non‐morph specific predation of peppered moths (Biston betularia) by bats

Abstract 1. Industrial melanism in the peppered moth, Biston betularia, is one of the foremost examples of natural selection in action. 2. Differential bird predation was suggested as the main agent for the evolution of melanism in the peppered moth by Tutt in the 1890s, with empirical support being published by Kettlewell in the 1950s. 3. Some recent critiques that have attempted to undermine Kettlewell’s work have lacked objectivity, and have been answered previously. 4. One criticism that has not previously been addressed is that of the role of bat predation in the case. 5. The difficulty of using non‐visual differential predation by bats to explain the increase and decrease in melanism in the peppered moth, correlated as it is to pollution levels, is outlined. 6. Predation experiments, in which moths of the typica and carbonaria forms of the peppered moth were released and observed at night, were used to determine whether bats differentially predate these forms. 7. Results of experiments at three sites showed no significant differences in the level of bat predation of the two forms of peppered moth.     

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.     

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.     

Behavioural studies on the peppered moth Biston betularia and a discussion of the role of pollution and lichens in industrial melanism

We examined the resting behaviour during pairing and egg-laying of bred female Biston betularia (L.) after they were set free at wooded sites in Somerset and Cardiff. Moths were not released immediately on emergence but were held for three nights to overcome an initial period of flight activity. Females (N= 257) were placed on trunks, and on large and small branches of trees, especially oaks, over periods of several weeks in 1984, 1985 and 1986. Releases were not made within 50 m of any known survivor. One hundred and thirty-one pairings were observed. We also examined oviposition behaviour in a cage experiment using different types of branches. Our observations support Mikkola's earlier conclusion from cage experiments with male moths that the species rests predominantly on branches and shows an appropriately specialized resting attitude, demonstrated here in a series of photographs. Many moths will rest underneath, or on the side of, narrow branches in the canopy. Once released females had settled and paired they only moved quite short distances, but positioning and egg-laying were profoundly influenced by the presence of foliose lichens. Females of all phenotypes show a strong preference to oviposit beneath a thallus of foliose lichens. Their tendency to rest against or close to the lichen during the day could markedly influence their crypsis. In the absence of foliose lichens, eggs are laid in cracks in the bark. The survivorship of released females both in different pairing combinations and during their whole reproductive life-span is analysed. These preliminary data suggest that bird predation can give rise to differential mortality of different pairing combinations and of the phenotypes. This technique provides a powerful method of measuring the relative crypsis and differential mortality of phenotypes with living females in natural resting sites. The significance of our observations on resting behaviour is discussed with regard to the role of air pollution and epiphytes in industrial melanism. Particular attention is given to the causal mechanisms underlying recent declines in the frequency of carbonaria melanics in parts of Britain. One important factor is the new growth made by trees since the marked reduction in particulate air pollution in the 1960s. It is emphasized that we need to know much more about the interactions between pollution, epiphytes and resting backgrounds, especially in the canopy, before we can be confident of our understanding of the evolution of industrial melanism.     

Experiments on resting site selection by the typical and melanic forms of the moth, Allophyes oxyacanthae (Caradrinidae)

Six families of the cryptic moth, Allophyes oxyucanthae , four of which contained both the typical and melanic forms of this polymorphic species, were tested for evidence of resting site selection in a large box lined with oak bark of three reflectances. Moths from different families were tested separately. Typicals from all four families tended to show a preference for the background which most closely matched their reflectance and on which they were most cryptic. Melanic moths from different families showed different resting site selection behaviour; those from two families preferring dark bark, on which melanics were most cryptic, while those from other families did not. The significance of these results in relation to previous suggestions about the control of resting site selection is discussed     

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.     

Industrial melanism and peppered moths (Biston betularia (L.))

The spread of melanic forms of the peppered moth ( Biston betularia (L.)) over polluted areas of Britain from the mid-nineteenth century onwards, has become widely known and quoted as a classical example of microevolutionary change. Probably the most important factor in the spread (and subsequent decline, following the Clean Air Act) of the melanics has been bird predation on less cryptic individuals, but a range of other factors may also affect the maintenance of allele frequencies at any one place (site selection, dispersion, heterosis, frequency dependent selection, larval hardiness, etc). The development of the "Peppered Moth Story" is described, and suggestions made about needed research.     

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.     

Industrial melanism in the moths, Diurnea fagella (Oecophoridae) and Allophyes oxyacanthae (Caradrinidae)

Comparative studies of melanism in the two cryptic moth species, Diurnea fagella (Denis & Schiffermüller) and Allophyes oxyacanthae (L.), have been carried out in southern England and south Wales. Estimates of the relative crypsis of the melanic and typical forms of both these species have been made at a number of sites and these were compared with the melanic frequencies in samples from these sites. These comparisons showed that selective prédation could be a major factor in the variation of melanic frequencies of both of these species. A consideration of the spread of melanism in these species suggests that non-visual selection may favour the melanics of D. fagella in urban areas and that non-visual selection, not closely associated with urban conditions, may be responsible for the restriction of melanics in A. oxyacanthae to Britain. The results for these two species are discussed in relation to investigations of melanism in other moth species.     

Industrial melanism: Do we have the answers?

Entomologists from the late 19th century onwards recognized the evolutionary interest of the association of black forms of the peppered moth with industrialization. They developed a qualitative explanation of the phenomenon involving a change in relative crypsis of the phenotypes due to the blackening of the moth's resting background by air pollution. More recently, ecological geneticists have obtained some estimates of predation by birds and of population parameters such as migration rate. Models incorporating these estimates have explored the ways in which natural selection influences spatial variation and the maintenance of polymorphism. Studies on the peppered moth and some of the many other insects exhibiting industrial melanism have concentrated on the variability and dynamics of adult populations. Recent work which has begun to examine the ecology and behaviour of individuals, complete life cycles, and gene-phenotype relationships, is refining our understanding of this adaptation and also of present-day declines in melanic frequencies in response to falling air pollution.     

Experiments on resting site selection by nocturnal moths

When different species of moths are presented with a choice between black and white resting backgrounds, there is a strong correlation between the colour selected and the reflectance of the forewings. Under more natural conditions, light-coloured moths usually rest on fresh vegetation whilst dark-winged species select tree bark or rest upon the ground, and different defensive strategies appear to have been adopted by species in these two latter situations. Studies on the mechanism of background selection, and on background selection in polymorphic species, are reviewed.     

Further experiments on resting site selection by the typical and melanic forms of the moth, Allophyes oxyacanthae (Caradrinidae)

The melanic and typical morphs from 13 families of the moth, Allophyes oxyacanthae (Linnaeus) were tested for evidence of resting site selection in a large box lined with oak bark of three reflectances. For 12 of the families, one or both of the parents originated from two families, 0/1 and 0/3, which had been tested in a similar manner in the previous year. In all of these 12 families typicals showed a preference for the palest background available, "light grey". The melanic results for the same families were significantly heterogeneous but could be partitioned on the basis of parentage, into six families in which melanics preferred light grey, and six families in which melanics preferred the background on which they are most cryptic, dark grey. In the only family in which neither of the parents originated from either 0/1 or 0/3, both typical and melanics showed a preference for the background on which typicals are most cryptic, bare bark. The results of the two years' experiments indicate that it is necessary to postulate that, in addition to the pleiotropic effects of alleles at the melanic locus, another locus (or loci) is involved in the control of resting site selection.     

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.     

The understanding of industrial melanism in the peppered moth (Biston betularia) (Lepidoptera: Geometridae)

Melanic polymorphism in B. betularia has been extensively studied. Correlations between high melanic frequency and high levels of air pollution have been demonstrated. Kettlewell and others have shown that differential bird predation has an important effect on the maintenance of the polymorphism, and coefficients of visual selection have been obtained on the assumption that the moth habitually rests on tree trunks. Computer models based on these selective coefficients show that they are not sufficient accurately to explain observed melanic frequencies. Other non-visual selective factors and weak frequency-dependent selection have been invoked to improve fits. Analysis of the resting positions of moths recorded in the wild demonstrates that B. betularia does not usually rest in exposed positions on tree trunks, but rather rests on the underside of branches, on trunks in shaded positions just below major branch joints or on foliate twigs. The results of a pilot selection experiment, while agreeing qualitatively with Kettlewell's results, suggest that fitness estimates that assume trunk-resting are quantitively incorrect. The error is greatest for melanic moths in rural areas. It is suggested that visual selective coefficients based on a true assessment of the resting behaviour of the moths may considerably improve the fit between computer predictions and observed phenotype frequency distributions.     

A Study of the Biological Preferences of Secondary School Pupils in Israel

An interactive computer simulation was designed as an aid to those teaching evolutionary theory. The simulation was inspired by (tie example of industrial melanism in the peppered moth. A BBC microcomputer is used to present a variegated display overlaid with moth shapes, where the colour of some moths is more like that of the background display than others. The student adopts the role of predator, and uses a mouse-pointer to ‘kill’ as many moths as possible in 18 sec. Surviving moths 'breed' to produce the next generation. Since practical trials demonstrated that moths are killed as an inverse function of degree of crypsis, progressive evolution occurs through the generations. The program also stores moth colour frequency data and, at a point chosen by the user, plots histograms for each generation. Development problems, and the simulation's value as a teaching aid, are discussed.     

Cryptically Patterned Moths Perceive Bark Structure When Choosing Body Orientations That Match Wing Color Pattern to the Bark Pattern

Many moths have wing patterns that resemble bark of trees on which they rest. The wing patterns help moths to become camouflaged and to avoid predation because the moths are able to assume specific body orientations that produce a very good match between the pattern on the bark and the pattern on the wings. Furthermore, after landing on a bark moths are able to perceive stimuli that correlate with their crypticity and are able to re-position their bodies to new more cryptic locations and body orientations. However, the proximate mechanisms, i.e. how a moth finds an appropriate resting position and orientation, are poorly studied. Here, we used a geometrid moth Jankowskia fuscaria to examine i) whether a choice of resting orientation by moths depends on the properties of natural background, and ii) what sensory cues moths use. We studied moths’ behavior on natural (a tree log) and artificial backgrounds, each of which was designed to mimic one of the hypothetical cues that moths may perceive on a tree trunk (visual pattern, directional furrow structure, and curvature). We found that moths mainly used structural cues from the background when choosing their resting position and orientation. Our findings highlight the possibility that moths use information from one type of sensory modality (structure of furrows is probably detected through tactile channel) to achieve crypticity in another sensory modality (visual). This study extends our knowledge of how behavior, sensory systems and morphology of animals interact to produce crypsis.     

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.     

Evolution of resting behaviour in polymorphic 'industrial melanic' moth species

The behaviour of individually marked melanic and typical forms of Allophyes oxyacanthae was followed in four successive tests in an apparatus comprising bark of three different reflectances. In samples of wild populations, melanics showed a moderate preference for dark bark, while typicals did not seem to prefer or avoid this substrate. However, in four of the eight families of known parentage tested, both melanics and typicals preferred to rest on dark bark. These four families were the progeny of dark typicals which had shown a preference for dark bark and were the families containing the darkest typicals. In A. oxyacanthae the variation in resting behaviour is not therefore closely associated with the melanic allele but may be linked to a 'dark typical' allele which, together with other loci, produces polygenic darkening of the normal phenotype. The results with A. oxyacanthae are compared with those obtained with Biston betularia and other moth species.     

Resting orientation enhances prey survival on strongly structured background

Prey can use various camouflage types as defense against predators. One of the most common and important types is background matching, which occurs if an animal matches the background in color, brightness, and pattern. Although background matching has been studied intensively, the effects of the resting orientation of prey on the effectiveness of camouflage through background matching are not well known in natural conditions. Several past experimental studies have been conducted on resting orientation in the lab often using the visual system of humans. Their results revealed that the detection rates of predators hinge on the combination of the resting orientation of artificial moths and their background. Here, we studied whether survival rates of artificial moth-like models depend on their resting orientation in the wild where the visual conditions and detection distances vary. We used a 2 × 2 design of two resting positions of a horizontally and a vertically striped morph on tree bark. Our results show that the survival probability of moths depended mainly on the orientation of stripes relative to the vertical structure of tree bark. Thus, resting orientation relative to background affected survival. After reanalyzing Endler’s (Biol J Linn Soc 22:187–231, 1984) data on resting habitats of 317 species of North American moths, we found that horizontally striped moths occurred frequently on small herbs and tree bark. We suggest that it would be beneficial for striped moths to orient non-randomly on strongly structured background, like furrows of tree bark. We further suggest that background matching was more important than coincident disruptive coloration in determining the survival rates of our artificial moths.     

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.