Melanic Morphs Research Articles

Theoretical models of melanism in Biston betularia-a review

The various theoretical models that have been constructed to explain the spatial and temporal changes in the frequency of the melanic morph of the moth Biston betularia are reviewed. The assumptions that are made in these models are discussed. It is shown that these models do explain the gross features of the spatial and temporal distribution of melanic variety in England and Wales. The models have been fairly successful in quantitative prediction of the observed decline in the frequencies of the melanic forms by relating selective differentials to the sulphur dioxide levels. Thus these models do yield a reasonable and consistent picture of the gross pattern of changes in melanic frequencies within the limitation of the available data. The models are robust and can thus accommodate changes to the data on the biology and behaviour of the moth. It is shown that neither heterozygote advantage nor non-Darwinian mechanism need to be invoked to understand the observed evolution of melanism in B. betularia.

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.

Background selection by the peppered moth (Biston betularia Linn.): individual differences

The hypothesis that dimorphically coloured, cryptic moths select appropriate rest sites by comparing their body scales to substrate reflectance was tested using typical and melanic morphs of the peppered moth, Biston betularia (L.). Experiments designed to block the individual's inspection of its inherited colour phenotype do not support Kettlewell's contrast/conflict (self-inspection) hypothesis. Instead, tracking of marked moths over successive days revealed individual differences in rest-site selection which were not related to treatments, experience (imprinting), nor closely to a moth's inherited colour pattern. Differences between family broods indicate that some genetic bias in background selection exists. The production of artificially selected lines with consistent but opposing preferences will allow us to investigate the co-evolution of pattern and behaviour.

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.

Postindustrial Melanism in the Peppered Moth

New data show the geographical pattern of frequency of the melanic morph carbonaria of the peppered moth, Biston betularia, in 1983-84. These frequencies are compared with data from 1952 to 1970. After 20 years of smoke control, the area of high melanic frequency has contracted to the northeast. The change indicates a disadvantage to carbonaria of about 12 percent compared with 20 years ago. Computer simulations, which do not include the assumption of heterozygote advantage, provide a good match to the surface for the period 1952 to 1970, and also the 1983-84 surface. Experiments on visual predation have been criticized as giving unrepresentative estimates of selection but they permit satisfactory simulations to be made.

Nymphal colour/pattern polymorphism in the leafhoppers Eupteryx urticae (F.) and E. cyclops Matsumura (Hemiptera: Auchenorrhyncha): spatial and temporal variation in morph frequencies

Variation in colour/pattern morph frequencies in Eupteryx urticae and E. cyclops is described for various field populations. Eupteryx urticae populations in S Wales exhibit a steep morph-ratio cline, such that black morph frequencies are positively correlated with altitude. High melanic frequencies at high-altitude sites, and the absence of the two darker morphs in lowland populations, suggest a similar trend in E. cyclops, but the data are insufficient to confirm this statistically. No differences in morph frequencies were detected on different parts of the primary host plant or on alternative host species. Similarly, there were no consistent trends within or between the two annual generations of either species, although melanic morph frequencies in one E. urticae population were heterogeneous over 10 generations. It is suggested that the polymorphism in E. urticae is strongly influenced by climate selection, darker morphs being at an advantage in cooler environments where their coloration enhances absorption of solar radiation. The advantage gained through thermal melanism is probably balanced by visual selection against black morphs by entomophagous parasitoids.

Differential winter mortality and seasonal selection in the polymorphic ladybird Adalia bipunctata (L) in the Netherlands

Seasonal selection acting on the melanic polymorphism in the two-spot ladybird Adalia bipunctata was investigated in The Netherlands. An increase in melanic frequency over the spring-summer reproductive period was quantified. The selective advantage gained by melanics averaged 9%, but significant heterogeneity occurred between populations. Adult hibernation behaviour is described. The beetles when outdoors show a highly clumped distribution both between and within trees. The distribution of the morph classes between aggregations is random. Survivorship in a hibernating cohort (initial n= 1898) on a grid of 70 lime trees near Utrecht was monitored by making three counts over the winter of 1981–1982. Intense selection favouring each melanic morph occurred during December and January. The relative fitness of non-melanics was 0.55 (melanics =1). The discovery of dead beetles in late January (about 5% of total losses) and the absence of spatially density-dependent mortality were consistent with a climatic stress rather than selective predation. The period of selection was associated with very cold temperatures averaging up to 4°C below normal and an overall mortality of nearly 75%. There was no change in morph frequency, near normal temperatures and a lower mortality from February to early April. Examination of groups of nearby trees in late January strongly suggested that similar differential mortality had occurred except on some willows. This difference was probably due to the more protected hibernation sites available on these trees. Samples of hibernating cohorts at three other sites showed no evidence of differential mortality. Laboratory experiments with hibernating beetles found no difference in survivorship or rate of weight loss between starved non-melanics and melanics in temperature regimes with and without periods of adult activity. It is concluded that the intense winter selection on the study limes is probably exceptional. Examination of changes in morph frequency through the annual cycle suggests that at some sites the selection favouring melanics during reproduction is counterbalanced by selection against melanics in late summer or early autumn. The results are discussed in relation to mathematical models of cyclical selection and to other field studies including that of Timoféeff-Ressovsky (1940), who found large decreases in melanic frequency during hibernation in Berlin.

The basis of thermal melanism in the ladybird Adalia bipunctata: Differences in reflectance and thermal properties between the morphs

The thermal properties of the red non-melanic and black melanic morphs of Adalia bipunctata were examined in the laboratory. Non-melanics have a higher cuticular reflectance than melanics. Under illumination similar to bright sunlight the temperature excess over ambient air (T. ex) gained by melanics is, on average, 2·1 °C larger than that of non-melanics. Initial rates of temperature change during heating and cooling are nearly 50 per cent faster for melanics. T. ex increases with weight in each morph class and is, on average, larger for females as these are larger. The larger melanics may have a higher risk of overheating than other Adalia in conditions of prolonged sunshine and very high ambient air temperatures. The results are discussed in relation to consequences of thermal melanism in natural populations and to the contribution of colour and size to insect heat budgets.

An Electrophoretic and Morphological Examination of Bothriembyron kendricki (Pulmonata: Bulimulidae), a New Species previously Considered Conspecific with B. bulla (Menke)

Bothriembryon bulla (Menke) has previously been thought to include pale and melanic morphs. Studies of morphology and allozymes reveal that the two forms are reproductively isolated. The melanic is herein referred to as B. kendricki, sp. nov. Both species show large amounts of variation among populations for both shell morphology and allozymes, emphasizing the need for comprehensive geographic sampling in studies of the genus Bothriembryon.

A theoretical study of morph ratio clines with special reference to melanism in moths

A migration–selection model with frequency-dependent predation is described that can explain all the features of morph ratio clines. The model is applied to the clines of the melanic morphs of the moth species Biston betularia and Phigalia pilosaria measured between Liverpool and northeast Wales. It is found that strong non-visual selection and weak frequency-dependent predation are needed to explain both these clines. The selection parameters for non-visual selection in both these species are obtained from the data.

Non-random mating in wild populations of polymorphic Adalia bipunctata

The nature of the factors maintaining the colour polymorphism in Adalia bipunctata has been the subject of much discussion and in some populations a tendency for the melanic morphs to mate more frequently than the non-melanics has been reported. The morph frequencies among mating pairs in wild populations of A. bipunctata in England have been scored and analysed in order to discover whether mating between morphs is a random process. Data from the literature are also analysed in the same way and the methods of analysis adopted by earlier workers are discussed. It is concluded that mating between morphs is frequency dependent so that it is the rare morph in the population, regardless of colour, which mates more frequently. This appears to be a further method by which the polymorphism in this species is maintained.

Thermal biology of the common garter snake Thamnophis sirtalis (L.) : II. The effects of melanism.

The thermoregulatory significance of a striped-melanic colour polymorphism in the common garter snake, Thamnophis sirtalis, was assessed through a combination of labortory experimentation and field study. In experiments with living snakes the melanic morph maintained a higher body temperature than the striped morph, when exposed to natural insolation. Experiments with excised skin showed that this thermal advantage is attributable to some integumental difference between the two morphs. Body temperatures of snakes in the field revealed that, during the colder part of the active season, melanics were able to stay warmer than striped snakes by an amount (1.24 C°) approximating the difference observed in the laboratory. Some evidence and argument is presented to suggest that melanism also may confer protection against overheating in warm periods.

Industrial melanism in Mesopsocus unipunctatus (Müll.) (Psocoptera) in northern England

Abstract. 1. The light and melanic morphs of M.unipunctatus are described, their genetic inheritance considered, and their geographic distribution over a pollution gradient in Yorkshire is examined.2. There is an obvious colour dimorphism. The light forms are disruptively patterned while the melanic forms are uniformly dark.3. Genetic crosses indicate that the melanic character is inherited as a Mendelian dominant and that separate alleles, linked on the same chromosome, control the respective melanisms of the head‐thorax and of the abdomen.4. In Yorkshire, there is a decrease in the frequency of melanics from the industrial south to the rural north.

Selection against the melanic morphs of Adalia bipunctata (two-spot ladybird): A review and some new data

Selection against the melanic morphs of Adalia bipunctata (two-spot ladybird): A review and some new data

Melanism and Selective Predation in three Species of Moths

In large-scale mark/release/recapture experiments with the polymorphic moth Biston betularia (L.), Kettlewell (1955a, 1956) convincingly demonstrated that birds were taking a disproportionate number of the form which was most conspicuous on the predominant backgrounds at different sites. Differential elimination of the morphs of B. betularia also occurred in experiments in which dead, frozen moths were exposed to predation on trees (Clarke & Sheppard 1966; Bishop 1972; Lees & Creed 1975). Bishop (1972) and Lees & Creed (1975) both found situations in which the melanic form, carbonaria, was present at a high frequency, even though it was at a cryptic disadvantage, relative to the typical form. This is consistent with Ford's (1937, 1975) suggestions that melanic polymorphism in moth species which rely on crypsis depends upon the balance between the selective advantages of both the non-visual and visual attributes controlled by the melanic genes. The results of Bishop (1972) and of Lees & Creed (1975) emphasize the need to estimate selective predation at a number of sites in order to examine the relative importance of non-visual and visual components of selection for different species. Lees, Creed & Duckett (1973) and Bishop et al. (1975) compared melanic frequencies with epiphyte cover (in addition to other variables) at a number of sites. Bishop et al. (1975) have emphasized the difficulties in deducing, from such analyses, whether non-visual or visual selective factors are more likely to be influencing melanic frequency. This can only be done by making more direct estimates of the selective predation of the morphs at different sites. Although predation experiments and mark/release experiments have been carried out with B. betularia at a number of sites, the time and resources required limits the number of sites that can be investigated during the flight period of the moth. However, Kettlewell (1955a, 1956) showed that the conspicuousness of the melanic and typical morphs of B. betularia to humans and to natural predators (birds) were related. Further confirmation of this was obtained by Lees & Creed (1975). The field experiments, described in this paper, were carried out to compare the consistency of different methods of estimating selective predation of the morphs. The work was undertaken in connection with surveys of the relative crypsis of the melanic and typical morphs of three species in southern Britain.

Melanism in Adalia bipunctata L. (Col., Coccinellidae) and its Relationship to Atmospheric Pollution

The use of lichens as biological indicators of air pollution is well established (Hawksworth & Rose 1970; Gilbert 1970, 1974). In the British Isles the melanic form of the peppered moth (Biston betularia L.) also seems to be a good indicator of air pollution and the same may be true of a number of other species of Lepidoptera (Kettlewell 1973). The relationship between melanism in B. betularia and air pollution does not appear to be a simple one as this melanism is related to the amount of lichen cover on tree trunks. Thus the proportion of the melanic form of this moth in the population cannot be used as an independent measure of air pollution. The discovery of other organisms that are influenced by air pollution but not by the condition of tree trunks would be invaluable. The two-spot ladybird (Adalia bipunctata L.) in common with many other Coccinellids (Hodek 1973) is a highly polymorphic species. The various morphs are under the control of a number of alleles at a single locus, with the black morphs being dominant to the red morphs (Lus 1928). In Britain these morphs can be conveniently classified into two groups, the typicals with red elytra and two black spots and the melanics with either four (quadrimaculata) or six (sexpustulata) red spots on black elytra. Intermediate forms, which are usually heterozygous, also occur, but are uncommon. Frazer & Rothschild (1960) have shown that A. bipunctata is distasteful, has a distinctive scent and is avoided by vertebrate predators. Both shiny black and red appear to be warning colours (Kettlewell 1973), so there can be little doubt that all the morphs are warningly coloured. Birds such as rooks and tits take occasional specimens of ladybirds, but the redstart (Phoenicurus phoenicurus L.) appears to be the only bird which has been recorded as regularly eating them (Buxton 1950). In Great Britain the Redstart is a summer visitor and it is doubtful whether it is abundant enough to be an important predator. Thus it would appear that selective predation of the morphs that blend less well with their surroundings, as has been shown for Biston betularia, would not operate on Adalia bipunctata and an alternative explanation for the existence of this polymorphism must be sought. Creed (1966, 1971a) postulated that melanism in A. bipunctata is directly related to air pollution and has produced evidence to link melanic frequency with smoke levels. He suggested that some unidentified component of the polluted atmosphere was less toxic to the melanic morphs than to the typicals (Creed 1971 a) and has proposed that the species may be used as an indicator of atmospheric smoke levels (Creed 1971b, 1974). We have challenged this view (Benham, Lonsdale & Muggleton 1974) and have produced evidence

Resting site selection in the geometrid moth Phigalia pilosaria (Lepidoptera: Geometridae)

When tested in a cylindrical apparatus, males of both typical and melanic morphs of the nocturnal moth Phigalia pilosaria (Schiffermueller) prefer to rest on light backgrounds. This preference is more pronounced when the moths have a rough surface on which to rest than when it is smooth. The two morphs show a distinct tendency to adopt resting positions close to the boundary between the shades presented in the experiments; this is interpreted as being a result of walking moths stopping at or close to the boundary when approaching it from the light side. The surface texture of the background influences the resting attitude of the moths in relation to vertical.

Polymorphism in the Two-Spot Ladybird Adalia bipunctata in Western Norway

Overall frequency of melanic morphs of the polymorphic Adalia bipunctata L. was 83 % in a coastal locality and 4 % in a locality inland. Monthly samples from the coastal locality showed no seasonal change in frequency and, since the area is relatively free from air pollution, industrial melanism is not involved. It is suggested that the coastal climate is responsible for the high frequency of melanic morphs.

Is polymorphism in two-spot ladybird an example of non-industrial melanism?

THE polymorphism exhibited by the two-spot ladybird, Adalia bipunctata Linn., in which specimens have a red ground colour and black spots (the typical morphs) or a black ground colour with red spots (the melanic morphs) is a well known phenomenon. The reasons for the existence of this polymorphism, however, are not well understood. Analogy to the situation in some melanic Lepidoptera, such as Biston betularia Linn.1,2, is unacceptable as it would seem that selective predation by birds upon the morphs which blend less well with their surroundings would not operate upon A. bipunctata. Ladybirds are distasteful, have a distinctive scent, are warningly coloured and therefore are avoided by vertebrate predators3,4.