A steep cline in ladybird melanism has decayed over 25 years: a genetic response to climate change?

A cline in the frequency of melanic morphs of the two-spot ladybird, Adalia bipunctata, was first surveyed in 1980 along a transect extending inland from the coast in the Netherlands. At that time, the frequency of melanics increased over some 40 km from 10% near the coast to nearly 60% inland. Additional surveys made in 1991 and 1995 demonstrated some progressive change in cline shape. New samples from 1998 and 2004 confirm these dynamics, and show that over a period of about 50 generations for the beetle, the cline had decayed rapidly to yield rather uniform frequencies of melanic morphs at around 20% along the whole transect by 2004. Climate data and evidence for thermal melanism in this species support our contention that these dynamics reflect a dramatic example of a rapid genetic response within populations to climate change and local selection.     

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.     

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.     

Relative fitness of near isogenic lines for melanic and typical forms of the oriental armyworm, Mythimna separata (Walker)

Melanism in the oriental armyworm Mythimna separata (Walker) is characterized by a black color in adults instead of the normal body color of off-white. Because inheritance of melanism in this moth follows Mendel's law, with the melanic allele (t) recessive to the normal allele (T) at a single autosomal locus, a backcrossing procedure was used to develop near isogenic lines (NILs) for melanic (NILs-tt) and typical forms (NILs-TT) of this moth. Melanic males (tt) and typical females (TT) were crossed in the laboratory, their progeny (F(1)) were interbred, and F(2) melanic males were backcrossed to typical female offspring of the parallel line. This cycle was repeated five times to create NILs for melanic and typical forms of this moth. The effects of the melanic allele on relative fitness were evaluated in terms of developmental and reproductive characteristics. Life tables of the NILs were constructed to determine net reproductive rate (R(0)). Results indicated that NILs-tt possessed apparent developmental and reproductive advantages, including shorter immature development time, higher larval and pupal survival, a shorter adult preoviposition period, and more eggs laid per female compared with the NILs-TT.     

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.     

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.     

Melanic moth frequencies in Yorkshire, an old English industrial hot spot

A survey has been carried out in Leeds, England, in the west Yorkshire industrial heartland, and in neighboring York, surrounded by agriculture, of melanic frequency in the moth species Biston betularia, Odontoptera bidentata, and Apamea crenata. All show a decline in melanics in the postindustrial environment, the first over almost the full range from nearly 100% to less that 10%, the others to smaller extents. Changes in several species over as great a magnitude and as wide an area must result from selection. The results are compared with others along a transect through northern England. The onset of response is progressively later from west to east. The rate of decline is lower at the extremes of the transect to west and east than it is in the center. We still do not have a clear picture of the causes of the changes. One major factor is likely to be selective predation, which is shown to be critically dependent on predation rate. As a consequence, differences in settling behavior between the species could account for different responses even if the species are attacked by the same predators.     

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.     

Little and large: body size and genetic clines in a New Zealand gecko (Woodworthia maculata) along a coastal transect

Clinal variation can result from primary differentiation or secondary contact and determining which of these two processes is responsible for the existence of a cline is not a trivial problem. Samples from a coastal transect of New Zealand geckos (Woodworthia maculata) identified for the first time a body size cline 7-10 km wide. The larger geckos are almost twice the mass of the small adult geckos. Clines in allele and haplotype frequency were found at two of the four genetic loci examined. Estimated width of the morphological cline was concordant with neither the narrower mtDNA cline (3-7 m) nor the wider nuclear cline (RAG-2; 34-42 km), and cline centers were not coincident. Although the body size cline is narrow compared to the entire range of the species, it is 2-3 orders of magnitude greater than estimates of dispersal distance per generation for these geckos. No evidence of assortative mating, nor of hybrid disadvantage was identified, thus there is little evidence to infer that endogenous selection is maintaining a hybrid zone. We cannot distinguish secondary contact from primary origin of this body size cline but conclude that secondary contact is likely due to the occurrence of mtDNA haplotypes from three distinct clades within the coastal transect and the presence of two frequency clines within this region.     

Mitochondrial DNA in the hybrid zone between Mus musculus musculus and Mus musculus domesticus: a comparison of two transects

We studied mtDNA introgression across the contact zone between Mus musculus musculus and M. m. domesticus in two independent transects in the Czech Republic and Bavaria, Germany. A total of 1270 mice from 98 localities in the Czech transect and 456 mice from 41 localities in the Bavarian transect were examined for presence or absence of a Bam HI restriction site in the mt-Nd1 gene. Using this simple mtDNA marker, variants that belonged to the M. m. domesticus lineage (presence of restriction site) could be unequivocally distinguished from those belonging to the M. m. musculus lineage (absence of restriction site). The extent of introgression of mtDNA, three autosomal allozymes and the X chromosome was compared. The introgression of X markers was more limited than was that of the allozymes and mtDNA. In the Czech transect, the centre for the mtDNA cline was shifted about 3.6 km to the west relative to the X chromosome cline, with asymmetric introgression from M. m. musculus to M. m. domesticus . Interestingly, in the Bavarian transect, the centre of the mtDNA cline was shifted about 10.9 km to the east relative to the X chromosome cline, with asymmetric introgression from M. m. domesticus to M. m. musculus, opposite in direction to that observed in the Czech transect.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 84 , 363–378.     

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.     

The peppered moth and industrial melanism: evolution of a natural selection case study

From the outset multiple causes have been suggested for changes in melanic gene frequency in the peppered moth Biston betularia and other industrial melanic moths. These have included higher intrinsic fitness of melanic forms and selective predation for camouflage. The possible existence and origin of heterozygote advantage has been debated. From the 1950s, as a result of experimental evidence, selective predation became the favoured explanation and is undoubtedly the major factor driving the frequency change. However, modelling and monitoring of declining melanic frequencies since the 1970s indicate either that migration rates are much higher than existing direct estimates suggested or else, or in addition, non-visual selection has a role. Recent molecular work on genetics has revealed that the melanic (carbonaria) allele had a single origin in Britain, and that the locus is orthologous to a major wing patterning locus in Heliconius butterflies. New methods of analysis should supply further information on the melanic system and on migration that will complete our understanding of this important example of rapid evolution.     

Cellular Basis and Developmental Timing in a Size Cline of Drosophila Melanogaster

We examined 20 Drosophila melanogaster populations collected from a 2600-km north-south transect in Australia. In laboratory culture at constant temperature and standard larval density, a genetic cline in thorax length and wing area was found, with both traits increasing with latitude. The cline in wing area was based on clines in both cell size and cell number, but was primarily determined by changes in cell number. Body size and larval development time were not associated among populations. We discuss our results in the context of selection processes operating in natural and experimental populations.     

Opposite Latitudinal Clines for First Mating and Second Mating (Remating) in Males of Drosophila melanogaster

Although female remating has been studied extensively in insects, few studies have been carried out for male remating (second mating). In this study, we analyzed Drosophila melanogaster males for their remating potential, using iso‐female line culture initiated with wild flies collected from eight Indian geographic localities. We examined the association of latitude and percent melanization with first and second male mating (including mating‐related traits). Our results indicated that second male mating has a negative latitudinal cline opposite to that of first mating. Body melanization is negatively correlated with second mating by male and positively with first mating (measured in terms of percent mated pairs). Mating latency during first (ML1) and second (ML2) male mating has a negative latitudinal cline, but slope values differ significantly as ML2 is great at higher latitudes as compared to ML1. The difference between ML1 and ML2 is non‐significant at lower latitudes. However, copulation period of second mating (CP2) has a negative latitudinal cline, whereas copulation period of first mating (CP1) has positive latitudinal cline. Next, the latency and copulation period differ significantly between first and second male mating treatments in within‐population analyses as well as in melanic strains. Furthermore, male remating ability (number of maximum successful remating attempts continuously by a male in 12 h) also follows negative latitudinal cline. The lower latitudinal lighter males have more remating ability as compared to darker males from higher latitudes.     

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.     

The evolution of extreme shell shape variation in the land snail Ainohelix editha: a phylogeny and hybrid zone analysis

Ainohelix editha from Hokkaido, Japan, exhibit great geographical variation in their shell morphology. In particular, A. editha in two quite separate locations, Shimamaki and Samani, are striking because they are extremely flat and have a sharp keel, whereas at adjacent sites the shells are globular or depressed-globular. We used mitochondrial 16S rRNA and nuclear ITS-2 sequences to infer a phylogeny among 47 snails from 29 locations. Snails from the two keeled-flat populations clustered separately in the phylogeny, suggesting that this unusual shell form could have evolved independently. A morphological analysis of shells collected along a transect between keeled-flat and globular snail sites showed a cline for shell shape and the angle of the keel. Two different mtDNA lineages were found across the transect, with a cline for an ITS-2 single nucleotide polymorphism. Together, the results may suggest a lack of reproductive isolation between keeled-flat and globular snails, with possible introgression by hybridization.     

Effects of polymorphic melanism and larval diet on life history traits of Malacosoma disstria moths

In this study we investigated the presence and possible genetic basis of polymorphic melanism in the forest tent caterpillar (Malacosoma disstria) moth. Adult moths were classified into pattern-based phenotypes and wing darkness was measured to quantify the degree of melanization. We found that two distinct phenotypes, melanic and simple, are present in these moths. Although the full melanic phenotype is sex-limited to males, it is partially expressed in females. We also provide support for the theory that the melanic allele is autosomal and dominant. The effects of larval diet quality on the survival, development and wing melanization of each phenotype were studied by rearing larvae on the foliage of either a primary or secondary host. Diet quality did not differentially affect the two phenotypes; however, melanic males were found to be smaller than simple males regardless of larval diet. Such inherent developmental differences between the two phenotypes could have important consequences for the frequencies of the two morphs.     

Clinal variation in morphology and assortative mating in the soldier beetle, Chauliognathus pennsylvanicus (Goleoptera: Cantharidae)

In north Georgia populations of the soldier beetle, Chauliognathus pennsylvanicus , the length of the elytral spot varies clinally. At the southern end of a 200 km cline the distribution of spot length is unimodal with longer spot lengths predominating while at the northern end of the cline the distribution is bimodal but with shorter spot lengths being more frequent. North of the cline only short elytral spot lengths are observed, while the converse is true south of the cline. The strength of assortative mating on the basis of elytral spot length increases from south to north along the cline, resulting in complete pre-mating isolation between short and long spot length morphs at the north end of the cline. Laboratory mate choice tests indicate that assortative mating in the field is not the result of differential timing of activity or microhabitat choice but rather that it represents a real behavioural preference. Individuals from monomorphic populations on either side of the cline do not mate assortatively in the laboratory, indicating that reproductive isolation has evolved on the cline.     

Changes in pools of depsidones and melanins, and their function, during growth and acclimation under contrasting natural light in the lichen Lobaria pulmonaria

This study analysed relationships between secondary chemistry, lichen growth rates and external habitat factors for two groups of UV-B-absorbing secondary compounds in the lichen Lobaria pulmonaria in order to test some hypotheses on their formation and function. Medullary depsidones and cortical melanins were quantified in thalli transplanted to three successional forest stands (shaded young forest, open old forest, sun-exposed clear-cut area) and subjected to different watering regimes (spraying with water, water + nitrogen, no spraying). Growth rates were already known. The total concentration of all seven depsidones was constant across the entire range of growth rates and sun exposures, showing that these depsidones serve functions other than photoprotection. Thalli from the well-lit transplantation sites had the highest synthesis of melanins. Within each forest type there was a trade-off between growth and melanin synthesis. Melanins and photosynthetic acclimation enhanced survival on a subsequent exposure to high light intensity, despite excessive temperatures resulting from higher absorption of solar energy in melanic thalli relative to pale thalli. In conclusion, the highly responsive melanic pigments play a photoprotective role in light acclimation, whereas the constant amount of depsidones across a wide spectrum of growth ranges and irradiances is consistent with herbivore defence functions.     

Chromosomal and allelic variation in Drosophila americana: selective maintenance of a chromosomal cline.

Geographically structured genetic variation, as represented by clines and hybrid zones, offers unique opportunities to study adaptation and speciation in natural populations. A hybrid zone has been reported between Drosophila americana americana and Drosophila americana texana, two taxa that are distinguished solely by the arrangement of their X and 4th chromosomes. In this study, samples of D. americana were collected along a latitudinal transect across the inferred hybrid zone, and the frequency of the alternative chromosomal arrangements is reported. These data illustrate that the alternative chromosomal arrangements are distributed along a shallow cline over a broad geographic region, and that the frequency of the arrangements is tightly correlated with latitude. Allelic variants at 13 RFLP loci in three genes on chromosome 4 exhibit no evidence of association with the cline. Presence of a cline for the chromosomal arrangements, as well as a general absence of geographic structure for variation at these genes, is interpreted as evidence that natural selection is responsible for the maintenance of this chromosomal cline. Furthermore, these results demonstrate that taxonomic subdivision of D. americana is unwarranted, because it exists as a cohesive species that is segregating a chromosomal fusion.