A theoretical analysis of the morph frequency variation in the peppered moth over England and Wales

The migration-selection model for the spatial and temporal variation of morph frequencies of Biston betularia over England and Wales (Cook & Mani, 1980) has been extended to include effects due to non-visual selection. The parameters for non-visual selection were chosen from the recent determination by Mani (1980) and by Creed et al. (1980). The morph frequencies over England and Wales were obtained through computer simulation and the results were compared with data along the Manchester-Yorkshire, Central Wales-East Anglia and South Wales-London transects. Best fits to the data were obtained by using the non-visual selective values of Mani for carbonaria and modified values of Creed et al. for insularia. It is concluded that the observed polymorphism could be well explained through a balance of migration and visual and non-visual selections.     

A migration-selection model for the morph frequency variation in the peppered moth over England and Wales

A migration-selection model for the spatial and temporal variation of morph frequencies over England and Wales for the peppered moth has been constructed. The morph frequencies have been obtained by computer simulation using the model for various assumptions regarding selection and migration within the boundaries of experimental observation. The selection, in this case, has been assumed to be wholly caused by bird predation and non-visual selection is taken to be same for all phenotypes. The results are compared with available experimental data along three transects. Reasonable agreement with the data is obtained for the Manchester-North Wales transect and the South Wales-London transect. The simulation fails, on the other hand, to reproduce observations along the Central Wales-Birmingham-East Anglia transect. Thus it is concluded that the hypothesis ol balance of selection and migration is untenable as it stands. Either the visual selection pressures are incorrectly or incompletely specified, or else non-visual selection is also operating.     

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.     

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.     

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.     

Peppered moth revisited: analysis of recent decreases in melanic frequency and predictions for the future

In a recent review article by Mani published in 1990, it was shown that the changes in the frequency of the morph carbonaria can be explained through a migration-selection model and the model is capable of reproducing the decrease in the carbonaria frequency since the enactment of Clean Air Acts. In this paper new data both for carbonaria and for insularia up to 1990 for West Kirby in NW England and for Cambridge are presented. In the earlier review, the data for West Kirby extended to 1987 and for Cambridge to 1985. Data not previously published for the carbonaria and the insularia frequencies at sites in Northwood in Middlesex, Egham in Surrey and Ringwood in Hampshire are also presented. A migration-selection model was used to predict the change in the frequencies of the melanic morphs up to the year 2010. All the data are compared with the predictions of the model. It is encouraging that the data follow the theoretical predictions.     

Patterns of differentiation in a colour polymorphism and in neutral markers reveal rapid genetic changes in natural damselfly populations

The existence and mode of selection operating on heritable adaptive traits can be inferred by comparing population differentiation in neutral genetic variation between populations (often using F(ST) values) with the corresponding estimates for adaptive traits. Such comparisons indicate if selection acts in a diversifying way between populations, in which case differentiation in selected traits is expected to exceed differentiation in neutral markers [F(ST )(selected) > F(ST )(neutral)], or if negative frequency-dependent selection maintains genetic polymorphisms and pulls populations towards a common stable equilibrium [F(ST) (selected) < F(ST) (neutral)]. Here, we compared F(ST) values for putatively neutral data (obtained using amplified fragment length polymorphism) with estimates of differentiation in morph frequencies in the colour-polymorphic damselfly Ischnura elegans. We found that in the first year (2000), population differentiation in morph frequencies was significantly greater than differentiation in neutral loci, while in 2002 (only 2 years and 2 generations later), population differentiation in morph frequencies had decreased to a level significantly lower than differentiation in neutral loci. Genetic drift as an explanation for population differentiation in morph frequencies could thus be rejected in both years. These results indicate that the type and/or strength of selection on morph frequencies in this system can change substantially between years. We suggest that an approach to a common equilibrium morph frequency across all populations, driven by negative frequency-dependent selection, is the cause of these temporal changes. We conclude that inferences about selection obtained by comparing F(ST) values from neutral and adaptive genetic variation are most useful when spatial and temporal data are available from several populations and time points and when such information is combined with other ecological sources of data.     

Genetic drift within a protected polymorphism: enigmatic variation in color-morph frequencies in the candy-stripe spider, Enoplognatha ovata

The candy-stripe spider, Enoplognatha ovata, exhibits a striking color polymorphism comprising three morphs. A number of lines of evidence strongly suggest that this polymorphism is maintained by natural selection: its presence in a sister species, E. latimana; the physical nature of the variation; the virtual lack of monomorphic populations; the highly consistent rank-order of morphs within populations; and the presence of large-scale clines associated with climatic variables. However, the absence of selection is equally strongly suggested by very local surveys of morph frequencies over space and time, perturbation experiments, and a variance in morph frequency between populations that is virtually independent of spatial scale. In addition, local spatial patterns in one study site (Nidderdale, Yorkshire, England) have been explained in terms of intermittent drift over half a century ago, a hypothesis supported here by the distributions of four other genetic markers (two allozyme and two visible polymorphisms). A heuristic model is suggested that reconciles these apparently contradictory messages regarding the importance of drift and selection in this system. It is proposed that when allele frequencies of the color morph redimita lie between approximately 0.05 and 0.3, the deltaq on q plot is very shallow, so that within this region, where the majority of populations lie, selection is weak and drift is the major force determining local morph frequencies. However, outside this range of frequencies, powerful selection acts to protect the polymorphism. This model may apply to polymorphisms in other species and explain why evidence of selection in natural populations is often elusive.     

Salamander morph frequencies do not evolve as predicted in response to 40 years of climate change

The global climate is changing rapidly, yet biotic responses remain uncertain. Most studies focus on changes in species ranges or plastic responses like phenology, but adaptive evolution could be equally important. Studying evolutionary responses is challenging given limited historical data and a poor understanding of genetically variable traits under selection. We take advantage of a historical dataset to test for an adaptive response to climate change in a widespread, polymorphic amphibian, the eastern red‐backed salamander Plethodon cinereus. We resurveyed color morph frequencies across New England to test for an adaptive shift in response to climate change. We modeled historical and present‐day morph proportions as a function of climate and tested the accuracy of predictions both within and across different time periods. Our models showed moderate accuracy when predicting morph frequencies within time periods, but poor accuracy across time periods. Despite substantial changes in climate and significant relationships between morph frequency and climate variables within periods, we found no evidence for the predicted shift in morph frequencies across New England. The relationship between climate and color morph frequencies is likely more complex than originally suggested, potentially involving the interplay of additional factors such as microclimate variation, land use changes, and frequency‐dependent selection. Model extrapolation and changes in the correlation structure of climate variables also likely contributed to poor predictive ability. Evolution could provide a means to moderate the effects of climate change on many species. However, we often do not understand the direct links between climate variation, traits, and fitness. Therefore, forecasting climate‐mediated evolution remains an ongoing and important challenge for understanding climate change threats to species.     

Gene frequency changes in Cepaea snails on the Marlborough Downs over 25 years

In 1985 we resurveyed the sites on the Marlborough Downs in southern England at which Cain and Currey in 1960/61 sampled Cepaea snails and thence introduced the term 'area effects' to describe large areas of uniform morph frequency. Some sites no longer harboured Cepaea and at others the species composition had changed, with a general spread of Cepaea hortensis at the expense of Cepaea nemoralis. The majority, however, permitted comparison of morph frequencies between the two surveys. In C. nemoralis, we detected a significant overall decrease in the frequency of the brown morph and estimate selection as 5–9% per generation. There was no apparent change in frequencies of banded morphs. In C. hortensis we detected a significant overall increase in the frequency of unbanded shells (1–3% selection per generation) and an almost significant decrease in the frequency of fusions within the banded class. There was insufficient colour polymorphism in C. hortensis to allow analysis of colour morph frequencies. These changes—all in the direction of reduced absorption of solar energy—resemble others detected in both species at other localities in southern England. Possible explanations include large-scale climatic effects and changes in vegetation.     

Celtic origin of the C282Y mutation of hemochromatosis

The main hereditary hemochromatosis mutation C282Y in the HFE gene was recently described, and the C282Y frequencies were reported for various European populations. The aim of this synthesis is to compile the Y allele frequencies of the C282Y mutation for 40 European populations. The most elevated values are observed in residual Celtic populations in Ireland, the United Kingdom, and France, in accordance with the hypothesis of Simon et al. (1980) concerning a Celtic origin of the hereditary hemochromatosis mutation.     

A note on the estimation of selective values from recaptures of marked animals when selection pressures remain constant over time

An equation is given for the estimation of selective values from data obtained by mark-recapture experiments, assuming that selective pressures remain constant while the experiments are carried out. The equation does not have an explicit solution but can readily be solved using a trial-and-error method. The use of the equation is illustrated on some data reported byKettlewell et al. (1969) from an experiment involving typica and edda morphs of the moth Amathes glareosa. It is found that the edda morph apparently had a selective advantage of about 12% per day compared to the typica morph and that this is significantly different from zero. Using another methodKettlewell concluded that the selective advantage of the edda morph was only 7% and that this was not statistically significant.     

THE INTERPLAY BETWEEN LOCAL ECOLOGY,DIVERGENT SELECTION,AND GENETIC DRIFT IN POPULATION DIVERGENCE OF A SEXUALLY ANTAGONISTIC FEMALE TRAIT

Genetically polymorphic species offer the possibility to study maintenance of genetic variation and the potential role for genetic drift in population divergence. Indirect inference of the selection regimes operating on polymorphic traits can be achieved by comparing population divergence in neutral genetic markers with population divergence in trait frequencies. Such an approach could further be combined with ecological data to better understand agents of selection. Here, we infer the selective regimes acting on a polymorphic mating trait in an insect group; the dorsal structures (either rough or smooth) of female diving beetles. Our recent work suggests that the rough structures have a sexually antagonistic function in reducing male mating attempts. For two species (Dytiscus lapponicus and Graphoderus zonatus), we could not reject genetic drift as an explanation for population divergence in morph frequencies, whereas for the third (Hygrotus impressopunctatus) we found that divergent selection pulls morph frequencies apart across populations. Furthermore, population morph frequencies in H. impressopunctatus were significantly related to local bioclimatic factors, providing an additional line of evidence for local adaptation in this species. These data, therefore, suggest that local ecological factors and sexual conflict interact over larger spatial scales to shape population divergence in the polymorphism.     

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.     

Social class differences in infant mortality in Sweden: comparison with England and Wales.

OBJECTIVES--To investigate social class differences in infant mortality in Sweden in the mid-1980s and to compare their magnitude with that of those found in England and Wales. DESIGN--Analysis of risk of infant death by social class in aggregated routine data for the mid-1980s, which included the linkage of Swedish births to the 1985 census. SETTING--Sweden and England and Wales. SUBJECTS--All live births in Sweden (1985-6) and England and Wales (1983-5) and corresponding infant deaths were analysed. The Swedish data were coded to the British registrar general''s social class schema. MAIN OUTCOME MEASURES--Risk of death in the neonatal and postneonatal period. RESULTS--Taking the non-manual classes as the reference group, in the neonatal period in Sweden the manual social classes had a relative risk for mortality of 1.20 (95% confidence interval 1.02 to 1.43) and those not classified into a social class a relative risk of 1.08 (0.88 to 1.33). In the postneonatal period the equivalent relative risks were 1.38 (1.08 to 1.77) for manual classes and 2.14 (1.65 to 2.79) for the residual; these are similar to those for England and Wales (1.43 (1.36 to 1.51) for manual classes, 2.62 (2.45 to 2.81) for the residual). CONCLUSIONS--The existence of an equitable health care system and a strong social welfare policy in Sweden has not eliminated inequalities in post-neonatal mortality. Furthermore, the very low risk of infant death in the Swedish non-manual group (4.8/1000 live births) represents a target towards which public health interventions should aim. If this rate prevailed in England and Wales, 63% of postneonatal deaths would be avoided.     

Analysis of polymorphic variation in different types of habitat

A correlation between the distribution of an organism and features of its environment can be taken as indirect evidence of natural selection. Biologists may therefore collect samples from polymorphic populations at a number of locations, classify the locations into habitat types, and consider whether the distribution of morphs varies with the habitat. Statistical aspects of this type of study are discussed in this paper. A randomization test for habitat effects is proposed and a negative binomial model is suggested for the distribution of morphs from random locations within one type of habitat. Data on the distribution of Cepaea hortensis and C. nemoralis snails in southern England provide an example. For both species there is clear evidence of differences between habitats, although the morph distributions are rather variable within habitats. The negative binomial model suggests that, for the snail data, variation in morph proportions is mainly due to location differences. The binomial sampling error is relatively unimportant unless the sample size at a location is very small. Therefore it is reasonable to analyse morph proportions by standard methods without giving different weights to data from different locations. The snail data are analysed in this way. Discriminant function analyses are used to test for habitat effects. The relationships between C. hortensis and C. nemoralis morph frequencies within one habitat are examined by a canonical correlation analysis.     

A countrywide survey of colour morph frequencies in the spider Enoplognatha ovata (Glerck) (Araneae: Theridiidae): evidence for natural selection

Data on the frequencies of the main colour morphs ( lineata, redimita and ovata ) of Enoplognatha ovata have been collected from a total of 454 Ordnance Survey 10 km squares distributed throughout Great Britain. Only c. 0.5% of samples were monomorphic, for lineata in each case. Multiple regression analysis has been used to assess possible associations between morph frequencies and principal components derived from nine environmental variables. The distribution of morph frequencies is not random but shows weak clines associated with certain climatic factors. These large scale clines indicate the action of natural selection although very local variations in morph frequencies may result from selection and/or drift.     

Geographic Variation in Genetic Dominance of the Color Morphs of the Red-Backed Salamander, PLETHODON CINEREUS

Female parent-offspring phenotypic data on color morph frequencies in the red-backed salamander, Plethodon cinereus, were obtained from two Virginia localities (164 broods from Greene County and 97 broods from Giles County). The color morph data indicate that the striped morph is genetically dominant in Giles County and recessive in Greene County. It is suggested that epistatic interaction of two or more loci is responsible for the difference between the localities.     

Alternative reproductive strategies and the maintenance of female color polymorphism in damselflies

Genetic polymorphisms are powerful model systems to study the maintenance of diversity in nature. In some systems, polymorphisms are limited to female coloration; these are thought to have arisen as a consequence of reducing male mating harassment, commonly resulting in negative frequency‐dependent selection on female color morphs. One example is the damselfly Ischnura elegans, which shows three female color morphs and strong sexual conflict over mating rates. Here, we present research integrating male tactics, and female evolutionary strategies (female mating behavior and morph‐specific female fecundity) in populations with different morph‐specific mating frequencies, to obtain an understanding of mating rates in nature that goes beyond the mere measure of color frequencies. We found that female morph behavior differed significantly among but not within morphs (i.e., female morph behavior was fixed). In contrast, male tactics were strongly affected by the female morph frequency in the population. Laboratory work comparing morph‐specific female fecundity revealed that androchrome females have lower fecundity than both of the gynochrome female morphs in the short term (3‐days), but over a 10‐day period one of the gynochrome female morphs became more fecund than either of the other morphs. In summary, our study found sex‐specific dynamics in response to different morph frequencies and also highlights the importance of studying morph‐specific fecundities across different time frames to gain a better understanding of the role of alternative reproductive strategies in the maintenance of female‐limited color polymorphism.