Disequilibrium in some Cepaea populations

Phenotypic disequilibrium is a measure available, along with morph frequency, in many published sets of sample data recording the polymorphism of shell colour and pattern in the snail Cepaea nemoralis. The relation of disequilibrium to morph frequency for the colour (pink/yellow) and banding (unbanded/banded) loci has been examined for a large and widespread set of data. The direction of disequilibrium is a function of frequency at the two loci in a way that suggests that selection favours combinations of common morphs, whichever they are. This could indicate that such combinations are common because they have been selected. The data are therefore consistent with the proposal that populations are generally subject to selection of varying directions at different places and times, acting on the phenotype. In combination with migration, such selection could lead to prolonged polymorphism.     

Temporal morph frequency changes in sand‐dune populations of Cepaea nemoralis (L.)

Changes in morph frequency over time in sand‐dune populations of Cepaea nemoralis in the British Isles have been examined using the Evolution Megalab database. Frequencies in colonies on a sand dune at Berrow, Somerset were estimated in 2008–9. This extends a survey started in 1926 by A. E. Boycott and C. Diver and continued in the mid‐20th Century by B. C. Clarke and J. J. Murray. An increase in the frequency of the mid‐banded morph, noted in earlier work, has continued. The apparent decrease in brown was not confirmed but the yellow frequency has increased. At a range of dunes in the British Isles, comparisons between data from the mid‐20th Century and the early 21st Century indicate an increase in yellow and mid‐banded morphs and a decrease in banding and brown morphs. These results differ from the overall trends derived from Europe‐wide comparison of early data with Evolution Megalab data, in which banded and mid‐banded increased in frequency, whereas there was no overall change in yellow. The general pattern of regional variation has been retained, although there is also high heterogeneity between samples, suggesting that a variety of factors are involved in explaining the changes. The mean shifts in frequency are consistent with climatic change over the period. Dunes are probably the most likely habitat in which to detect such a change. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 108 , 315–322.     

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.     

MORPH FREQUENCIES OF EMPTY INTACT SHELLS FROM CEPAEA NEMORALIS (L.) COLONIES ON SAND DUNES IN SOUTH WEST ENGLAND

In collections of empty intact shells of Cepaea nemoralis fromthe surface of grass swards on sand dunes of the south westpeninsula, yellow, unhanded and five-banded shells tend to beunderrepresented, while pink and mid-banded shells are over-representedin comparison with the live population. It is argued that thesedifferences occur because selective heat death is the majormortality factor. Little systematic geographical variation in polymorphism wasobserved. Braunton Burrows shows unusually high frequenciesof pinks and mid-bandeds, while on the St. Ives Bay dunes, Cepaeais uncommon and shows an unusually high frequency of effectively-unbanded,with frequent punctation, smudging or other reduction of bands. (Received 8 April 1978;     

Differences in predatory pressure on terrestrial snails by birds and mammals

The evolution of shell polymorphism in terrestrial snails is a classic textbook example of the effect of natural selection in which avian and mammalian predation represents an important selective force on gene frequency. However, many questions about predation remain unclear, especially in the case of mammals. We collected 2000 specimens from eight terrestrial gastropod species to investigate the predation pressure exerted by birds and mice on snails. We found evidence of avian and mammalian predation in 26.5% and 36.8% of the shells. Both birds and mammals were selective with respect to snail species, size and morphs. Birds preferred the brown-lipped banded snail Cepaea nemoralis (L.) and mice preferred the burgundy snail Helix pomatia L. Mice avoided pink mid-banded C. nemoralis and preferred brown mid-banded morphs, which were neglected by birds. In contrast to mice, birds chose larger individuals. Significant differences in their predatory pressure can influence the evolution and maintenance of shell size and polymorphism of shell colouration in snails.     

Variation in the distribution and fitness of the brown morph of Cepaea nemoralis (L.)

The distribution of the brown rnorph of the landsnail Cepaea nemoralis in part of the Brendon Hills, Somerset is described. The morph is at a high frequency on north-facing slopes, valley bottoms, and in a broad partly-enclosed basin. It is argued that low temperature is the selective agent. Alternative explanations are considered. In two population cages adult brown-shelled snails survived less well than pink five-banded ones during the summer months, this trend being reversed in the autumn. It is concluded that the experimental results support the 'climatic' interpretation of distributional data.     

Variation with altitude and habitat in Cepaea hortensis (Müll.)

Variation in morph, chromosome and gene frequencies in Cepaea hortensis are described for a valley system in the north slopes of the Brendon Hills, Somerset. Climatic data from three sites show that there are considerable differences in temperature regime at different altitudes and also that there are major climatic differences between the valley system and a more maritime district further west. Pronounced intercolony variation that can be related to altitude and geographical position suggests that temperature and/or rainfall may cause frequency differences. There is some variation with habitat suggesting the action of visual selection, but there are also consistent differences between habitats which are not in accordance with this hypothesis.
The linkage states between the alleles of the colour and banding genes vary with altitude.
Variation in the distribution of Arianta arbustorum and Cepaea nemoralis suggest that climatic differences are limiting factors.     

Difference in habitat selection by different morphs of the land snail Arianta arbustorum

The behaviour of morphs of Arianta arbustorum has been investigated using a new technique involving a paint which fades in daylight. The technique was used to study the response of different morphs to exposure to sunlight at a homogeneous woodland habitat and a heterogeneous open habitat. The yellow morph showed a significantly higher rate of fading than the brown morph. A mark, release and recapture programme was used to analyse the association between frequencies of different morphs recaptured and not recaptured. In both habitats, there was a general tendency for the frequency of recapture to increase with increasing age in both morphs. Habitat selection resulting from different behaviour of different morphs of A. arbustorum is discussed. Morph frequencies can be related to habitat, the brown morphs of A. arbustorum are significantly less frequent in the open habitat. The behavioural responses of Cepaea hortensis , present with A. arbustorum in two other populations, to the exposure of sunshine were also studied. Climatic selection by solar energy associated with different morphs, species and habitat are also discussed.     

Change and stability in a steep morph‐frequency cline in the snail Cepaea nemoralis (L.) over 43 years

Populations of the polymorphic land snail Cepaea nemoralis (L.) from Deepdale, Derbyshire, UK, sampled in 1965–67, showed a pattern of area effects, with steep clines among groups of populations differing in shell colour and banding morph frequencies. In 2010, most of these populations were resampled. In particular, a continuous transect made in 1967 of 42 quadrats (18.34 × 18.34 m) across a steep cline in several morph frequencies was completely resampled. In the dale as a whole, yellow shells had increased in frequency. In the transect, the frequencies of banding morphs showed no significant changes, although colour morphs showed some changes. Pink shells had increased in frequency in a section in which scrub had developed, and brown shells had increased in frequency in the area in which they had originally been at the highest frequency. In each case, the selection coefficients were of the order of 4%. Yellow had increased elsewhere. Nevertheless, both in the dale as a whole and in the transect, the pattern of geographical change in morph frequencies had remained essentially the same. Estimates of migration based on previous studies of marked snails and on modelling of the effect of drift and migration suggest that, regardless of whether the cline is a product of differential selection or of the gradual merging of previously separate founding populations, it has been in existence for a long time, and that migration occurs over greater distances than estimated from direct observation on marked snails. Although we can demonstrate that selection has occurred, the origin and maintenance of the cline and others similar to it remain in doubt; the development and maintenance of polymorphism in this species may require consideration of several processes operating on different time scales. © 2012 The Linnean Society of London     

The pale brown allele in Cepaea nemoralis

The pale brown colour morph in Cepaea nemoralis appears to be determined by an allele at the C (colour) locus ( C ^{P B}). Pale brown is dominant to yellow, codominant with pink and recessive to dark brown. It is linked to the B locus (which controls the presence or absence of banding on the shell), but not to the U locus, which determines whether there is one band or five. In segregations of pale brown and yellow there is a significant deficiency of pale brown, suggesting that there are differences in viability between the morphs.     

Wild birds prefer the familiar morph when feeding on pastry-filled shells of the landsnail Cepaea hortensis (Müll.)

Colour polymorphisms in prey could be maintained if predators concentrate on common morphs and confer a selective advantage on rare morphs. We describe experiments to test whether wild birds feed on pastry-stuffed shells of Cepaea hortensis in a manner that might lead to such apostatic selection. The birds were first given a 'pre-training' choice test of a shell population with equal numbers of yellow unbandeds and yellow five-bandeds; they were then trained on one morph alone, given a second choice test, trained on the other morph and, finally, given a third choice test. The birds preferred five-bandeds in five of the six pre-training tests. In all six experiments the first training session increased the birds' preferences for the morph that was familiar. The results were less clear-cut when selection during pre-training was compared with selection after the second training session. However, a comparison between selection after each of the two training sessions showed that in all six experiments the results were in the direction predicted from the hypothesis that familiar morphs are preferred. This set of experiments is one of the few in which behaviour which could lead to apostatic selection has been tested with morphs that differ in pattern. The findings support the idea that polymorphism in Cepaea could be maintained by apostatic selection.     

Rapid evolution in unstable habitats: a success story of the polymorphic land snail Cepaea nemoralis (Gastropoda: Pulmonata)

The present study reports on a natural experiment with twelve replicates in which rapid, predictable and consistent divergence of Cepaea nemoralis populations occurred in response to repeated selection gradient of adjacent open and shaded habitats. Because the frequencies of various genetically‐based phenotypes varied widely among surveyed populations, and there was a large overlap between habitat types, no overall association with habitat was apparent. In paired comparisons, however, significant changes were consistently towards higher frequencies of light morphs in the open than corresponding shaded habitats, and this result is attributable to natural selection. This shows that the knowledge of the genetic composition of reference populations is often essential for discerning selection from random processes. At each site, a different morph combination contributed to the divergence of populations, indicating that there are many genetic solutions to similar ecological problems; this likely enhances the maintenance of high levels of polymorphism. Adaptation of populations occurred in contemporary time and was fast. In one case where it was possible to follow changes, significant shifts in morph frequencies occurred within just two snail generations (selection coefficients of 0.404 and 0.518). High evolvability may be one of the factors contributing to the ecological success of Cepaea nemoralis. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 102 , 251–262.     

Flower Color Polymorphism in Rhododendron cyanocarpum (Ericaceae), an Endangered Alpine Species Endemic to NW Yunnan,China

Rhododendron cyanocarpum is a narrow endemic species with pink and white floral color. In the present study, to investigate the significance of petal color morphs, we examined color morph frequencies, petal color reflectance and other associated floral characters, effective pollinators, visitation frequencies, and fruit production in the field. In all surveyed known populations, plants with pink color morph dominanted and comprised 77%-100% of individuals. Two peaks at 430 nm and 650 nm were found in the petal color reflectance of pink flowers, and only one peak at 430 nm was found in the reflectance spectrum of white flowers. In addition, color morphs were also associated with colors of style and stigma, lengths of corolla, calyx and pedicel, the closest distance between stigma and stamen, but not with style length and nectar production. Moreover, higher visitation frequency of their shared pollinators (bumblebees) and fruit production were observed of pink flowers than white flowers. Despite a briefly temporal and spacial study, we suggest that color morph frequencies, visit frequencies of bumblebees and fruit production, all favor to be stablilizing selection for the pink color morph.     

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.     

滇西北特有植物蓝果杜鹃的花色多态性研究(英文)

蓝果杜鹃(Rhododendron cyanocarpum)为大理苍山特有的濒危植物,有粉色和白色两种花冠类型。为了探讨该物种花色多态性的意义,本研究调查了粉色花和白色花植株在已知的各居群的分布频率、花冠的反射光谱及其它的花部特征、有效传粉者及其访花频率与结实情况。结果表明:粉色花植株在所有调查的居群中占优势(77%~100%)。粉色花的花冠反射光谱在430 nm和650 nm有两个峰,而白色花只在430 nm有一个反射峰。同时,花特征如:花柱与柱头颜色、花冠长度、花萼长度、花梗长度以及雌雄蕊最短距离,两种花冠存在显著差异。另外,尽管熊蜂作为这两种花冠的主要传粉者,但粉红花的访花频率以及自然条件下的结实情况显著高于白色花。本研究结果推测粉红色花可能受到了稳定性选择的作用。     

Origin of the disjunct distribution of flower colour polymorphism within Limonium wrightii (Plumbaginaceae) in the Ryukyu Archipelago

The sea lavender, Limonium wrightii , has six morphs of flower colour variation. The geographical distribution of flower colour morphs is disjunct; the distribution of the pink flower morph is divided into two subregions, and that of the yellow flower morph intervenes between them. The present study aimed to examine the origin of this apparent distribution pattern of flower colour in L. wrightii . Two main hypotheses (i.e. past dispersal events and phenotypic changes by natural selection and/or stochastic processes) have been proposed to account for the origin of leapfrog distribution patterns. To determine which hypothesis was applicable, we conducted a molecular phylogenetic analysis using sequence variation in chloroplast DNA (three regions of intergenic spacers, trnG - trnfM , trnV - trnM , and psbA-trnH ). We sequenced 58 accessions of L. wrightii frin 28 islands in the Ryukyu Archipelago and the Izu-Ogasawara Islands, located south of the Japanese mainland, and 12 accessions of four congeneric species. Within L. wrightii , we obtained four lineages of ten haplotypes. These lineages and haplotypes did not correlate with the different flower colours. These results indicate that the formation processes of populations are complex. The haplotypes of the pink flower morph did not show a sister relationship between the two disjunct subregions, indicating that the disjunct populations of the pink flower morphs are unlikely to share the pink flower colour as a result of common ancestry. We conclude that the observed leapfrog distribution pattern is caused by natural selection and/or stochastic processes.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 97 , 709–717.     

A two-stage model for Cepaea polymorphism

The history of the study of snails in the genus Cepaea is briefly outlined. Cepaea nemoralis and C. hortensis are polymorphic for genetically controlled shell colour and banding, which has been the main interest of the work covered. Random drift, selective predation and climatic selection, both at a macro- and micro-scale, all affect gene frequency. The usual approach to understanding maintenance of the polymorphism, has been to look for centripetal effects on frequency. Possible processes include balance of mutation pressure and drift, heterozygote advantage, relational balance heterosis, frequency-dependent predation, multi-niche selective balance, or some combination of these. Mutational balance is overlaid by more substantial forces. There is some evidence for heterosis. Predation by birds may protect the polymorphism, and act apostatically to favour distinct morphs. Although not substantiated for Cepaea, many studies show that predators behave in the appropriate manner, while shell colour polymorphisms in molluscs occur most commonly in species exposed to visually searching predators. It is not known whether different thermal properties of the shells help to generate equilibria. Migration between colonies is probably greater than originally thought. The present geographical range has been occupied for less than 5000 generations. Climatic and human modification alter snail habitats relatively rapidly, which in turn changes selection pressures. A simple simulation shows that migration coupled with selection which fluctuates but is not centripetal, may retain polymorphism for sufficiently long to account for the patterns we see today. There may therefore be a two-stage basis to the polymorphism, comprising long-term but weak balancing forces coupled with fluctuating selection which does not necessarily balance but results in very slow elimination. Persistence of genetic variants in this way may provide the conditions for evolution of a balanced genome.     

Persistence and extinction in some Cepaea populations

Eight artificial populations of Cepaea nemoralis kept in enclosures excluding visual predators, built up to peak numbers and then crashed in periods of between 10 and 20 years. Fluctuations in numbers were caused by variation in recruitment, not in adult survival. One population survived through a very low population size (14 individuals or less), and shows no significant change in morph frequencies. The very slight fluctuation in morph frequency has a probabilty of chance occurrence of 7%. Studies of 250 years or more are probably needed to distinguish systematic effects on gene frequency from random variation. The lack of any major alteration in morph frequencies, however, suggests that there has been no genetic revolution in this population.     

Polymorphism in Cepaea Hortensis in marginal populations in Spain

Cepaea hortensis (Mull) is found in Spain only in the western part of the Iberian mountain range. This is the southernmost limit of its range. The most frequent colour is yellow, brown being very rare. The most frequent band patterns are 12345, fused banded and unbanded. There are associations between maximum and minimum temperatures and pink and effectively unbanded yellow phenotypes. The populations may be grouped into two zones, one located in the north and the other in the south of the sampled area. Visual predation was not observed. The marginal populations studied by us were compared with others from Northern Europe, and it was observed that pink snails are more frequent at the southern limit, and unbanded ones at the northern limit. This was interpreted as a climatic selection.     

Florivores prefer white versus pink petal color morphs in wild radish, Raphanus sativus

Many hypotheses suggest that pollinators act to maintain or change floral color morph frequencies in nature, although pollinator preferences do not always match color morph frequencies in the field. Therefore, non-pollinating agents may also be responsible for color morph frequencies. To test this hypothesis, we examined whether Raphanus sativus plants with white flowers received different amounts of florivory than plants with pink flowers, and whether florivores preferred one floral color over the other. We found that white-flowered plants received significantly more floral damage than pink-flowered plants in eight populations over 4 years in northern California. Both generalists and specialists on Brassicaceae preferred white petals in choice and short-term no choice tests. In performance tests, generalists gained more weight on white versus pink petals whereas specialists gained similar amounts of weight on pink and white morphs. Because our results suggest that florivores prefer and perform better on white versus pink flowers, these insects may have the opportunity to affect the frequency of color morphs in the field.