Interaction between area effects and variation with habitat in Cepaea

Populations of Cepaea nemoralis in Warwickshire occupy habitats of considerable temporal stability, most being at least 250 years old, and some much older. As expected from earlier work, shell pattern polymorphisms in these populations show variation with habitat of a kind suggesting the operation of visual selection for crypsis. They also show patterns of microgeographical variation unrelated to habitat. Although of a less extreme character, this variation resembles the 'area effects' seen in downland populations of Cepaea , in the lack of coincidence of variation at different loci, and in the existence of stronger and larger scale patterns in banding than in colour morphs. A similar explanation is advanced for their occurrence: previous bottlenecks and colonization from small relicts with founder effects. The less marked character of the variation is expected from the greater habitat stability and continuity in Warwickshire compared with downland. A clear colonization effect is seen in the inverse relationship between age of habitat and frequency of yellow in woodland populations. The results suggest that population histories affect variation in Cepaea even in areas of relative habitat stability.     

The distribution and variation of three species of land snail near Rickmansworth, Hertfordshire

The distribution of Arianta arbustorum, Cepaea nemoralis and C. hortensis , and the variations in the shell polymorphism of the Cepaea species have been studied in an area of low chalk downland and alluvial river valleys near Rickmansworth, Hertfordshire. The distribution of A. arbustorum is restricted, the species being largely confined to the bottoms of river valleys, especially in hedgerows. Variations in dampness seem the most likely limiting factor. C. hortensis is widespread and found in many habitats, but C. nemoralis is largely confined to woods, and is especially common in beechwoods. The morph-frequencies of C. nemoralis and C. hortensis vary in such a way as to suggest the operation of climatic and visual selection. In C. hortensis there are area effects for the presence or absence of pink five-banded shells, and there is an overall linkage disequilibrium resulting in an excess of unbandeds in non-yellow shells. Reasonable evidence for the operation of random factors in determining the morph-frequencies of populations of Cepaea in the area has been found only for three very disturbed populations. It is suggested that the large number of selective forces known to act on Cepaea populations in this area will account for the variation in morph-frequencies in populations from the same area or habitat.     

The habitats of the land snail Cepaea nemoralis (L.) on downland and their ecogenetie significance

Population densities of adult Cepaea nemoralis (L.) were estimated for nine chalk grassland sites on the South Downs. Estimates of cover, species composition and height of vegetation were also obtained. The density of C. nemoralis was positively associated with Bromus erectus cover, and negatively with Festuca spp. cover. This finding, confirmed by an earlier survey, suggests that the density of C. nemoralis in chalk grassland is directly or indirectly governed by mammalian grazing pressure, and not by the availability of suitable food. Intensely grazed downland is an unsuitable, or at best a marginal habitat for C. nemoralis. This conclusion is discussed with particular reference to 'area effects' in the shell character polymorphism, often found in downland populations of C. nemoralis.     

Contrasting patterns of variation in urban populations of Cepaea (Gastropoda: Pulmonata): a tale of two cities

The shell colour and banding polymorphisms in urban populations of Cepaea snails were studied in Sheffield (England) and Wrocław (Poland), which are two cities of similar size. Both Cepaea nemoralis and Cepaea hortensis were found in Sheffield, but only C. nemoralis was found in Wrocław. In neither city was variation with habitat apparent, nor was there evidence of large-scale geographical patterns. Spatial autocorrelation analyses revealed only vaguely defined local clusters. There was a greater degree of among-population variation in Sheffield, and more cases of extreme frequencies and low levels of polymorphism. Wrocław populations were more uniform and highly polymorphic. These differences can be related to the history of these snails in each city. Sheffield has been colonized mainly in the last 20 years, and some parts of the city with apparently suitable habitat remain unoccupied. Leptokurtic dispersal of small propagules followed by local spread appears likely. Values of F _ST resemble those seen in districts elsewhere with 'Area Effects'. Although there are ancient woodlands within the city, they have been colonized too recently for selection to achieve a new balance. Wrocław has held Cepaea populations for at least 100 years, and they are denser and more continuous. Shaded habitats are recent and temporary; the overall pattern of variation matches that seen in open habitats in districts elsewhere in which variation with habitat is apparent, and values of F _ST also correspond. Population history and the character of dispersal clearly affect the patterns of variation observed.  © 2009 The Linnean Society of London, Biological Journal of the Linnean Society , 2009, 97 , 27–39.     

Patterns of morphological and molecular polymorphism in the land snail Cepaea nemoralis

In the land snail Cepaea nemoralis , allele frequencies at loci controlling shell polymorphisms often show large areas of remarkable constancy which are separated by steep clines from neighbouring areas with strikingly different allele frequencies. It has recently been claimed that these 'area effects' exemplify a general tendency for population differentiation without geographic isolation in a variety of organisms of relatively low mobility. As such they could represent an early phase in the process of speciation. If this is true, population differentiation of shell polymorphisms in Cepaea would be expected to be accompanied by parallel differentiation at other gene loci, such as those detected by gel electrophoresis.
We have studied populations of C. nemoralis in North Wales and in the Valle de Aran of the Pyrenees. Levels of molecular heterogeneity are comparable to those found in related animals which show much less visible polymorphism. In spite of some statistical problems inherent in the analysis ol overlapping geographic patterns, there is no clear association between the patterns of geographic variation at the visible and molecular levels. Claims that genetic differentiation in the visible polymorphisms between C. nemoralis populations are a special case of the formation of geographic races are therefore probably not justified.     

Variation in body colour in western Irish populations of Cepaea nemoralis (L.)

Samples of Cepaea nemoralis (L.) taken from sand-dune, lowland and mountain populations show differences in body colour. Overall, sand-dune populations are much paler than those in the other two areas. On sand-dunes, darker and more variable populations are found in densely vegetated habitats; in the lowland, darker forms occur further inland but there is no correlation with vegetation; mountain populations are darker and less variable on north- than south-facing slopes. Overall differences between areas can be ascribed to climatic selection and differences in variation within areas to niche width.     

At the north-eastern extremity: variation in <Emphasis Type="Italic">Cepaea nemoralis</Emphasis> around Gdańsk,northern Poland

Variation in the shell colour and banding polymorphism in the land snail Cepaea nemoralis was studied in 260 populations in the region of Gdańsk, northern Poland. Unlike in other regions of Poland, many populations contain brown shells. Populations from shaded habitats have higher frequencies of brown than those from open and intermediate habitats, largely at the expense of yellow shells. Nearly all brown shells are also unbanded. Apart from this disequilibrium, banding morphs among yellow and pink shells show no relationship to habitat. There are no broad geographical trends in morph-frequencies, but there are very strong correlations among populations very close together, revealed both by pairwise analysis and Moran’s I. Principal Component Analyses show that these correlations relate to overall genetic similarity at the loci involved. The populations are at the north-eastern limits of the species’ range; habitats are mostly anthropogenic, and comparisons with studies in two urban areas (Wrocław, SW Poland, and Sheffield, central England) suggest that the patterns of variation seen are a product of human transport of propagules followed by local dispersal. The effect of habitat here is much less marked than in regions much further west, but it indicates that natural selection has occurred.     

Landscape scale genetic effects of habitat fragmentation on a high gene flow species: Speyeria idalia (Nymphalidae)

Detection of the genetic effects of recent habitat fragmentation in natural populations can be a difficult task, especially for high gene flow species. Previous analyses of mitochondrial DNA data from across the current range of Speyeria idalia indicated that the species exhibited high levels of gene flow among populations, with the exception of an isolated population in the eastern portion of its range. However, some populations are found on isolated habitat patches, which were recently separated from one another by large expanses of uninhabitable terrain, in the form of row crop agriculture. The goal of this study was to compare levels of genetic differentiation and diversity among populations found in relatively continuous habitat to populations in both recently and historically isolated habitat. Four microsatellite loci were used to genotype over 300 individuals from five populations in continuous habitat, five populations in recently fragmented habitat, and one historically isolated population. Results from the historically isolated population were concordant with previous analyses and suggest significant differentiation. Also, microsatellite data were consistent with the genetic effects of habitat fragmentation for the recently isolated populations, in the form of increased differentiation and decreased genetic diversity when compared to nonfragmented populations. These results suggest that given the appropriate control populations, microsatellite markers can be used to detect the effects of recent habitat fragmentation in natural populations, even at a large geographical scale in high gene flow species.     

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.     

The roles of geological history and colonization abilities in genetic differentiation between mammalian populations in the Philippine archipelago

Aim To test hypotheses that: (1) late Pleistocene low sea‐level shorelines (rather than current shorelines) define patterns of genetic variation among mammals on oceanic Philippine islands; (2) species‐specific ecological attributes, especially forest fidelity and vagility, determine the extent to which common genetic patterns are exhibited among a set of species; (3) populations show reduced within‐population variation on small, isolated oceanic islands; (4) populations tend to be most highly differentiated on small, isolated islands; and (5) to assess the extent to which patterns of genetic differentiation among multiple species are determined by interactions of ecological traits and geological/geographic conditions. Location The Philippine Islands, a large group of oceanic islands in Southeast (SE) Asia with unusually high levels of endemism among mammals. Methods Starch‐gel electrophoresis of protein allozymes of six species of small fruit bats (Chiroptera, Pteropodidae) and one rodent (Rodentia, Muridae). Results Genetic distances between populations within all species are not correlated with distances between present‐day shorelines, but are positively correlated with distances between shorelines during the last Pleistocene period of low sea level; relatively little intraspecific variation was found within these ‘Pleistocene islands’. Island area and isolation of oceanic populations have only slight effects on standing genetic variation within populations, but populations on some isolated islands have heightened levels of genetic differentiation, and reduced levels of gene flow, relative to other islands. Species associated with disturbed habitat (all of which fly readily across open habitats) show more genetic variation within populations than species associated with primary rain forest (all of which avoid flying out from beneath forest canopy). Species associated with disturbed habitats, which tend to be widely distributed in SE Asia, also show higher rates of gene flow and less differentiation between populations than species associated with rain forest, which tend to be Philippine endemic species. One rain forest bat has levels of gene flow and heterozygosity similar to the forest‐living rodent in our study. Main conclusions The maximum limits of Philippine islands that were reached during Pleistocene periods of low sea level define areas of relative genetic homogeneity, whereas even narrow sea channels between adjacent but permanently isolated oceanic islands are associated with most genetic variation within the species. Moreover, the distance between ‘Pleistocene islands’ is correlated with the extent of genetic distances within species. The structure of genetic variation is strongly influenced by the ecology of the species, predominantly as a result of their varying levels of vagility and ability to tolerate open (non‐forested) habitat. Readily available information on ecology (habitat association and vagility) and geological circumstances (presence or absence of Pleistocene land‐bridges between islands, and distance between oceanic islands during periods of low sea level) are combined to produce a simple predictive model of likely patterns of genetic differentiation (and hence speciation) among these mammals, and probably among other organisms, in oceanic archipelagos.     

The genetic structure of some steppe populations of the snailCepaea vindobonensis (Pf.)

The snailCepaea vindobonensis, which is polymorphic for the number of bands on the shell and for intensity of band pigmentation, was studied in an area of steppe in Eastern Romania. There are marked geographical variations in phenotype frequency which, unlike the similar variation found in populations from mountainous areas, show no detectable associations with any easily identifiable component of the environment. These patterns of variation may be due to selection by cryptic ecological factors or to a balance between climatic selection and selection by the genetic environment of the population.     

Genetic variation and structure in the expanding moss Pogonatum dentatum (Polytrichaceae) in its area of origin and in a recently colonized area

Genetic variation in the expanding moss species Pogonatum dentatum was studied using intersimple sequence repeat (ISSR) markers. The genetic consequences of range expansion were studied by comparing source populations in a mountain area with populations from a recently colonized lowland area in Sweden. Indices of genetic variation show slightly lower number of alleles per locus in the lowlands and a similar gene diversity in both areas. Three of four lowland populations had evidence of a recently passed bottleneck. Considerably higher haplotype diversity was found in the recently colonized lowlands compared to source populations in the mountains. Patterns of allelic diversity suggest that P. dentatum experiences loss of genetic variation through founder effects and genetic drift when expanding its distribution range. Higher haplotypic diversity, less linkage disequilibrium, and fewer compatible loci indicate that sexual recombination is relatively more important in the lowlands compared to the mountains. A likely explanation is higher success of establishment from spores in the lowlands, while clonal propagation predominates in the mountains. Less genetic differentiation among lowland populations indicates more gene flow in the lowland area, involving more spores and/or fragments moving among populations.     

SYMPATRIC SPECIATION VIA HABITAT SPECIALIZATION DRIVEN BY DELETERIOUS MUTATIONS

Theoretical studies have suggested that the evolution of habitat (host) races, regarded as a prelude to sympatric speciation, requires strong trade-offs in adaptation to different habitats: alleles that improve fitness in some habitats and have deleterious effects of similar magnitude in other habitats must be segregating in the population. I argue that such trade-offs are not necessary; the evolution of habitat races can also be driven by genetic variation due to loci that affect fitness in one habitat and are neutral or nearly so in others, that is, when performance in different habitats is genetically independent. One source of such genetic variation are deleterious mutations with habitat-specific fitness effects. I use deterministic two-locus and multilocus models to show that the presence of such mutations in the gene pool results in indirect selection favoring habitat fidelity or habitat preference over acceptance of both suitable habitats. This leads to the evolution of largely genetically isolated populations that use different habitats, from a single panmictic population of individuals accepting both habitats. This study suggests that the conditions favoring habitat race formation, and thus possibly sympatric speciation, are much less stringent than previously thought.     

Hierarchical habitat selection by woodland caribou: its relationship to limiting factors

Habitat selection is a hierarchical process that may yield various patterns depending on the scales of investigation. We employed satellite radio‐telemetry to examine patterns of habitat selection by female woodland caribou in central Saskatchewan at both coarse (seasonal range) and fine (daily area) scales. At each scale, we converted spatial data describing compositions of available and used habitat to standardised resource selection indices and examined them with multivariate analyses of variance. Seasonal ranges generally showed preferential inclusion of peatlands and black spruce dominated stands relative to recently disturbed stands and early seral stage forests. In all populations, caribou preferred peatlands and black spruce forests to all other habitat types at the daily area scale, in general, these patterns may reveal the effective avoidance of wolves, the primary factor limiting caribou throughout the boreal forest. In three populations where seasonal ranges showed the selective inclusion of either young jack pine stands or clearcuts along with peatlands and black spruce forests, we found a relative avoidance of the clearcuts and young jack pine stands at the daily area scale. As all caribou populations in the area are thought to be relics of a once more continuous distribution, the seasonal range selection by animals in disturbed areas may better describe historic rather than current habitat selection. We found inter‐annual variation in selection at the coarser spatial scale in one population, and inter‐seasonal variation in selection at the finer spatial scale in three populations, indicating that the relative grains of the spatial and temporal scales coincide. We were better able to explain the seasonal variations in finer scale selection by considering available forage, a factor less likely than predation to limit woodland caribou populations. The data agree with the theory that the spatial and temporal hierarchy of habitat selection reflects the hierarchy of factors potentially limiting individual fitness.     

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.     

The quality and isolation of habitat patches both determine where butterflies persist in fragmented landscapes

Habitat quality and metapopulation effects are the main hypotheses that currently explain the disproportionate decline of insects in cultivated Holarctic landscapes. The former assumes a degradation in habitat quality for insects within surviving ecosystems, the latter that too few, small or isolated islands of ecosystem remain in landscapes for populations to persist. These hypotheses are often treated as alternatives, and this can lead to serious conflict in the interpretations of conservationists. We present the first empirical demonstration that habitat quality and site isolation are both important determinants of where populations persist in modern landscapes. We described the precise habitat requirements of Melitaea cinxia, Polyommatus bellargus and Thymelicus acteon, and quantified the variation in carrying capacity within each butterfly's niche. We then made detailed surveys to compare the distribution and density of every population of each species with the size, distance apart and quality of their specific habitats in all their potential habitat patches in three UK landscapes. In each case, within-site variation in habitat quality explained which patches supported a species' population two to three times better than site isolation. Site area and occupancy were not correlated in any species. Instead of representing alternative paradigms, habitat quality and spatial effects operate at different hierarchical levels within the same process: habitat quality is the missing third parameter in metapopulation dynamics, contributing more to species persistence, on the basis of these results, than site area or isolation. A reorientation in conservation priorities is recommended.     

The response of bird populations to habitat loss

Environmental change through altered climate and land use could have a severe impact on bird populations. Predicting the consequences for the size of bird populations is one of the crucial problems for their conservation. We show how a population model based on the behaviour of individuals can be used to predict the consequences of habitat loss. For a wide range of conditions, loss of either wintering or breeding habitat results in population reduction. The approach is then extended to consider the impact of habitat loss in the wintering area on bird species with complex migratory systems. This shows that 'knock-on' effects may occur, so that habitat loss in a wintering area may affect populations which did not initially use that area. The ability to alter migration routes in response to environmental change may be crucial to the future viability of populations. Using a simple model combining genetics and population dynamics, we show that aspects of the biology of a species may affect whether or not its migration strategy is flexible enough to shift in response to habitat change. Some species may be able to adopt new migration routes and avoid the catastrophic consequences of habitat loss in traditional wintering areas; however, other species may lack this flexibility and may suffer severe population declines as a consequence.     

Evolution of intraspecific variation in the advertisement call of a cricket frog (Acris crepitans, Hylidae)

Advertisement calls of the cricket frog, Acris crepitans , show statistically significant variation among populations in all call variables measured. Call variables show strong clinal variation resulting in calls of lower frequency, longer duration and slower call rates produced by A. c. blanchardi in open habitat in the west of the range, and calls of higher frequency, shorter calls and faster call rates produced by A. c. crepitans in the pine forests in the eastern part of the range. This clinal variation does not result from pleiotropic effects of body size or any other morphological characters we measured.
The two subspecies usually reside in different habitats, but some A. c. blanchardi reside in an isolated pine forest in central Texas. By comparing the calls of this subspecies in open and forest habitat, and by statistically removing the effects of clinal variation for all populations, we determined that habitat explains some of the variation in call structure; this is not true of subspecies.
Our data reject several hypotheses that purport to explain the evolution of mate recognition signals. (1) We reject the notion of Paterson and others that there is strong stabilizing selection on species-specific mate recognition signals. (2) There is no support for the hypothesis that call variation is primarily due to pleiotropic effects of body size or other morphological characters over the geographic range we examined. (3) There is no evidence for reproductive character displacement. (4) Our data, as well as experimental studies of habitat acoustics, support the hypothesis that some differences in calls among habitats result from environmental selection on call structure to enhance call transmission. We suggest that the latter hypothesis does not explain the strong clinal component of call variation. This might result from the passive effects of gene flow between populations at the extremes of the range under selection generated by habitat acoustics.     

Intraspecific genetic analysis of the summer tanager Piranga rubra: implications for species limits and conservation

The summer tanager Piranga rubra is a Neotropical migrant that has experienced noted declines in the southwestern United States caused by extensive habitat loss of native riparian woodlands. This species is composed of two morphologically and behaviorally distinct taxa that traditionally have been recognized as subspecies, each occupying unique habitats in the southern part of North America. Genetic analyses of intraspecific variation are important in studies of threatened or endangered species because they can indicate whether smaller management units exist below the species level and they also provide estimates of within population variability. Using a mitochondrial DNA marker, the intraspecific genetic variation of this species is explored to determine whether the morphologically and behaviorally distinct subspecies are also genetically unique. By using traditional phylogenetic methods and building haplotype networks, results from this study indicate that the subspecies represent two phylogenetic species and should be managed as separate units. In addition, the level of gene flow among geographically isolated populations of the western subspecies is explored using Nested Clade Phylogeographic Analysis and population genetic tests. These analyses show that populations are genetically diverse and that haplotypes are shared across populations. Newly colonized populations are as diverse as older populations. This suggests that as habitat degrades in traditional breeding areas of the summer tanager, if suitable habitat elsewhere becomes available for new populations, these new colonies should be genetically diverse.     

Genetic effects of habitat fragmentation and population isolation on Etheostoma raneyi (Percidae)

The use of genetic methods to quantify the effects of anthropogenic habitat fragmentation on population structure has become increasingly common. However, in today’s highly fragmented habitats, researchers have sometimes concluded that populations are currently genetically isolated due to habitat fragmentation without testing the possibility that populations were genetically isolated before European settlement. Etheostoma raneyi is a benthic headwater fish restricted to river drainages in northern Mississippi, USA, that has a suite of adaptive traits that correlate with poor dispersal ability. Aquatic habitat within this area has been extensively modified, primarily by flood-control projects, and populations in headwater streams have possibly become genetically isolated from one another. We used microsatellite markers to quantify genetic structure as well as contemporary and historical gene flow across the range of the species. Results indicated that genetically distinct populations exist in each headwater stream analyzed, current gene flow rates are lower than historical rates, most genetic variation is partitioned among populations, and populations in the Yocona River drainage show lower levels of genetic diversity than populations in the Tallahatchie River drainage and other Etheostoma species. All populations have negative F_IS scores, of which roughly half are significant relative to Hardy–Weinberg expectations, perhaps due to small population sizes. We conclude that anthropogenic habitat alteration and fragmentation has had a profoundly negative impact on the species by isolating E. raneyi within headwater stream reaches. Further research is needed to inform conservation strategies, but populations in the Yocona River drainage are in dire need of management action. Carefully planned human-mediated dispersal and habitat restoration should be explored as management options across the range of the species.