Ned's+late+group


 * Introduction **

// Cepaea nemoralis // (common name Grove snail or Brown-lipped snail) is a species of small terrestrial pulmonate mollusc, belonging to the //Helicidae// family. This attractive snail is a successful coloniser and is found widespread across vast parts of Europe. Its aesthetic appeal has even seen it purposefully introduced to North America (Whitson, 2005). //Cepaea nemoralis// is found in a variety of habitats including woods and shrubs in plains and highlands, gardens, roadsides and even dunes. Like most terrestrial snails //nemoralis// is a hermaphrodite and must mate in order to produce fertilised eggs. It has a relatively slow development; usually taking up to three years to reach full, potentially breeding adulthood (Silvertown, 2011). It can be distinguished from its closest relative // Cepaea hortensis // (White-lipped snail) by the darkened apertural lip at the base of the shell. This particular species of gastropod exhibits a huge degree of variation in shell colour and banding. The presence of a range of phenotypes in a randomly mating population is known as genetic polymorphism and //Cepaea nemoralis// is a classic example that has been the subject of much research over the years. In the early 1900’s it was thought that this case of polymorphism could be explained by the rules of Mendelian genetics (Lang, 1904, 1908). However, simple laws of segregation cannot explain all the occurrences of variation. The development of techniques for the molecular detection of polymorphism has caused pronounced disagreement between scientists. The variation can be attributed to selective environmental pressures on the snails but also the natural drifting of genes through a population. The shells can range in colour; pink, brown and yellow, and can be banded with up to 5 bands. Research has shown that a set of alleles control ground colour, number of bands and type of band, forming a five-loci linkage group. Two additional loci modify the five-banded phenotype (reviewed by Murray, 1975, Cook 2007). This shows a huge degree of variation within a single population but what is it that has caused and maintained these phenotypic differences? Predation is likely to play a role. The Song thrush is the Grove snails’ principal nemesis and has a ‘searching image’ that can govern the way it predates on the snails, but to what extent is this relevant? The elements and parameters that make up the structure of the habitat can play a role in phenotypic variation. Are there any correlations that can help explain the phenotypic variety, is it a case of genetic drift, or is the variation between closely located populations kept low by gene flow? The snails’ ability to reserve the sperm it collects would be expected to help the latter process. <range type="comment" id="0_9">To find out what is dictating </range id="0_9">the varying frequency of the alleles that control shell colour and banding, samples were taken from Pulpit Hill, Lower Cadsen, Buckinghamshire, a study area that exhibits a good range of //nemoralis’// typical habitat; grassland, shrub area and woodland at a range of altitudes.