Snail+Snatchers+Introduction


 * Eric Boateng
 * Ollie Britton
 * Alex Laybourn*
 * Sophie Nash
 * Jamil Razzaque

//Cepaea nemoralis//is a species of snail widespread in the UK, found in high abundance at Pulpit Hill in Buckinghamshire. Previous studies on //C. nemoralis// have contributed greatly to our knowledge of natural selection. It is an ideal organism to study polymorphism, due to its highly visible traits – namely its shell colour; pink, yellow or brown and the presence of banding on the shell. These traits are representative of individual alleles.

Gene flow is the transfer of alleles from one population to another

//C. nemoralis// has a limited range of movement between generations, around 5-10 m per year (Jones //et al.)//, meaning different populations can be distinguished easily in a small area and the effects of gene flow are reduced. Polymorphism will always be affected by Genetic Drift, the effects of random processes on the frequency of alleles in a given population, however the strength of genetic selection in each case is difficult to determine. A number of selective forces have been proposed to explain the persistence of polymorphism within //C. nemoralis// and also why allele frequencies differ from place to place. Experimentation has shown that predators, birds, show density dependent predation. Analysis of thrush anvils has shown that abundant morphs are preferentially targeted, resulting in frequency dependent selection. The effects on microclimates within sand dunes in England and valleys in the Pyrenees in Southwestern Europe have shown that darker morphs are selected in frost hollows where they are able to absorb more solar radiation and become more active earlier.

In the investigation an interrupted horizontal transect will be performed that includes two of the different habitat sites found at Pulpit Hill; scrubland and woodland. Three samples of each habitat will be collected along the transect.

A transect has been chosen so that all sampling sites are at the same elevation, in order to monitor the effect of elevation in the investigation. The sampling sites are no less than twenty metres apart, so that each sample can be taken as a different micro-population; hence, the________ samples.**__Gene flow does occur , either directly or over time through intermediate populations, however, the effects will be minimal with distance in comparison to other processes such as physical barriers.__** **__is most likely that.__** There will be clear differences between the sampled habitats, including shade and background colour, therefore selection for polymorphisms in each habitat could vary greatly. **__A__** **__standard for distinguishing these polymorphisms observed in the snails has been devised, where we decided to compare the colour of each shell against the presence or absence of banding on them to distingush these polymorphisms observed.__** A thorough description of the habitat site, which we will be able to use in our analysis, will be taken. Though the grassland will not be sampled, the d ifferences observed between different habitats may not be affected by the variables we are controlling such as elevation and gene flow.

The hypothesis for our investigation are as follows;


 * Null Hypothesis - There <range type="comment" id="383938166_9">is no difference in underlying </range id="383938166_9">frequencies of polymorphisms between habitats; we will not be able to conclusively distinguish selection from genetic drift.
 * Alternate Hypothesis - There will be a significant difference in the frequency of polymorphisms in each habitat. Woodland //C.nemoralis// are expected to show higher frequencies of darker snails, due to visual predation, than scrub. <range type="comment" id="383938166_10">**__This finding will be consistent across our samples in s</range id="383938166_10">imilar habitats,__** reducing the likelihood that Genetic Drift (chance) is solely responsible for the frequency of polymorphisms.

__References__ Jones, J. S., Leith B.H. and Rawlings, P. (1977) //Polymorphism in// Cepaea//: A Problem with Too Many Solutions?// Annual Review of Ecology and Systematics, 8, 109-143 Wardhaugh, A. A., (1989) //Land Snails of the British Isles//, Aylesbury, Shire Publications Ltd
 * __Su H, Qu LJ, He K, Zhang Z, Wang J, Chen Z, Gu H (March 2003). "The Great Wall of China: a physical barrier to gene flow?". Heredity 90 (3): 212–9__**