Group+23

**__­­Introduction Group 23:__**

 * __Members:__**

Rupert Higgins Gemma Shireby Christina Pope Jacqui Moneke Jessica Patricot

// Cepaea nemoralis, // more commonly known as the Brown-lipped snail, is the most abundant species of land snail found in Western Europe. It is highly polymorphic for shell colouring and banding which is ideal for scientific study into genetics; the evolutionary forces vary so greatly from different populations that no single conclusion can be drawn. Each population needs to be accounted for separately as the selective mechanism acting on the polymorphism can differ depending on habitat, climate, founder effects, chance etc.

The genes for shell colour (yellow, pink and brown), banding (from zero to five bands) and lip colour are all closely linked so crossing-over rarely occurs. This is known as a supergene (Jones et al. 1977). This provides the notion that //C.nemoralis// populations living within close proximity of each other (even just a couple of hundred metres apart) can have fixed alleles on a certain locus differing from other populations.

The reason scientists study //C.nemoralis// over humans is the fact that the snails are easily obtainable and have a very limited travelling distance; usually about 20m in their lifetime. This restricts gene flow between populations (assuming samples are collected from over 20m’s apart) so you can study several populations separately within quite a small distance. As well as this they have very distinct polymorphic patterns which are easy to identify.

6 samples are to be taken from two habitats; grassland and shrub (3 samples from each). 20 snails are collected from each sampling area within a fixed radius (6m). The samples were taken 30m’s apart <range type="comment" id="383227426_9">to avoid the effects of gene flow</range id="383227426_9"> on any data collected (this ensures selection and drift are the selective mechanisms being studied). However <range type="comment" id="383227426_10">extraneous variables still may play a part in skewing the data. For example the predator of the //C.nemoralis//, the //Turdus ericetorum// (common name; Thrush) may drop shells in random areas or remove snails from the sampling area causing a slight skew in results. This is why repeats are done in order to collect reliable data which in turn can be valid; where the true conclusions can be drawn.</range id="383227426_10">

If we look at prior research such as that by Jones et al (1977) we can see <range type="comment" id="383227426_11">that the conclusions drawn shows</range id="383227426_11"> that the frequency of yellow shells depends on the cool climate and predation. The yellow protects them better against the green background however the predator’s frequency-dependant searching behaviour favours the rarer alternative colour alleles. Although in this study the yellow shell allele was favoured the importance of particular alleles <range type="comment" id="383227426_12">will</range id="383227426_12"> vary from population to population.


 * __ Hypothesis: __**

There will be a significant difference in polymorphism within snails between two different distinct habitats (grassland and shrub) and the variation can be explained due to either a selective pressure or genetic drift.


 * __ Null Hypothesis: __**

There will be no significant difference in polymorphism within snails between two different habitats and the lack of variation would be due to limited selection and genetic drift. The lack of variation is likely to be due to gene flow between populations ensuring similar characteristics are constantly flowing from population to population so no variation is acquired.


 * __References__**

Jones, J. S., Leith B.H. and Rawlings, P. (1977) //<span style="font-family: 'Calibri','sans-serif';">Polymorphism in // Cepaea//<span style="font-family: 'Calibri','sans-serif';">: A Problem with Too Many Solutions? // Annual Review of Ecology and Systematics, 8, 109-143

__**Sampling technique:**__

 * __ Distinguish between Genetic Drift, Selection and Gene flow: __**


 * // <range type="comment" id="383227426_13">Genetic drift: //** would be the effect of random processes causing the snails to differentiate from one another in different areas without some known founder effect or selective pressure.


 * // Selection: //** would be the idea that the polymorphism between snails is the outcome of some sort of selective pressure. For example predation; some snails may be more likely to survive with a particular colouring or pattern so selection has caused this phenotype to become prominent within the population.


 * //Gene flow:// ** This is the idea of populations being in close proximity causing the movement of genes between habitats, limiting the process of genetic drift.</range id="383227426_13">

We will have one horizontal line transect with three sample areas (each at least 20m apart) in shrub lands and grass. On arrival to the field we will choose a horizontal strip abundant in both scrubland and grassland, we will chose this area when we find at least one large populations of snails and conduct the line transect from there. We have combined with another group who will do a<range type="comment" id="383227426_14"> horizontal line transect at another altitude.</range id="383227426_14"> <span style="font-family: 'Calibri','sans-serif'; font-size: 13.3333px;">By collaborating with another group the human error in the results may be greater.
 * __ Sampling Method: __**
 * __ Advantages: __**
 * Each sampling area is 20m apart so the data will not be affecting by gene flow
 * Choosing to sample at one altitude eliminates altitude as a variable
 * Combining with another group who will carry out a line transect at a different height provides us with more data and helps us to see if altitude is an important factor in determining snail habitat and selection processes
 * We have specifically chosen sampling areas instead of random sampling so that we can see clear results in selection between grassy and shrub areas and to ensure each sample area consists of large populations that we can compare.
 * We have chosen an equal number of sampling areas for both grass and shrub and no woodland so that we have as much data as possible to compare between the two areas.
 * By choosing three sample areas in both the shrub and grass areas <range type="comment" id="383227426_15">we are able to replicate our results in order to increase its reliability.</range id="383227426_15">
 * __ Disadvantages: __**
 * There may not be a horizontal line of 3 areas of both grass and shrub land
 * <range type="comment" id="383227426_16">Not being completely random could bias </range id="383227426_16">our results in an unforeseen way
 * Choosing a single line may not be a<range type="comment" id="383227426_17"> true representation of the whole area that we are sampling in.</range id="383227426_17">