The+Fellowship+of+the+Snail


 * Marcus Yarwood *
 * Michael Willis
 * Clare Stockwell
 * Gideon Pomeranz
 * Miriam Harbourn


 * __Sampling Plan__**
 * Sample from 2 different habitats – 3 in grassland and 3 in woodland
 * Samples taken from the same altitude
 * 4x4 metre square for each sample
 * Samples must be at least 20 metres apart measured from the edge of the 4x4m square
 * No samples will be taken from any ground that is regularly used by humans, for instance foot paths
 * If possible measure along the side of woodland that receives the most sunlight which will reduce the chances of gene flow as it is assumed that there is a selection pressure on shell polymorphisms based on the amount of sunlight


 * __Strengths__**
 * Samples will only be taken from 2 different habitats
 * Due to the low sample size introducing other variables will complicate the study
 * Sampling the habitat of shrubs will not be included
 * Clear difference in the habitats of grass and woodland based on light levels
 * This will show clear differences in shell polymorphisms because it is assumed in the study that there will be different selection pressures in different light levels. This will also clearly highlight genetic drift if it is found that one sample is far different from the others
 * Shrubs are isolated within grassland and have similar light levels; this means there is a much higher chance of gene flow which is not going to be tested in this study
 * Sampling along the same altitude
 * This eliminates another variable from complicating the study
 * Sampling technique
 * The 4x4m square for each sample will ensure maximum numbers of snails in the study which will increase the accuracy of conclusions
 * The samples are 20 metres apart which will reduce the risk of gene flow between populations and confirm the samples are independent of one another
 * Measuring in woodland will increase the amount of snails caught
 * Woodland is generally much moister than grassland, increasing our chances of catching more snails


 * __Weaknesses__**
 * Number of samples is very limited
 * The study would benefit from pairing with another group to increase the number of samples from the specific altitude thereby increasing the accuracy of the study. However too many samples would increase the risk of comparing populations that undergo gene flow
 * The area of sampling is very small
 * A number of studies investigating //Cepaea nemoralis// have measured many hundreds of metres to determine an overall correlation. The small local population over a potential maximum of 100m is very small.
 * The number of variables is small
 * In an ideal world increase number of variables, such as measuring shrubs and also measuring the boundaries of different habitats to compare and be able to recognise gene flow.
 * Limit on space
 * In the area provided there may not be enough room to have each sample at 20metres apart. If this is the case the distance between samples will be reduced


 * __Distinguishing between genetic drift, selection and gene flow__**

//In an ideal situation// Selection – The same ratios of shell polymorphisms between all samples of differing populations e.g. 80% heavily banded snails in woodland and 80% lightly coloured snails in grassland

Gene flow – The same ratio of shell polymorphisms in adjacent samples on a boundary of environments e.g. 50:50 dark to light colouration in both woodland and grassland. //Note: The study hopes to eliminate gene flow between our samples//

Genetic drift – Different ratio of shell polymorphisms in a sample compared to the other two, so a very high ratio of dark banded snails in grassland which goes against our assumed selection pressures and is likely to be due to genetic drift

//What is expected// It is expected that all three of these processes will have an influence on all of the samples, producing results that are difficult to distinguish if gene flow or selection is having an effect. It could well be that all of the ratios in the samples are mainly due to genetic drift.