shell+wars

Lucy Vialls* Jennifer Mcguinness Elise Rees

Introduction:

In this project we are investigating polymorphism in the snail species // Cepaea nemoralis. // This species shows morphological differences in shell colour and banding patterns making it an ideal organism for this study. The snail is a good organism to use for data collection rather than a mammal species as it moves at a very slow rate meaning populations migrate to new areas very slowly too, moving only within tens of metres a year. The area we used for data collection was chosen as the ground had a high calcium content used by the snails for shell production. Our aim is to investigate whether these differences, across a small area, show a selective reason for the differing phenotypes e.g. adaptation to differing habitat/predation, changes due to genetic drift alone, or a combination of the 2. To achieve this we collected data of the snails in 6 populations recording whether they were dead or alive, adult or sub-adult, shell colour and number of bands. We had to consider when designing our experiment to collect our populations from specific areas so that the chance of pseudoreplication and sampling error was limited. Our experimental design originally was to collect from 3 different types of plant habitat (grassland, hedges and trees) at the same altitude however once visiting the site we found this impractical so instead collected 3 populations from hedge habitat at 2 different altitudes for our 6 samples. This was to see whether altitude has any effect on the average snail morphology and whether we can gain an understanding from this as to the evolutionary cause for the change in genetics. The samples collected at the same altitude were taken at approximately 10m apart and the difference in the altitude between the 2 sets of 3 was approximately 5m. Explanations for polymorphism in the snails include genetic drift, the changing frequency of alleles in a population due to random chance, this has a much greater affect within small populations. This could be affecting the snails as they move very slowly so are fairly isolated to mating within their own populations. Genetic bottlenecks can cause great differences between populations when within a population the number of individuals who mate is dramatically decreased, <range type="comment" id="469784336_1">then the alleles expressed in the subsequent generations</range id="469784336_1"> may change drastically from the original, with differing diversity among the individuals. Another explanation could be natural selection due to the pressures of surviving at different altitude (differing predation levels possibly). I<range type="comment" id="468808352_9">f these pressures are acting on the snails we should find evidence through our findings.</range id="468808352_9"> In our discussion we will analyse our data to try and find the best solution as to why polymorphism is exhibited among these snail populations.

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