Group+13

= Polymorphism in //Capaea nemoralis.// =

Introduction

What is a polymorphism?
How does it arise? Why is this important?

Why are we studying snails? What are their characteristics?

Polymorphism occurs when there are two or more visibly different phenotypes existing in the same population of a species. The different forms must be in the same habitat at the same time to be considered a morph. Polymorphism is an extremely important concept in genetics as it shows how important selection is in the evolution of natural populations, but also stochastic  forces (drift and mutations) can also introduce or sustain polymorphism in a population.  Polymorphism is seen in populations of many species, especially plants but we have chosen the snail Capaea  nemoralis  for this study.

The snail super gene makes them a good subject to study as the close linage in the chromosome produces a distinct and easily identifiable phenotype which allows for a __#|clear__ analysis. Shell colour is variable from pink, brown or yellow and the number of bands on the shell also ranges from zero to five, with 1, 3 or 5 bands being the most common. As well as having a number of observable changes within a certain population these snails also have limited locomotion. These snails have been reported to move no more than 20m in a generation; this means that it is possible to collect data from several populations as the populations are found in close proximity to each other due to their habitat types; woodland,shrubbery and grassland can be found beside each other in the environment.

We sampled on Pulpit Hill, Monk’s Riseborough, Buckinghamshire, which is a chalk land area. Our sampling design takes six samples of the //Capaea nemoralis// along a horizontal transect at 2 different habitats, scrub and grassland. Our sample size is 20 snails in each area. We will record the colour and the number of bands on the shells of each snail found. This design includes different environments for comparison and attempts to control other variables that we are not measuring, e.g. altitude. It is possible that there are factors that affect the snails that humans are not aware of, but we __#|try__ to control the variables that we can. Our sample areas are taken 20m apart to try to make them independent but if there is a large amount of gene flow this may not be a great enough distance apart because gene flow can smooth out the allele frequencies over distances given enough time. We could potentially combine our results with another group to increase our replications.



The null hypothesis states that polymorphism in //C.nemoralis// is du e to the processes of genetic drift and gene flow. If this proves to be true we would expect to see inconsistent differences in allele frequencies between our habitats. Our second hypothesis states that polymorphism in //C.nemoralis// is due to the processes of genetic drift, gene flow and selection. If this were true we would expect to see differences in allele frequencies among our habitats but they would be consistent because selection would be acting on the populations for the same advantageous or disadvantageous reasons. Selection can eventually lead to fixation or loss of alleles producing a common polymorphism in those populations.  From our results we hope to understand if random genetic drift, gene flow or natural selection pressures in different habitats cause polymorphism in snails.