Nephrops+-+Introduction

Liz Lynch Emily Carter Kirstie Loh Hannah Newcombe*

This experiment focuses on the snail //Cepaea nemoralisis//, using the clear visual polymorphisms to identify the effects of genetic drift and selection. Polymorphism is defined as the presence of multiple alleles within one species and in this experiment, the resulting phenotypes will be used to identify each allele. We are using snails rather than humans due to the easily identifiable polymorphisms, their high abundance in England, the short generation times and the short distance the average snail moves in a lifetime. We have allowed 20 meters of movement in one generation however studies suggest that generational movement is between 5 and 10 meters (Jones et. al. 1977, Wardhaugh 1989). The phenotypes we expect to observe are the presence or absence of banded pattening on the shell and different shell colours of brown, yellow, or pink.

From this study, we hope to identify whether there is selection acting on the distribution of these phenotypes or whether differences in distribution are due to genetic drift. Our hypothesis states that there will be selection acting on snails in different habitats.

Six samples of at least 30 will be taken from two environments, three from the grassland and three from the bushes, all along the same elevation. This means that we were only looking at one variable, the environment, increasing the internal validity of this study. It also allows us to increase the number of times we can replicate the sampling at each site, increasing credibility. A weakness of this sampling method is that we are not testing for all variables, elevation could have an effect on the polymorphisms present in different populations but we feel that with our time and resources that this is a reasonable compromise to take. We will also allocate one person to categorize each snail in order to maintain consistency. Each site will be at least 20m apart to eliminate the chances of gene flow between //C. nemoralisis // in different environments, this should give more defined results indicating either genetic drift or selection is occurring. The design is slightly biased as we will choose sampling sites that have a high population density, this allows us to get big enough samples in each group to be analysed, the bigger the samples hopefully the bigger any differences will be.</range id="380879196_4">



<span style="font-family: Arial,Helvetica,sans-serif;">The hypothesis for this is study is that there is significant selection acting on the populations of //<span style="font-family: Arial,Helvetica,sans-serif;">C. nemoralisis //<span style="font-family: Arial,Helvetica,sans-serif;"> in different environments of the same altitude. Evidence to support this hypothesis would be consistency in the allele frequency between all the //<span style="font-family: Arial,Helvetica,sans-serif;">C.nemaralisis //<span style="font-family: Arial,Helvetica,sans-serif;"> in the same environment but differences between those in different environments. This would suggest that selection is working on the populations and not genetic drift. Evidence supporting our null hypothesis of 'there is no selection acting on the populations of //C. nemoralisis//' would be no significant consistency between samples from the same habitat type. If this were found, we would assume that any differences observed are due to genetic drift and founder effects rather than selection acting on the species.

<span style="font-family: Arial,Helvetica,sans-serif;">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 <span style="font-family: Arial,Helvetica,sans-serif;">Wardhaugh, A. A., (1989) Land Snails of the British Isles, Aylesbury, Shire Publications Ltd