Paul_Roach+introduction


 * Evolutionary Genetics Field Trip Survey **


 * Introduction **

//Cepaea nemoralis // also known as the Grove Snail or Brown lipped snail is one of the most numerous and polymorphic species of land snail in Europe .I t is typically found to inhabit woodlands, meadows and residential or inner city gardens, it is nocturnal and usually feeds on dead or decaying vegetation. They have long been used by biologists as an research model in the study of heredity and evolutionary genetics. They have been extensively studied over the last 100 years and have subsequently become crucial in our understanding of natural selection in the wild (Clarke //et al// 1978). Since the 1940s, //C. nemoralis// has been used as a biological model species within the long held debate of selection versus genetic drift. It is an ideal species for this purpose due to its conspicuous polymorphism in both shell colour and number of bands. Individuals can easily be identified on site and then released. They exist in discrete populations with only occasional gene flow between populations, moving on average 5-10m per year (Cameron and Williamson, 1977). The study of this species provides valuable insights into the polymorphisms of other species that are not as easy to get inferable data from. Thousands of populations across Europe have been studied and still there is no conclusive answer as to the maintenance of the snail's polymorphism. Early studies have shown contradictory results in favour of both selection (Cain & Sheppard, 1954) and genetic drift (Lamotte, 1951). An even broader range of possible environmental factors has been discussed by Cook (1998), such as climatic fluctuations and human activities that both appear to rapidly alter snail habitats, which in turn has a knock on effect in terms of the selection pressures. Opinion has moved away from a single unifying solution, according to Jones //et al//. (1977), there are eight evolutionary processes that have been identified to be acting upon //C. nemoralis// populations. These processes are thought to act with varying intensities from population to population; therefore each must be treated as a unique case with its own unique evolutionary history. Modern evolutionary biology has followed this line of thinking; our explanation of observed patterns must be explained not only by natural selection but also by random events, migration, interbreeding, mutation and horizontal gene transfer (Lewontin, 2002). The purpose of this survey is to understand how polymorphism is maintained within a species. By conducting a comparative investigation of gene frequency distributions within //C. nemoralis// populations, we will be able to explain which evolutionary forces are acting upon the population and to what extent. This will be achieved by recording shell colour and banding patterns from individuals within various different populations located in the same meta-population. We will look for patterns in our results that correlate with patterns expected if visual or climatic selection was occuring. For example, if brown shelled snails are consistently found in areas with dark backgrounds and yellow shelled snails in open grassland, we can reasonably suggest that visual selection is taking place. If we rule out the significant effect of natural selection on the populations, we may then try to establish whether or not genetic drift is having a predominant effect. Lamotte (1951) devised a method of identifying genetic drift by looking for a signature pattern in gene frequency distributions. He suggested that if genetic drift was at work, the allele frequencies would be constantly fluctuating and would be more apparent in smaller populations than in larger ones. Therefore if he were to simultaneously sample multiple populations of different sizes, larger differences in allele frequencies would be observed between small populations than between large populations. In this way he was able to confidently claim that genetic drift was the main evolutionary force acting on //C. nemoralis.// However, due to the time constraints of this study we will be unable to replicate his methodology. As a consequence, we will only be able to establish the degree to which natural selection is acting upon the population. Therefore, our null hypothesis is that at each site there will be no significant difference between allele frequencies. <span style="font-family: 'Arial','sans-serif';">Word count: 679 <span style="font-family: 'Arial','sans-serif';">Contributing members <span style="font-family: 'Arial','sans-serif';">Paul Roach <span style="font-family: 'Arial','sans-serif';">Joshua Costa <span style="font-family: 'Arial','sans-serif';">Satyajeet Barot <span style="font-family: 'Arial','sans-serif';">Rajkrishnna Rajaratnam <span style="font-family: 'Arial','sans-serif';">Saran Kuttiyandisamy **<span style="font-family: 'Arial','sans-serif';">Refrences **

<span style="font-family: 'Arial','sans-serif';">Cain, A.J. and P.M. Sheppard. 1954. Natural selection in //Cepaea//. //Genetics// **39:**89-116

<span style="font-family: 'Arial','sans-serif';">Cameron, R.A.D. and P. Williamson. 1977. Estimating migration and the effects of disturbance in mark-recapture studies on the snail //Cepaea nemoralis//. //Journal of// //<span style="font-family: 'Arial','sans-serif';">Animal Ecology //<span style="font-family: 'Arial','sans-serif';"> **46**:173-80 47.

<span style="font-family: 'Arial','sans-serif';">Clarke B (1978) Some contributions of snails to the development of ecological genetics. In: Brussard PF (ed) //Ecological genetics – the interface//. Springer, New York, pp159–170

<span style="font-family: 'Arial','sans-serif';">Cook L. M. (1998). A Two-stage Model for //Cepaea// Polymorphism. //<span style="font-family: 'Arial','sans-serif';">Phil. Trans. R. Soc. B // <span style="font-family: 'Arial','sans-serif';"> **353** (1375): 1577–1593

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

<span style="font-family: 'Arial','sans-serif';">Lamotte, M. 1951. Recherches sur la structure génétique des populations naturelles de //Capaea nemaralis.// Bulletin Biologique de la France et de la Belgique **35**:1-239.

<span style="font-family: 'Arial','sans-serif';">Lewontin, R.C. 2002. Directions in evolutionary biology. //Annual Review of Genetics// **36:**1-18.

**Discussion**
Our results showed a significant difference when performing a two tailed chi squared test on the sample population of the top and the bottom of hill one. This is a similar case for when we compared the top and bottom of hill two but there was even more of a significant difference. But when we combined the results of top and bottom of hill one and two, the chi squared test showed no significant difference. This could be due to the difference in the two hills location or even the structural shape of the hills so visual selection could be a major factor. Selection seems more of a possibility across the two hills after testing the top of hill one and top of hill two which showed that the two tailed P –value is less than 0.0001 so the difference is considered to be extremely statistically significant. This suggests that random sampling from populations on top of different hills of similar locality would lead to a difference smaller than you observed in 99.99% of experiments and larger than you observed in 0.01% of experiments. The P– value provides us with a probability of the chance that the difference in our sample population can be observed in the actual population. But these results can be rejected due to sampling error because of the lack of repetitions in our results which would have given us more accurate mean values.

Although we had relatively low sample sizes and lacked multiple replications, there still appeared to be far more dead snails observed overall in the open areas and a high number dead adult light coloured snails to live ones, also a high number of dead dark snails compared to live ones in the wooded areas. Jones reported strong evidence of visual selection and a decrease in snails killed from April to May as the colour of the background changed from brown to green (1977), this indicates that seasonal differences are having a marked effect on predation and therefore serves to maintain the polymorphic variation in populations.The significant difference in allele frequencies between top and bottom of both hills and an insignificant difference in comparison between hills .This could possibly be evidence of natural selection occurring simultaneously at both hills overtime in both the open areas and wooded ones. We see slight evidence of visual selection in the open grasslands due to the amount of dead light coloured snails compared to live brown ones but this could also be a result of gene flow also Our results also illustrates flaws in our design plan which could have improved our results such as; comparing numbers of dead to alive in different sites and using larger sample sizes. for example, if light colored snails appeared to be surviving till adult hood in the open areas and the dark colored ones also doing the same in the more wooded areas, remains of predation and death would provide the evidence for this if compared with numbers and colours of both dead and alive snails .this would increases their chances of breeding and propagating their genes and would provide further evidence that polymorphism are pretty constant and non-random environmental factors were selecting for that particular beneficial trait that allows Some of them to evade predation before mating., as suggested by Lamotte (1951) we could the then attempt to rule out the effects of natural selection ,which in turn would allows us to investigate the degree to which genetic drift Or gene flow is acting on the different populations. However, as stated by Jones et al (1977) acceptance of one evolutionary factor need not necessarily involve rejection of the other non-random.

There seems to be evidence for visual selection in some cases (eg significantly more yellow at Top2, an open area, than at Bottom2, a bushy area near woodland.) → but even so, polymorphism remains in these populations – so the patterns observed cannot be explained only by visual selection – if this was the only force acting, eventually one colour or banding pattern wold have become fixed (or very dominant). → there must be other processes in action (gene flow, frequency-dependent selection, founder events). Without more data (replications) we cannot further our understanding of the cause of polymorphism in C.nemoralis, currently we don't have enough data to form a representative sample. - if after repeated sampling yellow-shelled snails were consistently found to predominate in more open, grassy areas then we could reasonably conclude that visual selection was taking place. Currently we are unable to rule out sampling error and design flaws as the cause of our observed patterns. As we did not sample the whole population we cannot fully reject the null hypothesis due to the 5% confidence limit, an increase the sample size to gain a larger insight into the true population could increase clarity. Further to this we already know the populations of the shell colour are affected by seasons, studying the polymorphs at spring and early summer would account for the maintenance of the snail's polymorphism. Another variable disregarded for within our study was the analysis of alive and dead snails observed. We could further investigate whether the grove snail found at each site compared with the knowledge that light shelled were more prevalent at the top of the hill (Cain & Sheppard, 1954) but whether they were more suspect to predation in the shaded woodland, by turning to association between morph and habitat. Our study was based on the altitude of the hills studied but temperature went unaccounted for. In Silvertown (2011) it was found that increased temperature increase all morphs, but these limits were unattained or measured within this study.

<span style="font-family: Arial,sans-serif;">Refrences Jonathan Silvertown, Laurence Cook, Robert Cameron, Mike Dodd, Kevin McConway, Jenny Worthington, Peter Skelton,Christian Anton,¤ Oliver Bossdorf, Bruno Baur, Menno Schilthuizen ,Benoît Fontaine, Helm. (2011). Citizen Science Reveals Unexpected Continental-Scale Evolutionary Change in a Model Organism. Available: http://hubs.plos.org/web/biodiversity/article/10.1371/journal.pone.0018927. Last accessed 30th Nov 2011.