Group+5+Intro


 * Snail Hunters**: Olivia Murrin, Olivia Niblock, Raveena Patel, Emma Patching, Felicity Hayter-Gare

Sampling Plan





We took samples in sites which were vertically in line with each other along a gradient at high, moderate and low gradient. We then replicated this twice along the hill. All sites were at least 10metres away from each other in order to avoid pseudo replication and to minimise gene flow between the populations found at each site. All samples were taken in grass sites as opposed to shrub and forest areas in the hopes of examining the effects of elevation and to eliminate selection due to environmental differences.

The experimental plan has weaknesses in that elevation is difficult to quantify and even though we selected grass areas only there is often slight variation in environment. The plans’ strengths lie in that it only examines one variable and that we have a total of 9 sites and 3 sets of experiments.

If drift alone were to explain the variation found in the populations we would expect to see sites of similar elevation with different ratios of colours and band numbers. If selection working on a background of drift were to explain the variation then we could expect to see similar populations in independent sites of the same elevation.

Introduction.

//Cepaea nemoralis// was used as a model organism for this experiment. The diverse populations of this species in different environmental conditions make it suitable as a case study for a project on evolution. Many factors contribute to the suitability of //C.nemoralis// such as their size, generation time, accessibility and manipulation. //C.nemoralis// is easily accessible in Western Europe which is why the experiment took place at the Pulpit Hill reserve and different populations are present in diverse environments. The polymorphic shell varies in shell colour and banding patterns with the adult having a black or brown lip around the shell. The shell may be pink, yellow or brown with up to five black or dark brown bands that may be fused together. The highly polymorphic shell of this species enables scientists to study the effect of genetic drift, selection and gene flow on the frequency of a certain phenotype. Due to the slow movements of snails scientists are able to see genetic pattern over short distances.

In 1954 Cain and Sheppard concluded that //C. nemoralis// is subject to natural selection in which the principal pressure that drives this selection is predation. Sheppard (1951) demonstrated that in a wooded habitat where thrushes are a predator of //C. nemoralis// the abundance of different variants of //C. nemoralis// is correlated to the changing of the seasons.¹ Sheppard postulated that visual selection is acting upon the snail as the changing ground colour from brown leaves in the autumn to a green background in late spring means certain colours are disadvantageous during certain seasons making them more visible to predators such as thrushes.

More recent studies have also indicated that selection is operating in //C. nemoralis.// Cameron et al (2012) analysed a changing morph frequency cline of //C. nemoralis// over a period of 43 years.² Samples were initially taken in 1965–67 in Deepdale, Derbyshire, UK in which populations demonstrated a steep cline morphologically, and in 2010 the site was resampled. It was hypothesised that if selection was the purpose of the cline than the cline should remain unchanged when the site was resampled, on the other hand if selection was non-existent the clines would deteriorate over time. The results revealed that the cline persisted in 2010 and concluded that selection was active at either end of the cline.

Genetic Drift has a part to play: However, it is also evident that Sheppard and Cain’s views were not shared amongst the entire scientific community. This is displayed in Roberta L. Millstein’s review article, ‘Distinguishing Drift and Selection Empirically: ‘‘The Great Snail Debate’’ of the 1950s’, as she describes the work of Maxime Lamotte.³ Lamotte concluded that his studies of //Cepaea nemoralis// strongly supported the idea that both selection and drift were at work on the populations of land snails, and it was certainly not just selection as Sheppard hypothesized, since the characteristics of a population were influenced by their relative sizes – i.e. there was a greater effect on snail populations if the population in question was small (this is of course a characteristic of random genetic drift).

This is further supported by work focusing on //Cepaea nemoralis// and genetic studies between populations of them in Belgium: “Genetic variation in two land snails, //Cepaea nemoralis// and Succinea putris from sites differing in heavy metal content” (2005, Kurt Jordaens et al.)⁴. The aim of the study is focused on genetic changes apparent between populations as a result of different soil constitutions, however it does <range type="comment" id="468650696_9">concede</range id="468650696_9"> that the species as a whole “are organisms with limited dispersal ability and analysis of the (micro) spatial genetic structure of several species have revealed <range type="comment" id="468650696_10">an important role for genetic drift in structuring the genetic variability of populations”</range id="468650696_10">

Therefore, there is also a strong case that this particular species is certainly subject to genetic drift, and it is this which has the most prominent effect, rather than selection.

In light of this we collected our own data in order to decide whether drift alone or selection and drift would explain variation in population samples. We took samples in at high, moderate and low elevation and replicated this twice in order to eliminate the possibility of variation due to stochastic changes.<range type="comment" id="468650696_11"> Each site was independent of one another a</range id="468650696_11">nd sample populations of 3o snails were taken. I<range type="comment" id="468650696_12">f there is no significant difference in</range id="468650696_12"> population variation there would be no evidence for selection pressures contributing to the variation of banding and color in the //Cepaea nemoralis// population at pulpit hill.

**References**


 * 1) Genetics (2013) http://www.genetics.org/content/39/1/89.full.pdf+html?ijkey=f27dd4582543b5b2512d7cd4927301b586a7ccaf&keytype2=tf_**.****accessed** **12/11/13**
 * 2) ** @http://onlinelibrary.wiley.com/doi/10.1111/j.1095-8312.2012.02033.x/pdf ****[]**
 * 3) Millstein, R.L (1983) ‘Distinguishing Drift and Selection Empirically: ‘‘The Great Snail Debate’’ of the 1950s’,
 * 4) ** Jordans, K. Et. Al (2005) **“Genetic variation in two land snails, //Cepaea nemoralis// and Succinea putris from sites differing in heavy metal content”