Carissa's+Group


 * Polymorphism in //Cepaea nemoralis//**

**Introduction:** //Cepaea nemoralis// is a small land snail that shows polymorphism – when two or more distinct phenotypes exist in the same population. The traits that we will be studying are shell colour (brown, pink or yellow) and number of bands on the shell. We have opted to study polymorphism in //C. nemoralis// since their phenotypes are easily identified and directly show their genotypes. Gene flow has a lower influence than compared to humans, which are more complex to study. //C. nemoralis// is therefore easier, less time-consuming to study, and results can also be applied to humans. The evolutionary processes that contribute to polymorphism are gene flow (migration of alleles from one population to another), mutation (changes in DNA sequence), genetic drift (random changes in allelic frequency) and selection (when a trait affects fitness and so becomes more or less common).

The main traits that show polymorphism in this species are colour of shell (pink, yellow or brown) and banding (between zero and five bands). These differences are thought to be a camouflage, to avoid predation, or are related to body heat (darker shells retain hea t easier) or may be selectively neutral. Polymorphism is thought to be affected by these environmental factors, which is supported by the fact that darker, banded snails tend to be found in woodlands whilst light and less banded individuals are found mainly in grasslands.

Apart from observing the colour and banding of //C. nemoralis//, we will also take into account the contour of the area (e.g. if it was flat or at an incline, etc), weather conditions (e.g. as wet conditions increases the number of snails visible, etc).

Differences in shell colour and band number in different populations may be due to either genetic drift or selection. If genetic drift affects this frequency, our results in one habitat would show no definite relation if compared to the other, since the change in frequency would be random, regardless of the environmental conditions. If selection was acting on phenotypic frequency, it would show ecological interactions with the population, therefore increasing frequency of advantageous phenotypes, and reducing that of disadvantageous phenotypes.

In order to determine this we will be observing six sample areas of more than twenty snails in shrubland and grassland habitats in two separate valleys. The samples are circles with radii of four metres and will be allocated by random sampling methods. We will locate the centre of each habitat-type and perform a 'random walk' where we will use a compass and randomly generate numbers between 0 and 360 (to give compass bearing) to determine the direction in which to go in and then continue walking in that direction until we find a snail, then look for more within the four metre radius. Location of subsequent sample areas are determined by the midpoint of the previous one, whilst avoiding overlap of areas.

The aim of our observations is to determine which of the two aforementioned factors is responsible for polymorphism in //Cepaea nemoralis//.

**Hypotheses:** For colouration Null hypothesis (Ho): There are no significant differences between the numbers of snails of different colours in different areas (woodland and grassland).. Hypothesis (H1): There are significant differences between the number of snails of different colours in different areas (woodland and grassland).

For banding: Null hypothesis (Ho): There are no significant differences between the numbers of snails of different banding in different areas (woodland and grassland). Hypothesis (H1): There are significant differences between the number of snails of different banding in different areas (woodland and grassland).

**Results:** We collected our results from the grasslands and woodlands of two separate valleys (Valley 1 and Valley 2) and recorded the numbers according to their colour (brown, pink or yellow) and banding (grouped as having 0 bands; 1 or 2 bands; 3 bands or 4 or 5 bands). The majority of the snails we collected were dead – the only exceptions were the few found alive under leaves and hibernating in the woodlands so we have decided to group all dead and live snails of the same colour together. We will be using the Chi-squared test to determine whether our hypotheses can be either accepted or rejected.

The first set of comparisons we did were to see if there are significant differences in the number of snails of each colour between the results of: And the Chi-squared results are as follows: > 35.35 > 9.21 (tabulated chi-squared value) therefore Ho is rejected at p=0.01 > > In Valley 2: grassland and woodland comparison – chi-squared = 28.42 > 28.42 > 9.21 (tabulated chi-squared value) therefore Ho is rejected at p=0.01 > > > 5.12 < 9.21 (tabulated chi-squared value) therefore Ho is accepted at p=0.01 > 2.48 < 9.21 (tabulated chi-squared value) therefore Ho is accepted at p=0.01
 * 1) the grassland and woodland within each of the valleys;
 * 2) the grasslands in both valleys;
 * 3) the woodlands in both valleys
 * 1) In Valley 1: grassland and woodland comparison – chi-squared = 35.35
 * 1) Grassland 1 and Grassland 2 comparison – chi-squared = 5.12
 * 1) Woodland 1 and Woodland 2 comparison – chi-squared = 2.48

Our second set of comparisons were to see if there were significant differences in the banding of the snails found regardless of their colouring between the results of: And the Chi-squared results are as follows: > 26.61 > 9.21 (tabulated chi-squared value) therefore Ho is rejected at p=0.01 > > In Valley 2: grassland and woodland comparison – chi-squared = 1.88 > 1.88 < 9.21 (tabulated chi-squared value) therefore Ho is accepted at p=0.01 > > > 5.10 < 9.21 (tabulated chi-squared value) therefore Ho is accepted at p=0.01 > > > 17.88 > 9.21 (tabulated chi-squared value) therefore Ho is rejected at p=0.01
 * 1) the grassland and woodland within each of the valleys;
 * 2) the grasslands in both valleys;
 * 3) the woodlands in both valleys
 * 1) In Valley 1: grassland and woodland comparison – chi-squared = 26.61
 * 1) Grassland 1 and Grassland 2 comparison – chi-squared = 5.10
 * 1) Woodland 1 and Woodland 2 comparison – chi-squared = 17.88

**Discussion:** The results of colour prevalence show a statistically significant difference between two different habitats in comparison to intra-valley. There is a selection pressure for yellow within the grassland habitat as opposed to more brown snails found in the woodland. This is seen across both valleys and there is no statistically significant different between the habitat type of different valleys. The factor causing this difference could be explained by selection pressure for colour type according to the environment. A preference for lighter colours in grassland in comparison to darker colours in woodland can be explained by camouflage adaptation, whereby there is a predation pressure for light in grassland and dark for woodland. The main predator, the thrush, would build up a search image for the more identifiable colour within the habitat. Another explanation for this colour distribution could be an adaptation to temperature absorption. //Cepaea nemoralis// is a cold blooded invertebrate; consequently the blood warms up by solar radiation. This means that the darker coloured shells will heat up quicker than the lighter colours, given the same amount of light. The woodland habitat has lower light exposure so a darker shell will be selected for. The reason that the darker shell colour is not more prevalent in the grassland is because there is a pressure for camouflage; therefore the lower predation pressure outweighs the sooner activity.

The results of banded in Valley 1 show a statistically significant difference between number of banded snails and woodland or grassland. There were higher numbers of banding on the snails within grassland in comparison to less number of banding within woodland areas. This could be accounted by predation and temperate pressure. However, there is no statistically significant difference in Valley 2, and there was a difference between the woodlands in different valleys – indicating that there is no factor causing differences in bandedness in relation to the habitat. The differences in bandedness could be due to genetic drift.

On the other hand, the results could be due to other factors such as inaccuracies in sampling and human error or selection. Sampling inaccuracies include the small sample size in both woodland areas, which were much lower than the sampling size of both grassland areas. Another inaccuracy is the small sample area and the number of areas measured. To increase either or both of these would increase the probability of identifying a higher number of snails, therefore increasing the precision of the statistical interpretation. The number of valleys is also the minimum number for comparison; therefore to increase accuracy further valleys could be measured. Human errors include inefficient searching for snails that are well hidden; also easily visible snails were more likely to be found which gives an unfair representation of snail distribution. Reducing any of the above errors would improve the statistical validity of this experiment. Further research could be conducted to test the theories of temperature dependency and camouflage pressure.

Word count: 485

Further reading: // Jones et al 1977. “ // Polymorphism in // Cepaea: A Problem with Too Many Solutions?” Ann Rev Ecol Syst //** 8 **// :109-143 //

Group Secretary: Carissa Chu bt09338@qmul.ac.uk Judit Bagi bt09254@qmul.ac.uk Helen Zhou bt09065@qmul.ac.uk Tom Gasan bt09351@qmul.ac.uk