Akhtar's+group.+Introduction.

//Cepaea nemoralis// are air breathing land snails which display a variety of band colours (usually pink brown or yellow) and band patterns on their shells (ranging from 0 bands to 5 bands). They show polymorphism; when two or more prominently or notably different phenotypes exist within the same population of a species.
 * __ Introduction __**

The primary factor in choosing to study //Cepaea nemoralis// is that their phenotypes display their genotypes clearly which allows rapid scoring of the genetic variation within and between the populations – allowing us to infer the effects of genetic drift and selection. Also, the findings and evolutionary principles we come across may be applied to human populations in some cases or may be used to make assumptions about these evolutionary principles in humans.

Genetic drift is a random process; it refers to changes in the frequency of those particular alleles that are caused by chance. Selection is the process whereby organisms better adapted to their environment tend to survive and produce more offspring i.e. when a certain trait is forced out or carried to the next generation depending on it’s’ fitness benefits. Gene flow is the movement of alleles from one location to another while mutation introduces alleles into the population via changes in the DNA sequence of bases.

Polymorphism is best observed using the characteristics earlier mentioned; colour of the shells and the number of bands. However, we will also take into account the altitude at which the snail is found. (I.e. was it found at the top of a hill or at the bottom?). This is an environmental difference which has previously been studied, for example a study conducted by Honek, A in Czech Republic found that Geographic trends (such as altitude) and habitat differences in morph frequencies may be affected by climatic selection.

In this example, the different habitat-types will be sampled to test two hypotheses:

__Null hypothesis__: There is no significant difference in the distribution of different coloured and banded snails found at high and low altitudes.

__Hypothesis__: there is significant difference in the distribution of different coloured and banded snails found at high and low altitudes.

To test these hypotheses, several samples will be taken from each habitat-type. This is because if there is no significant difference in findings from the same habitat-types, then we may be able to conclude that selection is occurring. If the results from several of the same habitat-type are not similar to each other, then we can conclude genetic drift is occurring.

The climate/environment may affect our results as dark coloured snails tend to retain heat better than those that are light in colour. This may affect the frequency at which certain phenotypes are found at high or low altitude (due to the difference in temperature/climate at these different altitudes).

We will be taking readings from three different altitudes from the left to the right of the hill to give our results more reliability and to cancel out any experimental deviations and human error. I.e. by taking samples from several different locations we are accumulating more results so can be more confident in our results. This will allow us to determine whether genetic drift or natural selection is taking place based on whether the results are the same at the different locations or not. If a decline or rise in frequencies is observed at different altitudes, the steepness can be used to judge the extent of gene flow between the different habitats.

__**Discussion**__

Looking at the chi-squared results, there is a significant difference between all three sample habitats in the high and low altitude areas. ‍ Based on the statistical findings, we can reject the null hypothesis of ‘there is no significant difference in the distribution of different colours and banded snails found at high and low altitudes’. Our initial aim was to examine colours and bands, looking at how these varied at the different altitudes. However, when analysing the data, there was no strong correlation between altitude and number of bands so it seemed more appropriate to investigate the link between colour and altitude.

Although, the experiment was to compare the differences in the populations at the high and low altitudes, when analyzing the results it became clear that there was also variation between the different populations at the same altitude. Due to the fact that we collected the samples from the same habitat type, it is not likely that this variation is due to selection, leading us to conclude that genetic drift is the more probably explanation.

With these sample groups living in the same habitats at the same altitude, it could be hard to comprehend the difference in phenotype variation. However, taking into account the sedentary lifestyle of //Cepea Nemoralis//, it is easy to assume that there will be little gene flow between populations living more than a few metres apart. Goodhart (1963) results showed a mean dispersal distance of 5.5m. This is a relatively small distance travelled over quite a large time period and could explain the degree of local genetic differentiation that is ‍ known to occur with //C. nemoralis//. It would possibly lead to frequent bottlenecks and founder effects. Another factor that contributes to this is the strong random genetic drift not being counterbalanced by gene flow (Le Mitouard, Bellido, et al, 2010). ‍

Another theory for the evolution of morphological area effects could be environmental selection by microclimatic features (Ochman, Jones, et al. 1983). This would mean that each of our sample groups would be living in their own specific microhabitat. ‍ This would mean the differing morphological features of the populations would not be caused by genetic drift but in fact natural selection in habitats that appear to be very similar but could be affected by microclimatic aspects.

Natural selection could also be a contributing factor to the morphological differences in the high and low altitude populations of //C. nemoralis//. The climatic differences between the two altitudes could include factors such as temperature and moisture. Another climatic factor is rainfall; studies have shown that with higher rainfall the frequency of yellow morph snails decreases (Mazon et al, 1987). The populations we were sampling were present at the top and bottom of a large hill, meaning that snails at the bottom of the hill are more likely to encounter more rain due to the steep incline which causes an accumulation of water at the bottom of the hill.

The main drawback in this experiment was that it may not be an accurate reflection of the results throughout the year. This is because the climate, rainfall, temperature, etc may vary depending on the time of year. To further this research we would repeat our investigation using the same experimental methods but at different times of the year, over a greater range of altitudes. This would help us come to a more solid conclusion by enabling us to observe the effects of the factors mentioned above.

__References__ CAMERON, R.A.D 1969, THE DISTRIBUTION AND VARIATION OFCEPAEA NEMORALIS L. NEAR SLIEVEGARRAN,COUNTY GLARE AND COUNTY GALWAY,EIRE. Proceedings of the Malacological Society of London, 38, 439-450 Chat conversation end

Honek, A (2003). Shell-band color polymorphism in Cepaea vindobonensis at the northern limit of its range, //Malacologia,// Vol. 45, Issue 1, 133-140.

Le Mitouard, E, Bellido, A, Guller, A, Madec, L, (2010) ‘Spatial structure of shell polychromatism in Cepaea hortensis in relation to a gradient of landscape fragmentation in Western France.’ //Landscape Ecology, // Vol. 25, Issue 1, 123-134

MAZON, L. I., M. PANCORBO, M. A., VICARIO, A., AGUIRRE, A. I., ESTOMBA, A. AND LOSTAO, C. M. 1987, Distributionof Cepaea nemoralis according to climatic regions in Spain. //Hereditary,// Vol. 58, 145-154.

