Discussion+Michelle+Catie+Priya+Liisa+Harriet+Grace

Michelle Hauge* Grace Challis Liisa Peltola Harriet Speed Catie Daubner Priya Rehal

This research was conducted to study polymorphism, genetic drift, gene flow and mechanisms of selection between different populations. The land snail Cepaea nemoralis was chosen specifically for this task due to its well established polymorphism. Data collection involved noting the most observable morphs: shell colour and banding. The data was used to explore the accuracy of the null hypothesis; populations of C. nemoralis at high and low altitudes have the same frequencies of phenotypes. Sampling error must be taken into consideration, as the sample we collect will not be representative of the entire population frequency. However this data collection was repeated at a series of sites at each altitude and all comparisons kept independent from one another, making this an unlikely event. A statistical test was carried out to evaluate the degree to which data was affected by sampling error, and to see if there were any statistically significant differences between sites at high altitude collectively – and again sites at low altitude. This test was the Chi squared test and after carrying it out, data could be grouped. It was found that there was no significant difference between the values of data observed, leading the null hypothesis to be accepted and the data grouped together as one. To account for the findings, several hypotheses can be put forward, as explained by Jones et al. 1977. Theories explaining the genetic variation in polymorphism suggest either natural selection or random events in nature are to blame. In addition, it is also suggested that there are at least eight different evolutionary forces known to influence the snail’s shell polymorphism. It is difficult to determine one sole cause of the polymorphisms C. nemoralis. As the null hypothesis was accepted, it can be implied that there is no selective pressure established between high and low altitudes and that any genetic variation is simply due to chance or random events. If there is a large population present, any genetic drift observed will be small. Gene flow can account for the results; it is able to change allele frequency in adjacent populations, making them more alike while distant populations may remain dissimilar. It is possible that over time, gene flow has made the phenotypes of the snails become more alike in the populations at high and low altitudes, however it can be said that because of the length of time this takes, it is actually selection maintaining the polymorphism after all. This means these two factors are working together as a combination of evolutionary forces. Further discussion of: • Genetic drift • Population bottleneck • How each factor can have affected the results obtained • Results of the Chi squared test