Team+Gastropoda*


 * Introduction**

This experiment concerns itself with polymorphism in snails, Cepaea nemoralis. Polymorphism refers to the presence of multiple alleles at the same locus of a chromosome within a population, governed by four major processes, mutation, gene flow, selection and genetic drift//1 //. The easiest polymorphic variations to identify are those that affect an organism’s phenotype, and it is the phenotypical variation of the snail’s colour, and number of bands on the shells that we are studying, with the aim to assign the differences to either selection or genetic drift.

The primary reason for our study of snails as opposed to humans is due to the visual phenotypic display of genotype on their shells. This allows for easy identification of the snails phenotype, both pre and post death. Another reason for the study of snails is their lack of locomotion. Snails travel no more than 20 metres in a lifetime and this lack of movement leads to sub-populations in geographically close proximities, allowing us to sample a representative number of snails in a small area. Their short life-span also means that we can observe snails from a number of different generations. Neither of these would be viable for human studies.

The logic behind our sampling method manifests itself in its control of confounding variables. By ensuring our samples are taken at the same altitude, we are also controlling humidity, temperature, moisture and wind exposure. This will lead to a greater ecological validity of our results. In order to allow time for a thorough sampling of the populations, we are only sampling two different types of habitat. This allows us to replicate our sampling to increase reliability. The allocation of these sites will be placed at least 20m away from each other to eradicate any effects of gene flow between the populations; they will all be independent of one another.

We hope to be able to identify if a change in habitat will lead to a significant difference in the frequency of phenotypes after sampling. <range type="comment" id="380841710_8">A chi squared test performed on the results will help evaluate whether the difference in frequencies is statistically significant. If it is significant then it suggests there is an actual difference in frequency between the populations, although there is the possibility (if unlikely) that the frequencies in the populations are actually the same.</range id="380841710_8">

Our hypothesis is that a change in habitat will lead to a significant difference in the frequency of phenotypes, due to selection over genetic drift. If the differences are due to selection we would still expect to see differences between samples, however, we would observe consistent differences between the same habitats. <range type="comment" id="380841710_9">Conversely, if it is due to genetic drift, there would still be differences between samples but there would also be inconsistent differences between the same habitant. </range id="380841710_9">Together, these observations will help us to work out if our results are due to either selection or genetic drift.

1. Jones, J.S., Leith, B.H., Rawlings, P. 1977. Polymorphism in Cepaea: a problem with too many solutions? //Ann. Rev. Ecol. Syst//. **8**:109-143.