Mashed+Potatoes

Group members:
1. Alima Toganassova 2. Shobiga Kuganesan 3. Madina Zhalbinova 4. Milena Simovic* 5. Javid Shafi

The aim of our study is to determine whether the differences in allele frequencies (polymorphism), between populations of the same species of organism are a result of genetic drift, natural selection or some other evolutionary forces. As the same principles of evolution apply to all organisms including humans, with different outcomes depending on the organism’s complexity and its population size, we have chosen to study polymorphism in //Cepaea nemoralism// hoping to understand those principles and then be able to apply them to the rest of organisms. //Cepaea nemoralism // is chosen for this study because it is known for being one of the most polymorphic members of the European fauna. It is known for its polymorphic shell and lip color as well as for its banding pattern. The major shell color classes of this snail are yellow, pink and brown whereas its band number can range from zero to five. Due to its low rate of movement ranging from 20 to 30m in its lifetime, all the genetic differences in this species accumulate within the short distances and we can therefore observe them in just one locality. This is a huge advantage of using snails rather than humans, as to observe the same amount of genetic diversity in humans we would have to travel across the whole Europe and further. Another advantage of using snails for this type of study is their short lifespan, thus by observing their dead shells we were able to see the specific pattern of polymorphism that existed through several generations and to see if it differed from the pattern observed in living snails. To get the same type of information in humans would be completely unethical and much more difficult.

Our sampling method was designed to obtain six large samples of snails from two habitat types. Three samples of snails were collected from grassland area and three from woodland allowing us to have multiple collections from each habitat. Sampling at the same altitude was our main control variable. The samples were taken across a horizontal line, where the sampling sites where at least 20m apart between any two adjacent samples. This allowed for independency of our samples, and made the effects of gene flow occurring between any two populations less extensive, so that differences in allele frequencies between any pair of populations regardless of their habitat type can be easily observed.  All the samples were large, with the minimum number of 20 snails in each sample. These large sample sizes helped us to reduce sampling variation as much as possible. Also, in order to avoid any sampling bias the same three people were recording the snails at all six sites after agreeing on the shell colors and number of bands. From our sampling we hoped to learn how sampling of snails in different habitats can help us to detect the different effects of gene flow, genetic drift and selection on establishing the polymorphism. The effects of these three evolutionary forces are chosen to be studied in this experiment but it is known that at least eight evolutionary forces affect shell polymorphism in Cepaea nemoralis including many types of selections such are visual, climatic, frequency-dependent, disruptive, density-dependent and stabilizing selection, heterozygote advantage, linkage disequilibrium and co-adaptation as well as other random processes such as genetic drift that we are studying (Jones et al 1977). Our null hypothesis is that the frequency of shell phenotypes does not differ between the populations. The main idea is that both the genetic drift and natural selection can result in the differences between populations if there is no extensive gene flow occurring between them. If on one hand genetic drift is the main force acting on polymorphism, than from our results we would expect to see that there is as much difference between two populations of the same habitat (e.g. two grassland populations) as there is between the two populations of two different habitats (e.g. grassland and woodland). However, even if we do observe the differences between the populations of the same habitat we cannot eliminate the possibility that natural selection resulted those differences due to different selective regimes that might exist between our sampling sites but are not detectable (e.g. differences in ecology between grassland areas that we sampled from). If on the other hand our results show that there is no significant difference in the frequencies of shell phenotypes between the populations of the same habitat, than we could be much more confident in concluding that natural selection is the most probable force acting on polymorphism in //Cepaea nemoralis// if our results also show the significant difference in the frequencies of shell phenotypes between the populations of two different habitats</range id="388230160_2">.