Selection

__**INTRODUCTION: Field Trip**__

Natural Selection is regarded as the main driving force behind evolution. Nevertheless in smaller populations gene flow and genetic drift also contribute to genetic variation drift goes on in all populations, and gene flow affects large populations and small in a similar manner. The effects of genetic drift are usually more prominent in smaller populations, where the gene pool is small enough to increase the chances of a certain allele becoming fixed drift is not only important when it leads to fixation, it can affect allele frequency. This is because in small populations the original gene pool is less genetically varied in comparison to large populations not really a precise explanation. Therefore the alleles that are successful are unlikely to be representative of the original population; this effect is demonstrated by the bottle-neck mechanism is it a mechanism? Look up the word. When gene flow occurs it hinders genetic drift really, what do you mean by that spell it out. Gene flow between populations introduces new alleles into the population not only that, it can change the frequency of existing allels ; this has the potential to rapidly change the allele frequency within populations.

Polymorphism occurs when two or more alleles exist in the same species in substantial frequencies. In order to be classified as a polymorph species, they must be at the same geographical location during the same time period. //Cepeae// //nemoralis// is a common land snail in Europe; closely linked genes determine the morphological variation of their shells. The phenotypic variations are relatively simple to recognise, as polymorphism between the species (this phrase does not make sense. I think you mean that the different phenotypes are readily distinguished. Look up the definition of a 'species' urgently!!) is readily visible. The observable traits of human phenotype are a result of interactions between many alleles. Whereas in the land snail //Cepeae// the interactions are less complicated as the genes are less closely linked. One reason that this study is based on snails is because they are less mobile; therefore collecting and observing the population is much easier. Snails are an ideal subject for experimentation of this type as opposed to humans, because the phenotypic outcome in snails is related to fewer genes. How do you know. I suspect there are more polymorphic loci in //Cepaea// than in humans? Why do you suspect the opposite?

The aim of this study is to find out if evolutionary processes such as genetic drift are responsible for geographic patterns of in frequency of phenotypes. Our sampling techniques will allow us to compare the phenotype of geographically different snail populations and investigate what evolutionary processes contribute to the observed patterns. The same number of snails will be collected at random from both the woodland and grassland; the frequency of each type will then be examined.

The study will be carried out by examining the band pattern and colour of //Cepeae// snail samples from different locations. It is expected that darker shelled snails will be found in the woodland whereas lighter individuals will be found on grassland. The two major known predators of the //Cepeae// are thrush and small mammals as well as various birds and beetles. It is known that predators select their prey according to the physical appearance, for examples thrush prefer darker shells as opposed to yellow shells. Therefore the predator preference is a major factor in determining where a polymorphic species is found.

Our hypothesis is that the woodland habitat would have more dark shelled individuals than the grassland. This hypothesis will be supported by results showing a trend of more brown shell snails in woodland and yellow shells more common in grassland. but how will you distinguish this explanation from the possibility that the differences in allele frequency were due to genetic drift?

Contributors: Mithusha Yogalingum Sinthujah Sivamohan Shahana Choudhury Sonia Begum Kirin Sultana