Norman+I



Alaina Morgan* Conor Ferguson Eleanor Matthews Veneta Prokopieva Tara Staunton

We are studying the organism //cepaea nemoralis// or more commonly, the grove snail. We used this organism because you can see the genetic polymorphisms in the form of banding patterns and shell colour. In our field experiment the focus will be on studying the effects of natural selection and genetic drift on the snail’s polymorphisms in the populations we will study. Additionally, the snails do not move far in one generation, and so they it should be possible to count all of the snails in a particular area. //Cepaea nemoralis// is distinctive against other land snail species (e.g //Cepaea hortensis//) mainly due to the presence of a dark lip. The species is widespread across Western Europe and lives in a variety of habitats ranging from woodland areas to shrub land. There are a number of physical colour variants within the species, for example pink, brown, yellow and also banded and un-banded snails.  We will test two different habitats: grass land and scrub land. Our sampling strategy is to use a horizontal line transect across the field site at the same elevation. We will take 3 samples from the grass habitat and 3 samples from the scrub habitat. Each sample will be at least 20m apart, to ensure the populations are independent. This will be to ensure there is no gene flow between populations, which can be found out by doing a preliminary study of the field site. We will be using a standard within the group, ensuring each members knows how to categorise each snail. Figure 1. A review of the sampling site and sampling strategy. We are taking a horizontal transect because it is useful for our experimental design as it allows us to control a variable, the altitude of the sampling site. This helps us to <range type="comment" id="385952442_1">eliminate changes i</range id="385952442_1">n weather conditions and other environmental differences due to altitude alteration as a reason for different polymorphism frequencies. The sampling of two types of area (grass and scrub) is also important as it allows repetition so <range type="comment" id="378106184_1">we can be <range type="comment" id="385952442_2">more sure that the results we get are more reliable</range id="385952442_2">. </range id="378106184_1"> Our null hypothesis is that the underlying frequencies of the phenotypic polymorphisms studied are not statistically different for any of the habitats and natural selection is not affecting them. Our experimental hypothesis is that the underlying frequencies of the polymorphisms <range type="comment" id="385952442_3">are statistically different for the two types of habitat studied, suggesting that selection has a stronger influence on them than genetic drift.</range id="385952442_3"> The observation of a <range type="comment" id="385952442_4">higher frequency </range id="385952442_4">of one particular polymorphism in one habitat would support our experimental hypothesis and having a mixture of polymorphisms in all habitat would no support our experimental hypothesis.

__**EDITS:**__

<span style="background-color: white; font-family: Arial,sans-serif; font-size: 10pt;">We are studying the organism //<span style="font-family: Arial,sans-serif;">Cepaea ////<span style="font-family: Arial,sans-serif;">nemoralis // or more commonly, the grove snail. We used this organism because you can see the genetic polymorphisms in the form of banding patterns and shell colour. In our field experiment the focus will be on studying the effects of natural selection and genetic drift on the snail’s polymorphisms in the populations we will study. Additionally, the snails do not move far in one generation, and so it should be possible see the phenotypic differences within a small spatial scale. The species is widespread across Western Europe and lives in a variety of habitats ranging from woodland areas to shrub land. There are a number of physical colour variants within the species, for example pink, brown, yellow and also banded and un-banded snail shells.

<span style="font-family: Arial,sans-serif; font-size: 10pt;">We will test the difference between allele frequency between two different habitats: grass land and scrub land. Our sampling strategy is to use a horizontal line transect across the field site at the same elevation. We will take 3 samples from the grass habitat and 3 samples from the scrub habitat. Each sample will be at least 20m apart. This will be to increase the potential independence between the populations, which can be found out by doing a preliminary study of the field site. This will minimise the impact of gene flow on our samples. We will be using a standard within the group, ensuring each member knows how to categorise each snail. We are taking a horizontal transect because it is useful for our experimental design as it allows us to control a variable, the altitude of the sampling site. Replication provides us with more sample sites to analyse. The more sites we have the less likely it is that the results we have are due to chance or sampling error as opposed to actual patterns established by either selection or drift.

Both genetic drift and selection have great impact on the phenotypic frequencies in the habitats. The way to distinguish between the two is by replication of the experiment and collecting data from as many sites as possible. If the differences in phenotypic frequency are due to genetic drift, the sample values for each particular phenotype across sites of the same kind of habitat will be statistically different. For example, genetic drift could cause one grass site to have a high or dominating frequency of yellow 5 banded snails, and another grass site - pink un-banded snails. If the frequencies are more influenced by selection than genetic drift, then there would be similar frequencies across sites within a habitat type e. g. most grass sites would have higher frequencies of un-banded snails, and most bush sites would have higher frequencies of banded snails. <span style="background-color: white; font-family: Arial,sans-serif; font-size: 10pt;">‍ <span style="font-family: Arial,sans-serif; font-size: 10pt;">Our null hypothesis is that the underlying frequencies of the phenotypic polymorphisms studied are not statistically different for any of the habitats and natural selection is not affecting them. Our experimental hypothesis is that the underlying frequencies of the polymorphisms are statistically different for the two types of habitat studied, suggesting that something other than error or chance caused the different allele frequencies. The observation of a higher frequency of one particular polymorphism in most populations of one habitat type would support our experimental hypothesis and having a mixture of polymorphisms in all habitat would not support our experimental hypothesis.

<span style="font-family: Arial,sans-serif; font-size: 10pt;">RESULTS:


 * - ||  || Habitat ||   ||   ||   ||   ||   ||   ||
 * Snail colour || Banded || Bush1 || Bush2 || Bush3 || Grass1 || Grass2 || Grass3 || Total ||
 * Brown || Yes || 10 || 2 || 1 || 0 || 2 || 1 || 16 ||
 * Yellow || Yes || 4 || 6 || 11 || 5 || 11 || 11 || 48 ||
 * Pink || Yes || 1 || 6 || 6 || 3 || 1 || 0 || 17 ||
 * Brown || No || 1 || 0 || 1 || 0 || 0 || 1 || 3 ||
 * Yellow || No || 1 || 1 || 1 || 10 || 4 || 5 || 22 ||
 * Pink || No || 4 || 5 || 0 || 2 || 2 || 2 || 15 ||
 * total ||  || 20 || 20 || 20 || 20 || 20 || 20 || 241 ||

Bush1


 * Bush1 ||  || observed || Expected || (O-E) || (O-E)2 || (O-E)2/E ||
 * Brown || Yes || 10 || 1.32 || 8.68 || 75.34 || 57.08 ||
 * Yellow || Yes || 4 || 3.98 || 0.02 || 0.0004 || 0.0001 ||
 * Pink || Yes || 1 || 1.41 || -0.41 || 0.17 || 0.12 ||
 * Brown || No || 1 || 0.25 || 0.75 || 0.56 || 2.24 ||
 * Yellow || No || 1 || 1.82 || -0.82 || 0.67 || 0.40 ||
 * Pink || No || 3 || 1.24 || 1.76 || 3.10 || 2.50 ||
 * Total ||  ||   ||   ||   ||   || **62.3401** ||

Bush2


 * Bush2 ||  || observed || Expected || (O-E) || (O-E)2 || (O-E)2/E ||
 * Brown || Yes || 2 || 1.32 || 0.68 || 0.46 || 0.35 ||
 * Yellow || Yes || 6 || 3.98 || 2.02 || 4.08 || 1.03 ||
 * Pink || Yes || 6 || 1.41 || 4.59 || 21.07 || 14.94 ||
 * Brown || No || 0 || 0.25 || -0.25 || 0.06 || 0.25 ||
 * Yellow || No || 1 || 1.82 || -0.82 || 0.67 || 0.37 ||
 * Pink || No || 5 || 1.24 || 3.76 || 14.14 || 11.40 ||
 * Total ||  ||   ||   ||   ||   || **28.34** ||

Bush 3


 * Bush3 ||  || observed || Expected || (O-E) || (O-E)2 || (O-E)2/E ||
 * Brown || Yes || 1 || 1.32 || -0.32 || 0.10 || 0.08 ||
 * Yellow || Yes || 11 || 3.98 || 7.02 || 49.28 || 12.38 ||
 * Pink || Yes || 6 || 1.41 || 4.59 || 21.07 || 14.94 ||
 * Brown || No || 1 || 0.25 || 0.75 || 0.56 || 2.25 ||
 * Yellow || No || 1 || 1.82 || -0.82 || 0.67 || 0.37 ||
 * Pink || No || 0 || 1.24 || -1.24 || 1.54 || 1.24 ||
 * Total ||  ||   ||   ||   ||   || **31.26** ||

Grass1


 * Grass1 ||  || observed || Expected || (O-E) || (O-E)2 || (O-E)2/E ||
 * Brown || Yes || 0 || 1.32 || -1.32 || 1.74 || 1.32 ||
 * Yellow || Yes || 5 || 3.98 || 1.02 || 1.04 || 0.26 ||
 * Pink || Yes || 3 || 1.41 || 1.59 || 2.53 || 1.80 ||
 * Brown || No || 0 || 0.25 || -0.25 || 0.06 || 0.25 ||
 * Yellow || No || 10 || 1.82 || 8.18 || 66.91 || 36.77 ||
 * Pink || No || 2 || 1.24 || 0.76 || 0.58 || 0.47 ||
 * Total ||  ||   ||   ||   ||   || 40.87 ||

Grass2


 * Grass2 ||  || observed || Expected || (O-E) || (O-E)2 || (O-E)2/E ||
 * Brown || Yes || 2 || 1.32 || 0.68 || 0.46 || 0.35 ||
 * Yellow || Yes || 11 || 3.98 || 7.02 || 49.28 || 12.39 ||
 * Pink || Yes || 1 || 1.41 || -0.41 || 0.17 || 0.12 ||
 * Brown || No || 0 || 0.25 || -0.25 || 0.06 || 0.25 ||
 * Yellow || No || 4 || 1.82 || 2.18 || 4.75 || 2.61 ||
 * Pink || No || 2 || 1.24 || 0.76 || 0.58 || 0.47 ||
 * Total ||  ||   ||   ||   ||   || **16.19** ||

Grass3


 * Grass3 ||  || observed || Expected || (O-E) || (O-E)2 || (O-E)2/E ||
 * Brown || Yes || 1 || 1.32 || -0.32 || 0.10 || 0.08 ||
 * Yellow || Yes || 11 || 3.98 || 7.02 || 49.28 || 12.38 ||
 * Pink || Yes || 0 || 1.41 || -1.41 || 1.99 || 1.24 ||
 * Brown || No || 1 || 0.25 || 0.75 || 0.56 || 2.25 ||
 * Yellow || No || 5 || 1.82 || 3.18 || 10.11 || 5.55 ||
 * Pink || No || 2 || 1.24 || 0.76 || 0.57 || 0.47 ||
 * Total ||  ||   ||   ||   ||   || **21.97** ||