GastroPosse

The land snail genus Cepaea has many polymorphisms, as they have a variety of different coloured shells with different banding patterns. Snails with five, one and zero bands are often found with shells coloured yellow, pink and brown. ‘C. hortensis shares nearly all the polymorphisms found in C. nemoralis (1)’. Ultimately polymorphism occurs because of mutation, but it is sustained for many reasons. These polymorphisms are very distinct from each other, implying that there is little gene flow between populations of Cepaea. To establish the amount of gene flow, samples of snails will be taken from different locations and their appearance will be recorded. Climate preferences of the snails can explain their distribution resulting in area effects. Polymorphisms can be driven by various different evolutionary forces including selection and drift. The song thrush is a predator of these snails and frequencies of shell appearance may be due to visual selection by the song thrush and other predators. It is also known that the song thrush exhibits frequency-dependent behaviour where a ‘search image’ is formed of the first prey encountered, this is usually the rarest allele in a particular location. ‘The case of Cepaea is regarded as a paradigm by selectionists (2)’ as the variety and prevalence of the different forms is a product of many evolutionary factors and environmental pressures merging.

1. Jones, J. S. et al 1977. Polymophism in Cepaea: A Problem with Too Many Solutions? Annual Review of Ecology and Systematics. Vol 8, 109-143 2. Lewontin, R. C. 1974. The Genetic Basis of Evolutionary Change. New York: Columbia Univ. Press. 346 pp.

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From Cara Hitt (120022636) and the group Gastroposse (Cara Hitt, Anjalikaa Thaker, Katie Pritchard, Jamie Rowland, Joel Brown, Sean Mauremootoo)