The+Dim


 * A study of polymorphism in //Cepaea nemoralis//**

Babbington, E. A., Curran, F. L., McMahon, L. N., Pollock, T. G. and Vincent-Piper, A.

Polymorphism describes a situation in which there are many variants of a particular characteristic in one biological species. Polymorphism is observed in some organisms such as humans, bees and the species that this study focuses on, //Cepaea nemoralis.// The phenomenon of polymorphism has posed a troubling question to scientists as at first it may seem that the sustained existence of polymorphism within a species directly contradicts the action of natural selection, which is expected to favour one phenotype that has an advantage above other possible variants, eventually fixing just one allele in the genome of that species. The answer to this question lies in the frequencies of the different alleles within a population. An example of how frequency dependent polymorphism can be beneficial to individuals among a species can be seen in the human population in the form of left handedness. Scientists have determined that the frequency of left handedness has stayed at a low and consistent level for thousands of years, this is explained by the way humans and especially males fight with each other: when the majority of the population are right handed, fighters are well versed in how to fight a right handed fighter, however when they are put up against a left handed fighter they are more vulnerable to attacks that they are less able to defend themselves from and so the left handed fighter is more likely to survive the fight, but this only remains true if left handedness remains rare. Based on the principle that frequency dependent polymorphism can confer an advantage to a species, this study will seek to determine whether or not the polymorphism seen in //C. nemoralis// seems to be selected for or if it is due to genetic drift. An indication that polymorphism is selected for will be if the observed frequencies in separate populations (that are not connected by gene flow) are the same or very similar, therefore our hypotheses are as follows: If there are significant differences in the frequencies of each population, selection is not affecting the polymorphism, alternatively, if there are not significant differences in the frequencies of each population, selection is affecting the polymorphism.

Our study on //C. nemoralis// shells was carried out at Pulpit Hill, Monk’s Riseborough, Buckinghamshire which is a chalk land area. //C. nemoralis// are cross-fertilising hermaphrodites that travel up to 20m in their lifetime and have a large ecological niche in terms of habitat. They prefer warmer microclimates than their sibling species //C. hortensis// as they conserve water well in warm, dry environments (Jones et al. 1977). The variety of shells present in this species show different colours and banding patterns. The different colours seen are pink, brown and yellow and the different banding patterns are unbanded, midbanded, with 3 or 5 bands or rarely with 4 bands.

The distribution of //C.nemoralis// can be affected by two main factors: predation and microclimate. Individuals with darker coloured shells or more bands are likely to be found in sheltered areas that are cooler, as their darker colouring makes them more able to absorb heat whereas those with lighter coloured shells are less able to absorb light and so benefit from being in open areas where there is direct sunlight. An observation has been made that the more Northwards a population of //Cepaea// and the colder the climate becomes, the higher the frequency of darker shells. Predators such as //Turdus philomelos// (common thrush) influence their distribution in several ways. Firstly, the darker shelled individuals are camouflaged in shelter against the dark background and the lighter coloured ones are more likely to be camouflaged in open areas. As well as this, it’s been observed that //T. philomelos// are more likely to prey on the colour of shell that has the highest frequency; this may be part of an optimal foraging strategy.

**References:** Jones, J.S., Leith, B. H. And Rawlings, P. 1977. Polymorphism in Cepaea: A problem with too many solutions? //Annual Review of Ecology and Systematics.// 8. 109-143.