Download Evolutionary Genetics Field Trip Survey Introduction Cepaea

Evolutionary Genetics Field Trip Survey
Introduction
Cepaea nemoralis also known as the Grove Snail or Brown lipped snail is one of the most
numerous and polymorphic species of land snail in Europe .It is typically found to inhabit
woodlands, meadows and residential or inner city gardens, it is nocturnal and usually
feeds on dead or decaying vegetation . They have long been used by biologists as an
research model in the study of heredity and evolutionary genetics. They have been
extensively studied over the last 100 years and have subsequently become crucial in our
understanding of natural selection in the wild (Clarke et al 1978).
Since the 1940s, C. nemoralis has been used as a biological model species within the long
held debate of selection versus genetic drift. It is an ideal species for this purpose due to
its conspicuous polymorphism in both shell colour and number of bands. Individuals can
easily be identified on site and then released. They exist in discrete populations with only
occasional gene flow between populations, moving on average 5-10m per year (Cameron
and Williamson, 1977). The study of this species provides valuable insights into the
polymorphisms of other species that are not as easy to get inferable data from.
Thousands of populations across Europe have been studied and still there is no
conclusive answer as to the maintenance of the snail's polymorphism. Early studies have
shown contradictory results in favour of both selection (Cain & Sheppard, 1954) and
genetic drift (Lamotte, 1951). An even broader range of possible environmental factors has
been discussed by Cook (1998), such as climatic fluctuations and human activities that
both appear to rapidly alter snail habitats, which in turn has a knock on effect in terms of
the selection pressures.
Opinion has moved away from a single unifying solution, according to Jones et al. (1977),
there are eight evolutionary processes that have been identified to be acting upon C.
nemoralis populations. These processes are thought to act with varying intensities from
population to population; therefore each must be treated as a unique case with its own
unique evolutionary history. Modern evolutionary biology has followed this line of thinking;
our explanation of observed patterns must be explained not only by natural selection but
also by random events, migration, interbreeding, mutation and horizontal gene transfer
(Lewontin, 2002).
The purpose of this survey is to understand how polymorphism is maintained within a
species. By conducting a comparative investigation of gene frequency distributions within
C. nemoralis populations, we will be able to explain which evolutionary forces are acting
upon the population and to what extent. This will be achieved by recording shell colour and
banding patterns from individuals within various different populations located in the same
meta-population. We will look for patterns in our results that correlate with patterns
expected if visual or climatic selection was occuring. For example, if brown shelled snails
are consistently found in areas with dark backgrounds and yellow shelled snails in open
grassland, we can reasonably suggest that visual selection is taking place.
If we rule out the significant effect of natural selection on the populations, we may then try
to establish whether or not genetic drift is having a predominant effect. Lamotte (1951)
devised a method of identifying genetic drift by looking for a signature pattern in gene
frequency distributions. He suggested that if genetic drift was at work, the allele
frequencies would be constantly fluctuating and would be more apparent in smaller
populations than in larger ones. Therefore if he were to simultaneously sample multiple
populations of different sizes, larger differences in allele frequencies would be observed
between small populations than between large populations. In this way he was able to
confidently claim that genetic drift was the main evolutionary force acting on C. nemoralis.
However, due to the time constraints of this study we will be unable to replicate his
methodology. As a consequence, we will only be able to establish the degree to which
natural selection is acting upon the population. Therefore, our null hypothesis is that at
each site there will be no significant difference between allele frequencies.
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Contributing members
Paul Roach
Joshua Costa
Satyajeet Barot
Rajkrishnna Rajaratnam
Saran Kuttiyandisamy
Refrences
Cain, A.J. and P.M. Sheppard. 1954. Natural selection in Cepaea. Genetics 39:89-116
Cameron, R.A.D. and P. Williamson. 1977. Estimating migration and the effects of
disturbance in mark-recapture studies on the snail Cepaea nemoralis. Journal of
Animal Ecology 46:173-80 47.
Clarke B (1978) Some contributions of snails to the development of ecological genetics. In:
Brussard PF (ed) Ecological genetics – the interface. Springer, New York,
pp159–170
Cook L. M. (1998). A Two-stage Model for Cepaea Polymorphism. Phil. Trans. R. Soc. B
353 (1375): 1577–1593
Jones J. S., Leith B. N., Rawlings P. (1977). Polymorphism in Cepaea: A Problem with Too
Many Solutions. Annual Reviews in Ecology and Systematics 8: 109–143
Lamotte, M. 1951. Recherches sur la structure génétique des populations naturelles de
Capaea nemaralis. Bulletin Biologique de la France et de la Belgique 35:1-239.
Lewontin, R.C. 2002. Directions in evolutionary biology. Annual Review of Genetics.
36:1-18.