DRIFT+area+effects


 * __Area Effects and Polymorphism (17/12/10)__**

Snails (Cepaea) have been studied for decades as organisms that can help us understand the effects of polymorphism. Originally, it was suspected that the different phenotypes in particular areas and their frequencies are random, perhaps indicating genetic drift, but after some studies (Cain and Sheppard, 1950) in many English populations of Cepaea, it was suggested that a particular phenotype is advantageous depending on the background colour of the environment. A certain colour may be more apparent than another and is therefore an easier target for preying birds.

If this was the case, selection would take place due to survival pressures and advantageous individuals would have a greater chance of living and producing offspring which would in turn inherit these ‘successful’ alleles, whereas the other individuals with ‘disadvantageous’ coloured shells would be eaten at a greater rate and would, with time, disappear from the population.

Yet, populations remain polymorphic.

Sometimes, a particular phenotype will be present regardless of the background colour of the ground it lives on, which would suggest that this variation does not have a dramatic effect on the organism’s survival; the level of advantage provided is not significant for it to be totally selected for over other traits. And these phenotypes can be present in a number and cover an area greater than that of a panmitic population (a population where all the individuals are potential mating partners for each other and there are no genetic, behavioural or physical restrictions to prevent them). Such a predominance of different morphological frequencies over a diverse area despite the effects of visual selection is known as an area effect.

Area effects can be seen as a pattern within the polymorphism phenomenon. But an actual reason as to why these effects happen is not clear and has puzzled evolutionary biologists for some time. Polymorphism can occur from other evolutionary processes, such as drift.

Populations in areas are subject to genetic drift. Genetic drift occurs in all populations, but when selection is happening at the same time, we could predict certain relationships. In particular environments, for example woodland area, you would expect to see darker phenotypes due to selection, but lighter phenotypes are also found. In areas in which the phenotypes vary and no particular phenotype is advantageous to the area to others, then there would be no preference for a certain phenotype. If there is no preference, there is no reason why one phenotype should be more common than another.

However, in smaller populations, the effects of genetic drift can be more dramatic. This could be seen in naturally small numbers of organisms or after an event which dramatically reduced the population size, such as a flood (known as founder events). Eventually, fixation of one particular allele at a locus can occur and all the members of that population will show a particular phenotype.

Fixation could therefore provide an explanation for area effects. If a founder event occurred, or we began with a small population size and one trait became fixed, we could expect the population size to eventually increase over time. It could increase to such a dramatic size that it covers a wide area of greater coverage than a typical population size. This is one possible theory for area effects, and would account for the prevalence of a particular phenotype over such a large area when it does not appear to have any selection benefits. It was simply lucky.

 You have put forward a coherent argument here. You need to think more about why the area effects cover such large areas. This could be explained by population expansion into unoccupied territory - but why and when was it unoccupied (perhaps due to the last major episodes of climate change). However, the sharp transitions between allele frequencies where the areas meet require some explanation... why have the two sets of genotypes introgressed.

 You have not done outside reading on these issues... and not provided references for your assertions here.

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Tasneem Mithawala (secretary) bt09068@qmul.ac.uk Aisha Mayet bt09367@qmul.ac.uk James O'Shea bt09192@qmul.ac.uk Maria Nadeem bt09247@qmul.ac.uk Peta Lough bt09354@qmul.ac.uk