With the exception of rare or endangered species, which have limited distributions, virtually every species investigated to date has revealed some level of genetic differentiation between populations. A lack of population differentiation would mean that all populations had the same allele frequencies. This would be possible only if the entire species constituted a single group of randomly mating
Molecular Ecology Joanna Freeland © 2005 John Wiley & Sons, Ltd.
individuals, which will not occur in species that comprise multiple populations. For many years, a frequently cited exception to this rule was European eels (Anguilla anguilla). These eels overwinter in numerous, geographically distinct sites across Europe, but regardless of their overwintering site, all individuals migrate to the Sargasso Sea in the summer to reproduce. This common mating ground provides a mechanism for species-wide panmixia, a hypothesis that was supported by a complete lack of population genetic structure across Europe according to either allozyme or mitochondrial data (Avise et al., 1986).
More recently, however, data from seven variable microsatellite loci revealed weak but significant genetic differences between groups of eels from several overwintering sites. These genetic differences suggest that eels from across Europe do not, after all, form a single panmictic population in the Sargasso Sea, possibly because eels migrating from different latitudes reproduce at different times, in which case reproduction should occur more often between eels that overwinter at similar latitudes (Wirth and Bernatchez, 2001). Population divisions such as these are often very difficult to detect without the aid of appropriate molecular markers; bearing this in mind, we shall look now at some methods of analysing genetic data that allow us to determine whether or not populations are genetically distinct from one another.
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