Dillon and Frankis 2004

Dillon RT, Frankis RC. 2004. High levels of mitochondrial DNA sequence divergence in isolated populations of freshwater snails of the genus Goniobasis Lea, 1862. American Malacological Bulletin 19: 69-77.

These authors examined sequence divergence within and between populations and species of “prosobranch” freshwater gastropods in the south-eastern USA. These species are relatively well studied, and previous work by other authors had suggested very high levels of mtDNA heterogeneity, between populations and between species.

Part of the justification for this work is as a way of calibrating a new measurement tool: DNA sequences may be useful for systematic assignment, but if levels of sequence divergence are to be used to distinguish species, first the levels of divergence between species well-established by other means (e.g. interspecific hybridization trials) must be determined. These snails had previously been well studied for traits associated with concepts of species such as pre- and post-mating isolation (e.g. Dillon 1986).

The other major reason for this paper was to test the hypothesis that freshwater snail populations may be so old, so large, and so isolated that intrapopulation mtDNA sequence divergence is likely to swamp interpopulation (and interspecies) mtDNA divergence. Under this hypothesis, within-population levels of divergence will overlap with between-population levels. Populations studied here are extremely isolated, with no freshwater connections between them; this area was not inundated nor was it ice-bound by the Pleistocene glaciations, suggesting that some populations may have been isolated for millions of years.

The levels of divergence found in COI and 16S mtDNA sequences were extremely high, especially among populations of one species, Goniobasis proxima. In one population, individual conspecific snails collected from adjacent rocks may have more divergent mtDNA than individual snails collected from populations separated by over 400km of (impassable) land. In that same highly heterogeneous-mtDNA population, morphology and seven nuclear markers (allozymes) were essentially homogeneous.

The authors end with a caution that systematic inference must be made with care when faced with high levels of intrapopulation divergences. This paper appears to provide an example situation in mtDNA phylogeography that does not show the “barcode gap”, i.e. a clear distinction between intra- and interpopulation divergences.