| Summary: | The pattern of genetic diversity in many species observed today can be traced back to historic ecological events that influenced the distribution of species not only on a global but also a local scale. For example, historical events such as habitat fragmentation, divergence in isolation, and subsequent range expansion, can result in a recognisable pattern of genetic variation which can be used to infer ecological factors (e.g. effective population size, dispersal capacity), as well as those affecting speciation processes. This thesis examines these issues from a phylogeographic and phylogenetic perspective by analysing patterns of variation in the mtDNA cytochrome c oxidase sub-unit 1 (COI) gene in two co-occurring Antarctic endemic arthropods in Southern Victoria Land, Ross Dependency. Within the Southern Victoria Land Dry Valleys of Garwood, Marshall and Miers, populations of the springtail Gomphiocephalus hodgsoni (Collembola: Hypogastruridae) and mite Stereotydeus mollis (Acari: Prostigmata) revealed consistently dissimilar patterns of genetic structure. COI divergence within G. hodgsoni was less than 0.7%, while divergence within S. mollis reached upwards of 17%. Within our study area G. hodgsoni and S. mollis harboured 10 and 22 haplotypes, respectively and showed links to previously sampled populations across Southern Victoria Land. The distribution of G. hodgsoni haplotypes across sites was homogenous while those of S. mollis were distinctly heterogenous. The extremely low genetic variation and links to previously sampled populations suggest that G. hodgsoni is a relatively recent colonist within our study area and/or the victim of an extreme bottleneck event. On the other hand, the extreme levels of genetic diversity observed for S. mollis, and the occurrence of two highly divergent haplotypes that were unique to our study area, suggest that: (1) S. mollis may have had a longer association in isolation with our southern study area; and/or (2) S. mollis has colonised our study area on more than one occasion via multiple extant refugial populations. Throughout its entire Southern Victoria Land range S. mollis is characterised by extremely high levels of mtDNA (COI) divergence (greater than 17%), suggesting a possible multi species complex. To examine this issue, I used both Neighbour Joining (NJ) and Maximum Likelihood (ML) methods to construct a phylogeny utilising all 50 known unique S. mollis sequences with other Victoria Land congeners including an available S. belli sequence, and several new S. shoupi sequences. Both NJ and ML analyses revealed significantly congruent trees with strong bootstrap support. The morphologically similar S. shoupi was placed as a monophyletic sister group, basal to S. mollis in both analyses with strong support. However, there was disagreement between the two methods in the placement of the single S. belli sequence within in the resulting phylogenies which was not possible to resolve with the current data. Despite this latter uncertainty, the possibility of cryptic species within S. mollis remains. Collectively, these studies have demonstrated differences in the genetic structure between two co-occurring species and suggested how similar historic processes, combined with differing life history attributes can lead to that differentiation. Furthermore, genetic analyses were used to identify isolated and unique populations, which are likely to be of high conservation value.
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