This study used allozyme and mitochondrial DNA variation to examine genetic structure in the Oxleyan Pygmy Perch Nannoperca oxleyana. This small-bodied freshwater ®sh has a very restricted distribution occurring only in some small coastal streams in south-east Queensland and northern New South Wales. It was expected that subpopulations may contain little genetic variation and be highly di erentiated from one another. The results, based on allozyme and mitochondrial DNA control region variation were in agreement with these expectations. Allozyme variation was very low overall, with only one locus showing variation at most sites. The high di erentiation was because a di erent locus tended to be polymorphic at each site. Mitochondrial variation within sites was also low, but some sites had unique haplotypes. The patterns of similarity among mitochondrial DNA haplotypes were not as expected from geographical proximity alone. In particular, although some northern sites had unique haplotypes, four sites spread along 200 km of coastline were remarkably similar, sharing the same common haplotype at similar frequencies. We suggest that these four streams may have had a con¯uence relatively recently, possibly when sea levels were lower, 8000±10 000 BP.

No abstract

The upland mesic rainforests of eastern Australia have been described as a "mesothermal archipelago" where a chain of cool mountain "islands" arise from a warm "sea" of tropical and subtropical lowlands. An endemic freshwater crayfish belonging to the genus Euastacus is found on each of these mountain "islands." The Euastacus are particularly suitable for the study of evolution because each mountain harbors a unique species, there are many taxa present providing replication within the group and, most importantly, their distribution is linear, extending along a south-north axis. This group could have evolved by "simultaneous vicariance" where there was one vicariant separation event of a widespread ancestor, or by "south to north stepping stone dispersal" where there were long distance dispersal events from neighboring mountain islands, starting in the south and proceeding north in a dispersal-colonization wave. We used pairwise genetic distances between nearest geographic neighbors as a novel way to test the two hypotheses. If diversification was due to "south to north stepping stone dispersal," then pairwise genetic distances between nearest geographic neighbors should decrease progressively the farther north the taxon pairs are found, reflecting the decreasing periods of isolation. In this case there should be a negative correlation between the south to north rank order of nearest neighbors and pairwise genetic distances. A Spearman's correlation on 16S mtDNA pairwise genetic distances and geographic rank order was not significant, indicating there was no support for the south to north stepping stone dispersal hypothesis. If simultaneous vicariance was responsible for diversification then all nearest geographic neighbor taxon pairs should have similar genetic distances and, therefore, the variance in nearest neighbor distances should be zero, or close to it. To test if the observed variance was tending towards zero we developed a randomization test where nearest neighbor taxon pairs were assigned random genetic distances and the variances calculated. The observed variance lay in the < 0.05 range of the simulated variances, providing support for the simultaneous vicariance hypothesis. The data also suggest there was simultaneous vicariance of at least two ancestral Queensland lineages. The timing of this vicariant event was probably in the Pliocene, which is consistent with the divergence times reported for other Australian mesic rainforest restricted taxa.

Phylogenetic evidence suggested that the Queensland Euastacus diversified through ‘simultaneous vicariance’, where the range of a widespread ancestral Euastacus receded to tops of mountains with the Pliocene warming of the continent and subsequent isolation lead to speciation. Implicit in the simultaneous vicariance hypothesis are three postulates on ancestral history: (1) warm temperatures were effective barriers to ancestral gene flow; (2) the ancestral Euastacus had an extensive contiguous distribution; and (3) there was a single vicariant event associated with Pliocene warming. It is argued that if there was interspecific diversification due to simultaneous vicariance then, within extant species, there are three predictions on current population structure. First, lowland areas, even those connected by streams, would be barriers to contemporary dispersal. Second, there would be contemporary dispersal between catchments covered by mesic rainforests. Third, there would have been recent Pleistocene intraspecific vicariant events. The population structure of E. robertsi, E. fleckeri, E. hystricosus and E. sulcatus was investigated with mtDNA and allozymes and it was found that the intraspecific data were consistent with these predictions. Furthermore, the Euastacus underwent limited range expansions during the cooler Pleistocene glacial cycles, and it is hypothesised that during cooler glacial periods, lowlands were still effective barriers to dispersal because of increased Pleistocene aridity.

Species invasions are occurring at an increasing rate in coastal environments. Accurately identifying introductions is a critical issue to take full advantage of the new invasion databases.