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The evolutionary consequences of restrictions on gene flow are discussed in relation to the population genetic structure and speciation of four Australian hydrobiid snail faunas. The studied faunas comprise: (1) the brackish water genus Tatea; (2) species of Fluvidona in freshwater streams at Wilsons Promontory; (3) species of Fonscochlea and Trochidrobia in artesian springs near Lake Eyre in South Australia; and (4) species of an undescribed genus at Dalhousie Springs in northern South Australia, another arid zone artesian spring complex. Gene flow in these hydrobiids is very variable. It is high in Tatea, relatively high at Dalhousie Springs and extremely low in Wilsons Promontory and the Lake Eyre springs. Levels in the latter two faunas are similar despite a great disparity in geographic area. In these four faunas, the detail of gene flow patterns is complex, emphasising the dependence of population structure on the interaction of current and historical factors. This is illustrated by speciation patterns, the numbers of species and their distributions usually correlating well with observed levels of gene flow. However, there are examples, among species groups with comparable, but very low gene flow, in which some taxa have undergone speciation yet others have not.

The data were analysed using F-statistics and the private allele frequency approaches. Whilst the qualitative conclusions from the two approaches were generally similar, the exceptions usually indicated (on biogeographic grounds), that the F-statistics approach is the more reliable estimator of gene flow. The private alleles approach is dependent on a coincidence of the scale of sampling with the biological scale of population sub-division. Intensive sampling schemes, as utilised in our studies, tend to find even rare alleles in more than one population even though they may be quite restricted in geographical distribution. An analytical method for treating conditional allelic frequencies would not be as sensitive to this problem as the private alleles approach.