THE NAUTILUS, Supplement 2:25-43, 1994 Page 25 The Evolutionary Consequences of Restrictions on Gene Flow: Examples from Hydrobiid Snails D. J. Colgan W. F. Ponder The Australian Museum P.O. Box A285 Sydney South, Australia 2000 e ABSTRACT 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 stud- ied faunas comprise: (1) the brackish water genus Tatea; (2) species of Flu vidona in freshwater streams at Wilsons Prom- ontory; (3) species of Fonscochlea and Trochidrobia in artesian springs near Lake Eyre in South Australia; and (4) species of an unde:cribed 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 Prom- ontory 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 dis- tributions 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-sta- tistics 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- di: :sion. 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 distribu- tion. An analytical method for treating conditional allelic fre- quencies would not be as sensitive to this problem as the private alleles approach. Key words: Gene flow, snails, freshwater, isozymes, Hydro- biidae, evolution, population structure. I NTRODUCTION The evolutionary fate of populations is largely deter- mined by two sets of factors. The first may be charac- terised as - coping factors - -those determining the sur- vival of a population. This set includes external pressures, such as predation, parasitism and disease, extremes of, or changes in, climate, competition from other species or resource diminution. It also includes endogenous prop- erties such as the amount of local inbreeding or the ca- pacity of the breeding system to engender genetic re- combination. The second set may be characterised as - isolating factors - -those factors which affect the evo- lutionary history of a population relative to other, orig- inally con-specific, populations. In general, this history depends on how effective gene flow is in overcoming differentiation inevitably arising from genetic drift or responses to local selection. Conversely, speciation pro- cesses are contingent on an - evolutionarily sufficient" restriction of gene flow between populations that have successfully accommodated the first set of factors. These t wo sets of factors are, however, also inter-related. For instance, it may be only through the introduction of a novel gene from another area that a population is enabled to withstand a climatic change. Many studies have shown that speciation is directly related to reductions in gene flow (reviewed by Grant, 1980; Porter, 1990). It is difficult, however, if not im- possible, to predict what restrictions on gene flow over what period of time constitute evolutionarily significant barriers. The importance of the evolutionary conse- quences of restriction on gene flow is such that its esti- mation remains a goal of many experimental studies (e.g. Skibinski et al., 1983; Waples, 1987; Johnson et al., 1988; Mitton et a/., 1989; Arter, 1990; Porter, 1990; Preziosi & Fairbairn, 1992). There is also continuing interest in the development of mathematical models for the analysis of gene flow (e.g. Slatkin, 1985a; Barton & Slatkin, 1986; Slatkin & Barton, 1989) and/or the the genetical sub- division of species (Cavalli-Sforza & Feldman, 1990). We have been investigating the hydrobiid gastropod faunas of a variety of habitats to characterise their tax- onomic and population genetic structure. We were par- ticularly interested in how biological and environmental differences between these faunas are reflected in the