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Figure 4.5 The presence and absence of isolation by distance in 39 phytophagous insect species with varying dispersal abilities. Isolation by distance is defined as a significant relationship (P< 0.05) between gene flow and geographical distances, and is most prevalent in species with moderate mobility. Data from Peterson and Denno (1998a)

In a review of phytophagous insects, Peterson and Denno (1998a) compared dispersal ability with the patterns of IBD revealed by allozyme data from 39 species. Based on the significance of associations between gene flow and geographical distance (see Figure 4.5), and also on the slopes and the intercepts of the IBD regressions, the authors concluded that from distances of tens of kilometres to more than 1000 km, the IBD was more pronounced in moderately mobile insects compared with both sedentary and highly mobile species. This pattern, which has also been found in studies on other taxonomic groups, can be explained by the high levels of genetic differentiation that may exist between even neighbouring populations of sedentary species, and the low levels of genetic differentiation that may be found between even distant populations of highly mobile species.

It therefore seems that, whether we are looking at single estimates of gene flow or at patterns of isolation by distance, dispersal ability can be a useful predictor of the extent to which genes are exchanged between populations. We are, however, talking about generalities here, and there are many exceptions to this rule. To give just one example, the soil collembolan Orchesella cincta is a small, wingless insect that inhabits soil and leaf litter and is generally considered to be stationary. The Fst values calculated from AFLP data revealed no significant genetic differentiation between several pairs of O. cincta populations that were separated by distances of approximately 10 km, which suggests surprisingly high levels of gene flow (van der Wurff et al., 2003). Conversely, in a comparison of gene flow within different species of carabid beetles, a fully winged species showed similar levels of population differentiation (and hence inferred gene flow) to those of a species with vestigial wings (Liebherr, 1988). Clearly, dispersal ability is only a partial explanation for gene flow, and we shall now take a look at the importance of barriers.

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