Determinants of difference in genetic diversity between populations
The log response ratio values for the genetic diversity measures included in our analysis ranged from 0, indicating cases with no difference between populations, to 4.32, which corresponds to a ratio difference of approximately 75:1 between measures of genetic diversity between two populations. In the main model we found no evidence for a strong effect of geographic or macroclimatic distance, system type or kingdom on the log ratio of mean genetic diversity measures between populations, as the posterior distributions of all parameters overlapped with zero (Table 1b, Table S6.2a). Of all variables, geographic distance had a weak, positive influence on the difference in mean genetic diversity between populations, with the posterior distribution for all 100 combined models slightly overlapping zero, and an increasingly larger range of inter-population differences in neutral genetic diversity observed at higher geographic distances (Table 1b, Table S6.2a, Fig. S6.2b). Across the random terms included in the model, most of the variation was associated with the residual terms, less variation was associated with the response type, and very little effect was attributed towards the phylogenetic term, study and species (Table 1b).
In the model that did not include geographic distance, we detected higher differences in genetic diversity between islands than between mainland populations (Table S6.6b), but this effect did not persist in the model that included geographic distance (Table S6.5b). The zero adjusted model (TableS6.3b), the reduced model of heterozygosity (Table S6.4b) and the models which excluded environmental distance (Table S6.5b) produced qualitatively similar results to the main genetic diversity model.