COS 150-4 - Effects of Insularity on Genetic Diversity Within and Among Natural Populations

Background/Question/Methods

We conducted a quantitative review of genetic diversity (GD) within and among populations in relation to categorical population size and isolation (“insularity”). Insular populations are often thought to have reduced within-population GD, and increased among-population GD due to higher rates of fixation and drift, and lower rates of mutational and migration input. Because of this, populations identified by biogeography as insular are often highlighted as having inherent conservation importance or value due to either their lack of GD or unique adaptive alleles. However, it has also been established that the range of GD within natural populations is much less than the range in population sizes, often referred to as “Lewontin’s Paradox”. Given GD’s clear importance to evolution, ecology, and conservation, we set out to test these assumptions regarding GD and establish what proportion of natural populations actually do have reduced within-population and increased between-population GD due to a priori identified insularity. Using a paired design which considered populations from within the same studies, we were able to control for between-study variation in methodology, as well as demographic and life histories. Studies which included both insular as well as non-insular populations were identified, and deviations from equality between these were identified.

Results/Conclusions

Contradictory to typical expectations, insularity had relatively minor effects on genetic diversity (GD) within and among population, which points to the more important roles of other factors in shaping evolutionary processes. Only 26 of 54 studies had reduced average GD within insular populations compared to non-insular populations in the same study. Nor were these studies randomly distributed – they were biased towards studies that found relatively high levels of GD to begin with. That is, insularity influenced GD in a study system only when GD was already high in non-insular populations of the system. We propose several explanations for these findings, including the idea that evolutionary and demographic histories of those populations may mean the GD “available” to the meta-population is easily supported by even the small and isolated sub-populations. We also discuss who certain life histories may be slowing declines in GD, such that while at equilibrium the insular populations will have lowered GD, that may be much longer that generally intuited. These conclusions were more robust for within-population GD than among-population GD, although a number of biases might underlie this difference. Overall, our findings indicate that population-level genetic assumptions need to be tested rather than assumed in nature.