Diversity Estimators for Locating Glacial Refugia

This next post is going to focus on how species diversity is assessed in order to infer areas of glacial refugia. A common assumption is that glacial refugia harbor higher levels of genetic diversity than areas of subsequent colonization. This is the basis on which many glacial refugia are located. However Comps et al. (2001) believe this to be too simplistic, as the way in which genetic diversity is estimated can influence the results. To give a brief background of Comps et al. (2001), they studied European beech (Fagus sylvatica) across Europe. As F. sylvatica is wind pollinated a detailed pollen record is available in which the genetic data can be compared against. In addition it was chosen as its closest relative is Fagus orientalis so there are no problems of interspecies introgression.

The question is what pattern of diversity is expected from recolonization of a refugium? Colonizations of new areas normally consist of populations of a few individuals therefore showing a small sample of the genetic diversity of the source population. How much of a reduction is dependent of diversity estimators. For the new population, allelic richness (which is the number of alleles per population) would be expected to be lower than genetic diversity (which is the probability that two alleles sampled at random from a population are different). In F. sylvatica, allelic richness was lower in populations in Northern Europe, consistent with loss of rare alleles due to recolonization. 

However a reduction in genetic diversity was not found. Comps et al. (2001) found a negative correlation between allelic richness and genetic diversity. This meant areas with a decreased allelic richness displayed an increased genetic diversity. Since the majority of papers only calculate genetic diversity, academics may be misled in the identification of glacial refugia.

Personally I take the findings of Comps et al. (2001) with a pinch of salt. I have used this example not to show that a wrong estimator is being used, just that other estimators are available and might give slightly different results. I believe that the increase in genetic diversity when allelic richness decreased was a conclusion unique to the species in question and general conclusions should not be taken from this case study. It does bring an extra addition to the glacial refugia debate and I believe that diversity estimators should be taken into account when discussing methods of glacial refugia identification.