Diverging trends between heterozygosity and allelic richness during postglacial colonization in the European beech

Variation at 12 polymorphic isozyme loci was studied in the European beech on the basis of an extensive sample of 389 populations distributed throughout the species range. Special emphasis was given to the analysis of the pattern of geographic variation on the basis of two contrasting measures of genetic diversity, gene diversity (H) and allelic richness, and to their relationship. Measures of allelic richness were corrected for variation in sample size by using the rarefaction method. As expected, maximum allelic richness was found in the southeastern part of the range (southern Italy and the Balkans), where beech was confined during the last ice age. Surprisingly, H was lower in refugia than in recently colonized regions, resulting in a negative correlation between the two diversity measures. The decrease of allelic richness and the simultaneous increase of H during postglacial recolonization was attributed to several processes that differentially affect the two diversity parameters, such as bottlenecks due to long-distance founding events, selection during population establishment, and increased gene flow at low population densities.