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 througho ut the species range. Special emphasis was given to the analysis of the pat tern of geographic variation on the basis of two contrasting measures of ge netic diversity, gene diversity (II) and allelic richness, and to their rel ationship, Measures of allelic richness were corrected for variation in sam ple size by using the rarefaction method. As expected, maximum allelic rich ness was found in the southeastern part of the range (southern Italy and th e 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 al lelic richness and the simultaneous increase of FI during postglacial recol onization was attributed to several processes that differentially affect th e two diversity parameters, such as bottlenecks due to long-distance foundi ng events, selection during population establishment, and increased gene fl ow at low population densities.