Recent methodological advances permit refined inferences of evolutionary processes from the fine-scale spatial genetic structure of plant populations. In this issue of Molecular Ecology, Born et al. (2008) exploit the full power of these methods by examining effects of ancient and recent landscape histories in an African rainforest tree species. The authors first detected admixture of distinct gene pools that may have formed in Pleistocene forest refuges. Then, comparing across six study populations in Gabon, the authors found similar patterns of fine-scale spatial genetic structure despite natural and anthropogenic variation in population density. The latter results suggest that enhanced gene dispersal may compensate for low population densities in fragmented landscapes.Keywords: assignment methods, forest history, Gabon, spatial genetic structure, tropical tree Received 31 December 2007; revision received 19 January 2008; accepted 3 February 2008Limited dispersal in plants can generate clusters of closely related individuals, which manifests as fine-scale spatial genetic structure (SGS). SGS develops at landscape levels over ecological timescales (a few generations) and may be represented as a regression of kinship over the logarithm of distance (kinship-distance curve). Development of SGS is the first stage in Wright's (1943) model of isolation by distance (IBD) for continuous populations. The recently developed Sp statistic, which quantifies the strength of SGS, can be used to compare SGS among populations and species (Vekemans & Hardy 2004). Recent meta-analyses have shown that species with self-compatibility, low population densities or poorly dispersed seeds tend to exhibit strong SGS (Vekemans, Hardy 2004;

To select candidate populations of wild species to be given priority for conservation, genetic criteria gained from the study of molecular markers may be useful. Traditionally, diversity measures such as expected heterozygosity or percentage of polymorphic loci have been considered. For conservation we propose instead that priority should be given to measures of allelic richness. To standardize the results of allelic richness across populations, we used the technique of rarefaction. This technique allows evaluation of the expected number of different alleles among equal-sized samples drawn from several different populations. We also show how the contribution of each population to total diversity can be partitioned into two components. The first is related to the level of diversity of the population and the second to its divergence from the other populations. For conservation purposes the uniqueness of a population-in terms of its allelic compositionmay be at least as important as its diversity level. These new descriptors are illustrated by means of isozyme and chloroplast DNA data obtained for an endangered tree species, the argan tree of Morocco ( Argania spinosa (L.) Skeels). With these analyses the conservation value of the argan tree populations, especially those of two isolates present in the north of the country, can be better appreciated. The methods proposed to identify priority areas for conservation of the genetic resources of the argan tree are compared to those sometimes advocated in the case of reserve design, where one of the goals is to maximize species richness. Identificacón de Poblaciones para su Conservación en Base a Marcadores GenéticosResumen: Los criterios genéticos obtenidos del estudio de marcadores moleculares podrían ser útiles para seleccionar poblaciones de vida silvestre como candidatos con prioridad para su conservación. Tradicionalmente se consideran medidas de la diversidad como son la heterocigocidad esperada, o el porcentaje de loci polimórficos. Para medidas de conservación, nosotros proponemos en su lugar que la prioridad se enfoque en medidas de riqueza alélica. Para estandarizar los resultados de riqueza alélica entre problaciones, utilizamos una técnica de vacuidad. Esta técnica permite evaluar el número esperado de alelos entre muestras de igual tamaño obtenidas de diferentes poblaciones. Mostramos como la contribución de cada población a la diversidad total puede ser repartida en dos componentes; el primero esta relacionado con el nivel de diversidad de la población y el segundo con su divergencia de las otras poblaciones. Para propósitos de conservación, la singularidad de una población (en forma de composición alélica) puede ser por lo menos tan importante como lo es su nivel de diversidad, Estos nuevos elementos descriptivos son ilustrados mediante el uso de datos de DNA de isozima y cloroplasto para una especie de árbol en peligro, el árbol argan de Morocco ( Argania spinosa (L.) Skeels). Con estos análisis, el valor de conservación de las poblaciones del árbol argan...

The Mediterranean Basin is expected to be more strongly affected by ongoing global climate change than most other regions of the earth. Given the magnitude of forecasted trends, there are great concerns for the particularly rich biodiversity found in the region. Studies of the consequences of past climate shifts on biodiversity represent one of the best sources of data to validate models of the ecological and evolutionary consequences of future changes. Here we review recent findings from palaeoecology, phylogeography and climate change research to (1) explore possible antecedents of the predicted climate warming in the younger geological history of the Mediterranean Basin, (2) assess how tree populations have reacted to them, and (3) evaluate the significance of the evolutionary heritage that is at stake. A major question of our retrospective approach is whether Quaternary tree extinctions took place primarily during glacial or during interglacial episodes. Available data are scanty and somewhat conflicting. In contrast, abundant phylogeographic evidence clearly indicates that the bulk of genetic diversity in European temperate tree species is almost invariably located in the southernmost part of their range. Long‐term persistence of isolated populations have been common phenomena in the Mediterranean, to the point that the current genetic structure in this area probably often reflects population divergence that pre‐dates the onset of the Mediterranean climate in the Pliocene. In particular, Tertiary migrations into the Mediterranean of tree taxa originating from Asia seem to have left their footprints in the current genetic structure in these slowly evolving organisms. Moreover, phylogeographic studies point to heterogeneous rates of molecular evolution across lineages that are inversely related with their stability. We conclude that relict tree populations in the Mediterranean Basin represent an evolutionary heritage of disproportionate significance for the conservation of European plant biodiversity.

Patterns of chloroplast DNA (cpDNA) variation were studied in eight white oak species by sampling 345 populations throughout Europe. The detection of polymorphisms by restriction analysis of PCR-amplified cpDNA fragments allowed the identification of 23 haplotypes that were phylogenetically ordered. A systematic hybridization and introgression between the eight species studied is evident. The levels of subdivision for unordered (G  ST) and ordered (N  ST) alleles are very high and close (0.83 and 0.85). A new statistical approach to the quantitative study of phylogeography is presented, which relies on the coefficients of differentiation G  ST and N  ST and the Mantel's test. Based on pairwise comparisons between populations, the significance of the difference between both coefficients is evaluated at a global and a local scale. The mapped distribution of the haplotypes indicates the probable routes of postglacial recolonization followed by oak populations that had persisted in southern refugia, especially in the Iberian peninsula, Italy and the Balkans. Most cpDNA polymorphisms appear to be anterior to the befinnina of the last recolonization. A subset of the preexisting haplotypes have merely expanded north, while others were left behind in the south.

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