Abstract
Genetic diversity is key to survival of species. In evolutionary ecology, the general centre–periphery theory suggests that populations of species located at the margins of their distribution areas display less genetic diversity and greater genetic differentiation than populations from central areas. The aim of this study was to evaluate the genetic diversity and differentiation in six of the main pine species of the Sierra Madre Occidental (northern Mexico). The species considered were Pinus arizonica, P. cembroides, P. durangensis, Pinus engelmannii, P. herrerae and P. leiophylla, which occur at the margins and centre of the geographic distribution. We sampled needles from 2799 individuals belonging to 80 populations of the six species. We analysed amplified fragment length polymorphisms (AFLPs) to estimate diversity and rarity indexes, applied Principal Coordinate Analysis (PCoA), and used the Kruskal–Wallis test to detect genetic differences. Finally, we calculated Spearman’s correlation for association between variables. The general centre–periphery model only explained the traits in P. herrerae. The elevation gradient was an important factor that influenced genetic diversity. However, for elevation as partitioning criterion, most populations showed a central distribution. This information may be useful for establishing seed collections of priority individuals for maintenance in germplasm banks and their subsequent sustainable use.
Highlights
Ecological research focuses on how environmental conditions and population processes regulate the abundance and distribution of species [1], which reach their highest abundance at the centre of their range and decrease towards the edges [2]
The levels of genetic diversity and differentiation observed in this study suggest that isolation or proximity to marginalisation, probable wood exploitation due to the economic value of these species, together with population decline and fragmentation, may have little effect on the short-term or immediate-term evolutionary potential of central or peripheral populations of five of the six pine species under study
Genetic diversity was high and evenly distributed within populations of all the pine species evaluated, the geographically peripheral populations set a priori did not coincide with the genetic diversity expected for peripheral populations because all populations displayed a predominantly central distribution after running all the analyses; considering the central–marginal theory, it is possible that the populations of the six species studied are central populations because they all exhibit higher genetic diversity and extremely low genetic differentiation
Summary
Ecological research focuses on how environmental conditions and population processes regulate the abundance and distribution of species [1], which reach their highest abundance at the centre of their range and decrease towards the edges [2]. In some specific cases, the geographical and ecological areas are not congruent [5,7,9,10] This incongruent pattern may be explained by one of the three main hypotheses concerning trends in genetic diversity across the central–peripheral clines, each of which has different spatial implications [14]. The first hypothesis, developed by Carson [15], argues that genetic diversity will increase from the periphery of the distribution range towards the centre. Carson suggested that the more continuous, denser and central populations undergo balancing selection and are expected to display higher levels of within-population genetic diversity [15,16]
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