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Nine wild Iberian provenances of Castanea sativa Mill. grouped in two gene pools, North Iberian Peninsula and Mediterranean, were evaluated for several adaptive traits in two provenance–progeny trials with the aim of evaluating the role of natural selection in shaping adaptive variation and increasing our understanding of the genetic structure of this species, as well as reporting complete information on the genetic variation among and within the studied populations. An annual growth rhythm experiment was evaluated during the first 3 years after establishment for phenology, growth, stem form and survival, and a periodic drought-stress experiment was evaluated for dry weight, growth, survival and other related drought traits in both well-watered and drought-stress treatments. The high genetic variability reported in both trials is largely due to the genetic variation among populations. The significant differences reported between quantitative genetic and neutral marker differentiation indicated the local adaptation of these populations through directional selection, mainly for phenology, growth and biomass allocation. A clinal variation among populations was determined through correlations of phenology with latitude and xerothermic index of the provenances, showing that central and southern Mediterranean populations had earlier phenology than northern populations and that drought played a relevant role in this differentiation. The significant correlation between phenological traits and the ancestry values in the Mediterranean gene pool supported the different pattern of behavior between both gene pools and also indicated the existence of two ecotypes: xeric and mesophytic ecotypes, corresponding to Mediterranean and North Iberian gene pools, respectively. The results obtained in the drought-stress experiment confirmed that, in general terms, xeric populations showed a greater adaptability to drought, with more developed root systems and higher survival than northern populations. Moreover, the genetic variability observed within populations indicated the potential response capacity of Iberian C . sativa populations to undergo fast adaptive evolution.

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In domesticated species, translocation of materials can alter natural demographic patterns; this may have occurred in Castanea sativa (European chestnut), a species conserved in several refuges, with a long domestication history for nut production. Bayesian analysis of population genetic structure in marginal areas and in the centre of C. sativa range, considering spatial information and making corrections for unbalanced size, allowed visualization of a genetic structure that was subsequently analysed by approximate Bayesian computation to assess its natural demographic history and test the origin of some hypothetical translocated germplasm. We obtained evidence of C. sativa population contraction during the earliest Pleistocene, resulting in a split into eastern (Greek) and western (Italian and Iberian) populations. The northern Iberian population, in the Eurosiberian area, is one of the vestiges that remained after the contraction that split the global Tertiary population. A secondary encounter, occurred from Middle to Upper Pleistocene, which explains the hybrid origin of the Western Mediterranean population present in Italy and in the centre and south of the Iberian Peninsula. It has been demonstrated that a germplasm translocation from Italy to the Central Iberian Range may have occurred. Recent translocations have hybridized with the local northern Iberian population, producing naturalized populations with high diversity. The populations of C. sativa in southwestern Europe have a genetic structure compatible with a natural origin, in which signs of population contractions and expansions caused by climatic oscillations since the Late Miocene have been imprinted.