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20 pages, 5 figures, 4 tables, supporting information https://doi.org/10.1111/ddi.13382.-- Data availability statement: Microsatellite genotypes are available on Dryad (DOI https://doi.org/10.5061/dryad.xgxd254gw), Aim: How historical and contemporary eco-evolutionary processes shape the patterns of genetic diversity and differentiation across species’ distribution range remains an open question with strong conservation implications. Focusing on the orange stony coral, Astroides calycularis, we (a) characterized the pattern of neutral genetic diversity across the distribution range; (b) gave insights into the underlying processes; and (c) discussed conservation implications with emphasis on a national park located on a hotspot of genetic diversity. Location: South Mediterranean Sea and Zembra National Park. Methods: We combined new data from 12 microsatellites in 13 populations located in the Centre and in the Western Periphery of the distribution range with a published dataset including 16 populations from the Western and Eastern Peripheries. We analysed the relationship among parameters of genetic diversity (He, Ar(g)) and structure (population-specific FST) and two measures of geographic peripherality. We compared two estimators of pairwise genetic structure (GST, DEST) across the distribution range. The evolutionary and demographic history of the populations following the Last Glacial Maximum was reconstructed using approximate Bayesian computations and maximum-likelihood analyses. We inferred the contemporary connectivity among populations from Zembra National Park and with the neighbouring area of Cap Bon. Results: We demonstrate a decrease in genetic diversity and an increase in genetic differentiation from the Centre to the Eastern and Western Peripheries of the distribution range. Populations from Zembra show the highest genetic diversity reported in the species. We identified a spillover effect towards Cap Bon. Main conclusions: The patterns of genetic diversity and differentiation are most likely explained by “the postglacial range expansion hypothesis” rather than the “central–peripheral hypothesis.” Enforcement of conservation measures should be considered to protect this genetic diversity pattern, in particular when considering the low effective population size inferred at many sites, JBL was funded by a Postdoctoral Grant (SFRH/BPD/74400/2010) from the Portuguese Foundation for Science and Technology (FCT). This research was supported by national funds through FCT within the scope of UIDB/04423/2020 and UIDP/04423/2020 and by the MIMOSA project funded by the Foundation Prince Albert II Monaco. JG, DGG and PL acknowledge the funding of the Spanish government through the “Severo Ochoa Centre of Excellence” accreditation (CEX2019-000928-S). This work was partially supported by the European Union's Horizon 2020 research and innovation program under grant agreement SEP-210597628 (FutureMARES)

European genetic gradients of modern humans were initially interpreted as a consequence of the demic diffusion of expanding Neolithic farmers. However, recent studies showed that these gradients may also be in fl uenced by other evolutionary processes such as population admixture or range contractions. Genetic gradients were observed in the Americas, although their speci fi c evolutionary causes were not investigated. Here we extended the approach used to study genetic gradients in Europe to analyze the in fl uence of diverse evolutionary scenarios on American genetic gradients. Using extensive computer simulations, we evaluated the impact of (i) admixture between expansion waves of modern humans, (ii) the presence of ice-sheets during the last glacial maximum (LGM) and (iii) long-distance dispersal (LDD) events, on the genetic gradients (detected by principal component analysis) of the entire continent, North America and South America. The specific simulation of North and South America showed that genetic gradients are usually orthogonal to the direction of range expansions — either expansions from Bering or posterior re-expansions to recolonize northern regions after ice sheets melting — and we suggest that they result from allele surfing processes. Conversely, our results on the entire continent show a northwest-southeast gradient obtained with any scenario, which we interpreted as a consequence of isolation by distance along the long length of the continent. These findings suggest that distinct genetic gradients can be detected at different regions of the Americas and that subcontinent regions present gradients more sensible to evolutionary and environmental factors (such as LDD and the LGM) than the whole continent.

Various academic disciplines shed light on human migrations, helping us to reconstruct the past. Studying the genetic diversity of human populations today reveals past demographic and migratory events that have left an imprint on our genome. Armed with knowledge of migrations in prehistoric times, we can test hypotheses put forward in other scientific disciplines. Similarly, the distribution of genetic diversity in the future will largely depend on today’s extensive human migrations, facilitated by technological advances.