Biogeographically marginal: source of evolutionary novelties and future potential

Abstract

Towards distribution margins, populations are thought to be genetically impoverished due to isolation and genetic drift acting in often small border populations. Here we analyse the hypothesis that contrary to expectations, populations towards the dry Nothofagus pumilio range are relicts of a past distribution, and thus hold comparable levels of genetic diversity than central ones using conserved chloroplast DNA (cpDNA) sequences. We sampled fresh leaf tissue from a total of 70 natural populations along a precipitation gradient in Northern Patagonia, Argentina, from central (N = 33), i. e. humid, to marginal (N = 37), i. e. dry, locations which were analysed by cpDNA sequencing and phylogeographic methods. We also analysed the fate of populations under global scenarios by ecological niche models. The analysis of a total of 186 cpDNA sequences yielded nine different haplotypes, eight and six of which were present in marginal and central regions, respectively. We identified four new haplotypes for the species, three were exclusive to the marginal region and one was also new for the Nothofagus subgenus. Haplotypes were latitudinally structured as previously documented, yet those from marginal and central regions diverged synchronically at ~3.5 Ma. Coalescence analyses yielded larger effective population sizes for marginal populations, except for the northernmost group, and divergence between regions occurred from early to late Pleistocene. Neutrality tests developed to infer past population size changes showed population stability for marginal and central regions yet current range retraction. Future distributions by ecological niche models predict population decay in both central and marginal regions with a stronger effect in the latter. Our results show that marginal populations are as genetically diverse and demographically stable as central ones and their genetic makeup may assure their long-lasting persistence. Unique variants and genetically-based adaptive traits of the former may be used as germplasm sources for restoration under forecasted droughts.