Everchanging range: how the changing environment has influenced the genetic diversity and differentiation of an iconic North American palm species.

Background and aims: Understanding spatial patterns of neutral and adaptive genetic variation and linking them to future climate change have become crucial in assessing the genetic vulnerability of species and developing conservation strategies. Using a combination of genomic approaches, this study aimed to explain the demographic history of Washingtonia palms, predict the adaptive potential in Washingtonia palm populations on the Baja California Peninsula (BCP) and southern California, and determine the geographical areas where climate change will have the most drastic effects.

Methods: We used over 5000 single nucleotide polymorphisms from 155 individuals across 18 populations spanning the entire distribution range of Washingtonia palms on the BCP and southern California. We examined past and current genetic diversity distribution patterns and identified outliers using genetic differentiation and genotype-environment association methods. Genetic vulnerability was predicted, and species distribution modelling was utilized for the geographical regions that will be at risk under future climate scenarios.

Key results: Demographic modelling supported a bottleneck related to the Wisconsin glaciation, which was stronger and longer in northern Washingtonia populations. Genomic diversity presented a strong relationship to geography and provided evidence for range expansions from several refugia. Gradient Forest Analysis revealed that the genetic variation was shaped primarily by variables related to latitude and temperature during the coldest quarter, indicating adaptation to local thermal environments. We found limited adaptive potential and high levels of genetic vulnerability in lowland southern and central populations. Accordingly, species distribution modelling found that the southern distribution range will be affected by climate change, particularly under a high-emissions scenario.

Conclusions: Our findings include a history of population bottleneck related to postglacial range expansion, population divergence with limited gene flow, and probable future changes in distribution under changing conditions. Under long-term climate change, southern and central lowland populations of Washingtonia will experience harsher climate conditions and strong genomic offset.