Pleistocene climatic fluctuations promoted alternative evolutionary histories in Phytelephas aequatorialis, an endemic palm from western Ecuador

Abstract

Aim: Pleistocene (2.58 Ma–11.7 ka) climatic fluctuations have shaped intraspecific genetic patterns worldwide; however, their impact on species in many regions remains unknown. In order to determine the impact of Pleistocene climatic fluctuations on the tropical rain forests of western Ecuador, we explored the evolutionary history of the endemic palm Phytelephas aequatorialis. Location: Western Ecuador, north-western South America. Taxon: Phytelephas aequatorialis (Arecaceae). Methods: One hundred and seventy-six nuclear genes were sequenced in 91 individuals for phylogenomic and population structure analyses. The time of divergence between identified genetic lineages was estimated using a coalescent phylogenomic analysis. Palaeoecological niche modelling analyses were performed to determine areas of historical climatic suitability since the Last Glacial Maximum (LGM; 22 ka) that potentially acted as forest refugia during the Pleistocene. A Wilcoxon test and Pearson correlations were used to explore how current levels of genetic diversity, in terms of expected heterozygosity (Hs), have been shaped by several palaeoclimatic and geographic factors. Results: Phylogenomic and population structure analyses revealed two main genetic lineages with a north–south distribution, which diverged 1.14 Ma during the Pleistocene. Two potential Pleistocene refugia were identified, one along the Pacific coast of Ecuador and one in the Andean foothills of south-western Ecuador. The location of these refugia agrees with the spatial location of the two genetic lineages. Within the Andean foothills, Hs was lower for the southern lineage than for the northern lineage. Hs significantly increased with decreasing latitude across the species as a whole. Main conclusions: Pleistocene climatic fluctuations promoted intraspecific divergence in P. aequatorialis within the rain forests of western Ecuador. The Andean foothills of south-western Ecuador could be an important area for rain forest evolution because they potentially remained climatically suitable throughout the Pleistocene. Lower genetic diversity in the southern lineage, which apparently remained isolated in the Andean foothills during glacial cycles, adds evidence to the presence of a forest refugium in south-western Ecuador. The geographic pattern in genetic diversity suggests that P. aequatorialis colonized western Ecuador from the north. This study supports the role of Pleistocene climatic fluctuations in promoting intraspecific divergence, and for the first time, we show their impact west of the Andes.