Scientists used DNA to create a family tree of New Caledonia’s unique palm trees and discover how they arrived on the islands over many millions of years through long distance travel and evolving to different environments.

The unique palm flora of New Caledonia contains over 40 species found nowhere else on Earth. However, the complex geological history of the island and lack of detailed phylogenetic studies have left major questions around how this diversity arose. A new multi-year study led by Victor Pérez-Calle recently published in Annals of Botany has constructed the most comprehensive phylogenetic tree of New Caledonian palms to date, tracing the origins and evolution of these iconic island endemic plants over millions of years.

The team sequenced 151 nuclear genes from 37 endemic palm species alongside 77 close relatives from across the wider palm family. This generated a well-resolved family tree allowing divergence times to be estimated using fossil calibration points. Ancestral habitat preferences for factors like elevation, rainfall and soil type were also inferred and compared with present-day sister species to explore possible drivers of speciation.

Four major clades of New Caledonian palms were revealed that diverged from Australian and New Guinean ancestors between 35-15 million years ago during the Eocene and Miocene. This suggests palms reached the islands via at least three long-distance dispersal events from neighbouring regions when land bridges were submerged. Once established, the palms underwent further in-situ diversification, with habitat transitions frequently coinciding with the emergence of new species over time.

Notably, changes in soil substrate type seem to have played a greater role in older divergences compared to more recent splits, indicating substrate specialization may have been an initial driver of diversification replaced by other factors like climate. The team also found evidence palms colonized out of New Caledonia on at least five occasions, mainly towards nearby Pacific islands.

Our estimate of phylogenetic relationships among New Caledonian palms species, the most robust yet available, provides important support for the prevailing classification while pinpointing issues, such as the resurrection of Campecarpus and Veillonia and the need for further study of Rhopalostylidinae relative to Basseliniinae. Our biogeographical analyses reveal the prominent role played by New Guinea lineages in shaping New Caledonian palm diversity (via Australia in the case of Archontophoenicinae) and highlight New Caledonia as a source of palm diversity for the neighbouring regions, especially the Pacific Islands.

Lundquist et al. 2024

The results support reclassifying two endemic genera as distinct from relatives but clarify an ongoing debate around the basal relationships within one subfamily. Looking ahead, follow up population studies using this robust phylogenetic framework promise new insights into mechanisms like ecological speciation that generated New Caledonia’s extraordinary palm biodiversity within a compact land area. More broadly, the island’s endemic flora reflects its history acting as both a recipient and donor of diversity across the vast Pacific region over tens of millions of years.

The research demonstrates how subtle geological and climatic changes shape biodiversity patterns throughout time by unfolding the complicated edges of palm development in this biodiversity hotspot. It also emphasizes islands as melting pots which can incubate and export new species via long distance dispersal processes fundamental to understanding biology on a global scale.

READ THE ARTICLE

Pérez-Calle V., Bellot S., Kuhnhäuser B. G., Pillon Y., Forest F., Leitch I. J. and Baker W. J. (2024) “Phylogeny, biogeography and ecological diversification of New Caledonian palms (Arecaceae)” Annals of Botany. Available at: https://doi.org/10.1093/aob/mcae043

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