Abstract
Oceanic islands originate from volcanism or tectonic activity without connections to continental landmasses, are colonized by organisms, and eventually vanish due to erosion and subsidence. Colonization of oceanic islands occurs through long-distance dispersals (LDDs) or metapopulation vicariance, the latter resulting in lineages being older than the islands they inhabit. If metapopulation vicariance is valid, island ages cannot be reliably used to provide maximum age constraints for molecular dating. We explore the relationships between the ages of members of a widespread plant genus (Planchonella, Sapotaceae) and their host islands across the Pacific to test various assumptions of dispersal and metapopulation vicariance. We sampled three nuclear DNA markers from 156 accessions representing some 100 Sapotaceae taxa, and analyzed these in BEAST with a relaxed clock to estimate divergence times and with a phylogeographic diffusion model to estimate range expansions over time. The phylogeny was calibrated with a secondary point (the root) and fossils from New Zealand. The dated phylogeny reveals that the ages of Planchonella species are, in most cases, consistent with the ages of the islands they inhabit. Planchonella is inferred to have originated in the Sahul Shelf region, to which it back-dispersed multiple times. Fiji has been an important source for range expansion in the Pacific for the past 23 myr. Our analyses reject metapopulation vicariance in all cases tested, including between oceanic islands, evolution of an endemic Fiji–Vanuatu flora, and westward rollback vicariance between Vanuatu and the Loyalty Islands. Repeated dispersal is the only mechanism able to explain the empirical data. The longest (8900 km) identified dispersal is between Palau in the Pacific and the Seychelles in the Indian Ocean, estimated at 2.2 Ma (0.4–4.8 Ma). The first split in a Hawaiian lineage (P. sandwicensis) matches the age of Necker Island (11.0 Ma), when its ancestor diverged into two species that are distinguished by purple and yellow fruits. Subsequent establishment across the Hawaiian archipelago supports, in part, progression rule colonization. In summary, we found no explanatory power in metapopulation vicariance and conclude that Planchonella has expanded its range across the Pacific by LDD. We contend that this will be seen in many other groups when analyzed in detail.
Highlights
long-distance dispersals (LDDs) and local ecological dispersal have never been formulated; but Heads (2017, p. 423) has attempted to identify differences between the two mechanisms
Heads (2016, 2018) speculated that the Vanuatu–Fiji–Tonga island groups did not inherit the majority of their plants and animals from Australia or Asia, but that the biota developed as metapopulations that have survived and evolved on ephemeral islands of the ancient Vitiaz island arc system, possibly before the formation of the Pacific Ocean, until Fiji and associated archipelagos were formed
Molecular dating of the plant genus Cyrtandra (Gesneriaceae) indicates that the genus originated in Fiji around 9 Ma and that the archipelago played an important role in the evolution of the genus with onward dispersals, founder events, and diversifications in French Polynesia, Hawaii, and Vanuatu (Johnson et al 2017)
Summary
LDD and local ecological dispersal have never been formulated; but Heads (2017, p. 423) has attempted to identify differences between the two mechanisms. Heads (2018) stated that the biota of the Loyalty Islands is more similar to that of Vanuatu than to the geographically closer Grande Terre He explained this by metapopulation vicariance, arguing that the Loyalty–Three Kings volcanic arc separated from the Vitiaz–Tonga–Kermadec Ridge around 50 Ma, followed by westward plate rollback, and that the Loyalty Islands came into juxtaposition with Grande Terre around 35 Ma (see Schellart et al 2006). This biotic affinity has been demonstrated in crickets, but the divergence time was estimated to around 2 Ma (Nattier et al 2011). It remains unclear whether this observation was an artifact of incomplete sampling, island-hopping, or metapopulation vicariance that stretches back to the Cretaceous sensu Heads (2018)
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
