About the author
Brock Mashburn is a PhD Candidate at Washington University in St. Louis and the Missouri Botanical Garden, working in the labs of Dr. Christine Edwards and Dr. Ken Olsen. He is interested in applying genetic methods to practical conservation, as well as questions regarding speciation and biogeography.
On August 7, 1947, the raft Kon-Tiki (Fig. 1) smashed into Raroia Atoll in French Polynesia. The crew of six, captained by naturalist Thor Heyerdahl, had constructed Kon-Tiki from natural materials and set sail from Peru 101 days prior, travelling nearly 7000 km on ocean currents. The trip was verification of Heyerdahl’s theory that pre-Columbian South Americans could have also travelled on ocean currents, making contact, trading, and intermarrying with Polynesian cultures centuries before European exploration (Heyerdahl 1950, Moreno-Mayar et al. 2014). The success of the Kon-Tiki expedition also supported Charles Darwin’s hypothesis that plant seeds could make similar long-distance dispersal events floating on water or debris rafts (Darwin 1856). More recently, genetic studies have provided numerous examples suggesting that long-distance dispersal is a significant contributor to biotic communities on disparate islands around the world (de Queiroz 2005).
Long-distance dispersal events add complexity to our understanding of the evolution of oceanic island biotas. Oceanic islands are landmasses that rise from the ocean via volcanic or tectonic activity and, as such, initially have no plants or animals inhabiting them. Thus, in accord with Island Biogeography Theory, we might expect the first and, indeed, the majority of arriving species to come from the nearest ‘source’ landmasses, either continents or nearby islands (MacArthur & Wilson 1967). This is a logical expectation, which makes it even more exciting when exceptions to the rule are found. One striking example occurs in the plant genus Acacia, where, within the last 1.4 million years, the species Acacia koa dispersed from the Hawaiian islands to the Mascarene islands, ca. 18,000 km away (Le Roux 2014). Such discoveries lead one to question: how connected, really, are oceanic islands across the globe? Do extreme long-distance dispersal events, particularly direct dispersal between oceanic islands, contribute more to the evolution of island biotas than we thought, or is the example of Acacia koa simply an exciting anomaly?
My PhD research examines the history of long-distance dispersal in the island-specialist group Hibiscus section Lilibiscus, which includes the famous Hibiscus flowers found in most tropical gardens under the name Hibiscus rosa-sinensis. What makes Hibiscus sect. Lilibiscus so fascinating is that, of the ca. 28 species in the group, only two occur on a continent (East Africa), with six species in Madagascar and 20 species spread across three volcanic archipelagos (Mascarenes, 6; Fijian, 5; Hawaiian, 9). That means that this group has managed to disperse and inhabit oceanic islands in the Indian and Pacific Oceans while somehow avoiding the large landmasses in between, such as Australia and Southeast Asia (Fig. 2).
In addition to being a group of biogeographic oddities, 13 of the oceanic island species are listed as critically endangered, largely due to rapid habitat loss that has taken place on these islands in the last 100 years and more. So, while my work uses Hibiscus sect. Lilibiscus to test evolutionary theories of island biogeography and speciation, I also attempt to contribute to their conservation. In fact, we have recently published a conservation genetics study of Hibiscus liliiflorus, a species from the island of Rodrigues, in the Journal of Heredity (Fig. 3).
I will soon head to Madagascar for field work, funded by the AGA’s Evolutionary, Ecological, or Conservation Genomics (EECG) Research Awards. My goal is to collect material of the six species in Madagascar to add them to the global phylogeny we are constructing for Hibiscus sect. Lilibiscus. With these samples from Madagascar, we will have complete species sampling of the group, allowing us to understand the patterns and tempo of dispersal and speciation on volcanic archipelagos across the world. We hope that this will allow us to approach an answer about how Hibiscus sect. Lilibiscus became a group of oceanic island specialists and how it came to defy biogeographical expectations by spreading so far around the world.
References
Heyerdahl, T. 1950. The Kon-Tiki Expedition: By Raft Across the South Seas. (Allen and Unwin).
Moreno-Mayar JV, Rasmussen S, Seguin-Orlando A, Rasmussen M, Liang M, et al. 2014. Genome-wide Ancestry Patterns in Rapanui Suggest Pre-European Admixture with Native Americans. Current Biology 24(1): 2518–2525.
Darwin, C. 1856. On the Action of Sea-water on the Germination of Seeds. Botanical Journal of the Linnean Society 1(3): 130–140.
de Queiroz, A. 2005. The resurrection of oceanic dispersal in historical biogeography. Trends in Ecology and Evolution 20(2), 68–73.
MacArthur RH, Wilson EO. 1967. The Theory of Island Biogeography. Princeton: Princeton University Press.
Le Roux JJ, Strasberg D, Rouget M, Morden CW, Koordom M, Richardson DM. 2014. Relatedness defies biogeography: the tale of two island endemics (Acacia heterophylla and A. koa). New Phytologist. 204(1): 230–242.