PLUNKETT, GREGORY M.1*, PORTER P. LOWRY2, and JONATHAN M. EIBL1. 1Department of Biology, Virginia Commonwealth University, Richmond, VA 23284-2012; 2Missouri Botanical Garden, St. Louis, MO 63166-0299. - Evolution and Speciation in New Caledonian Araliaceae.
The location and geological history of New Caledonia has made this
Pacific island one of the most intriguing spots on earth to study
plant evolution. The island separated from Australia roughly 65 mya,
carrying with it many ancient lineages of angiosperms, some of which
are left nowhere else on earth (e.g., Amborella). Although its
tectonic history has predisposed this island to high levels of
paleoendemism, the unique combination of geography, geology, and
ecology found in New Caledonia has also provided many opportunities
for more recent speciation. Araliaceae have a near-cosmopolitan
distribution, but their generic and species diversity is nowhere
greater than in New Caledonia, making the family an ideal model for
studying both paleo- and neoendemism. To infer the role of geological
history and physical conditions on evolutionary patterns, we present
data on three araliad lineages. The generic diversity of one lineage
(comprising Mydocarpus, Delarbrea, and Pseudosciadium)
appears to pre-date the late Cretaceous separation of New Caledonia
from Australia, but species diversity is likely due to radiations onto
ultramafic substrates, especially in Myodocarpus. For two other
araliad lineages, Polyscias and Schefflera, our data
suggest an arrival in New Caledonia via long-distance dispersal (most
likely after the Cretaceous), followed by relatively recent
radiations. Speciation patterns in these genera, especially in
Polyscias, are closely correlated with the physical diversity
of the island. Species pairs that are morphologically coherent and yet
clearly distinguishable are allopatrically isolated on the basis of
elevation, soil types, and/or local geography. The single species
Polyscias dioica, however, combines high levels of
morphological variability and a broad distribution across the island
with complex patterns of phylogenetic relationships. Species
"breakdown" (via hybridization among once distinct species)
may account for this exceptional pattern.
Key words: Araliaceae, molecular systematics, Myodocarpus, New Caledonia, Polyscias, Schefflera