Comparative phylogeography of California Channel Island endemic Eriogonum (Polygonaceae)

Riley, Lynn

Although islands have long been recognized as ideal natural laboratories for studying evolution, the patterns and processes of evolution have not been well described on near-shore island systems relative to the tremendous amount of research on isolated oceanic islands, like the Hawaiian Archipelago. Less isolated systems, such as California's Channel Islands, have higher potential for both chance colonization and subsequent gene flow. The extent to which this potential is realized is rarely tested, and its evolutionary impact is largely unknown. The current study investigated patterns of colonization and divergence among Eriogonum (Polygonaceae) taxa endemic to California's Channel Islands to determine the role of geographic isolation in driving diversification in a system unusually close to the mainland. Lineages within two clades (Latifolia and Fasciculata) of the traditionally circumscribed subgenus Eucycla were analyzed separately and compared to identify general biogeographic patterns. Combining data from molecular markers variable across a broad range of taxonomic divergences allowed inferences about the phylogeographic structure of Eriogonum at many scales. Phylogenetic relationships within Eriogonum were found to be substantially influenced by isolation, even at these modest geographic distances. Island lineages were generally monophyletic, suggesting that both successful colonization of, and gene flow to, the islands are infrequent. Additionally, colonization was found to follow a stepping-stone pattern through the archipelago. There was no evidence of successful colonization from the mainland to the southern islands, but rather, colonization apparently followed a north to south pattern across the islands. The stepping-stone pattern, coupled with relatively little post-colonization inter-island gene flow, particularly among southern islands, has generated a pattern of more genetically depauperate and divergent lineages on the isolated southern islands. Populations on northern islands are less divergent from one another, but still significantly structured. The combined founder effects, strong inter-island barriers to gene flow, and historic anthropogenic demographic declines have resulted in taxa that are characterized by low observed heterozygosities, extensive population structuring, and extremely small effective population sizes. Although the census numbers have been increasing for all focal taxa since the removal of exotic herbivores from the California Channel Islands, the long-term persistence of the endemic Eriogonum is uncertain.