Repeated range expansion and niche shift in a volcanic hotspot archipelago: radiation of Hawaiian Euphorbia (Euphorbiaceae)

Abstract

Aim The taxon cycle hypothesis describes the cyclic movement of taxa during range expansion and contraction, accompanied by an evolutionary shift from open and often coastal vegetation to closed, and often inland forest vegetation in island systems. The Hawaiian Archipelago is an ideal system to test this hypothesis given the linear fashion of island formation and a relatively well-understood geological history.\n\nLocation Hawaiian Islands.\n\nMethods We sampled 153 individuals in 15 of the 16 native species of Hawaiian Euphorbia section Anisophyllum on six major Hawaiian Islands, plus 11 New World close relatives, to elucidate the biogeographic movement of this lineage along the Hawaiian island chain. We used a concatenated chloroplast DNA data set of more than eight kilobases in aligned length and applied maximum likelihood and Bayesian inference for phylogenetic reconstruction. Connectivity among islands and habitat types was estimated using BayesTraits. Age and phylogeographic patterns were co-estimated using BEAST. In addition, we used nuclear ribosomal ITS and the low-copy genes LEAFY and G3pdhC to investigate the reticulate relationships within this radiation.\n\nResults We estimate that Hawaiian Euphorbia first arrived on Kauai or Niihau ca. 5 million years ago and subsequently diverged into 16 species on all major Hawaiian Islands. During this process Euphorbia dispersed from older to younger islands in a stepping-stone fashion through open, dispersal-prone habitats. Taxa that occupy closed vegetation on Kauai and Oahu evolved in situ from open vegetation taxa of the same island. Consequently, widespread species tend to occupy habitats with open vegetation, whereas single island endemic species predominantly occur in habitats with closed canopy and are only found on the two oldest islands of Kauai and Oahu.\n\nMain conclusions The spatial and temporal patterns of dispersal and range shifts in Hawaiian Euphorbia support an intra-volcanic-archipelago version of the taxon cycle hypothesis.