Plant Speciation on Lord Howe Island

Abstract

Lord Howe Island (LHI) is an ideal location for researching the speciation process. The diversity of unique species, its isolation and minute size offer a rare opportunity to investigate the contribution that speciation has made to the entire flora of an ecosystem. On LHI, speciation in sympatry has been documented previously in Howea palms and this project sought to investigate whether this divergence was an exceptional occurrence or if the process is more general. A phylogenetic approach was used to acquire the first estimates of the frequency of sympatric speciation and speciation with gene flow in a community of island plants. The results indicate that speciation with gene flow may be relatively common on LHI. Biogeographic patterns show that Australia is a major source of species for LHI and that, for a given region, the number of immigrants that can establish and speciate is dependent on dispersal limitation and niche conservatism. Speciation events in two genera (Metrosideros and Coprosma) were examined in greater detail to determine whether ecological divergent selection may have promoted the evolution of reproductive isolation. In both cases, evidence is presented demonstrating that natural selection, habitat isolation and competitive exclusion may have played vital roles in these speciation events. Closer examination of speciation in Coprosma revealed that six species have evolved following a single colonisation of LHI, the first documented evidence for a sympatric radiation in plants. Four of these species have evolved via speciation with gene flow and two species are derived from hybrid speciation events; supporting theories that speciation with gene flow and hybrid speciation may be integral to the onset of an adaptive radiation. Together with speciation in Howea, these new cases show that ecologically driven speciation with gene flow is an important source of biodiversity on LHI and potentially in other botanical communities.