The monophyly of island radiations: an evaluation of niche pre-emption and some alternative explanations

It has been argued that niche pre-emption is not the only possible explanation for monophyly among Macaronesian endemic plants because (i) interspecific competition is diffuse, not species-specific, (ii) the radiations in question may not in fact be monophyletic, and (iii) later colonists may have hybridized with earlier ones, making a small and undetected contribution to the gene pool of lineages that appear to be monophyletic. The niche pre-emption mechanism does not, however, require species-specific competitive interactions. It merely proposes that the clade created by adaptive radiation will occupy more niche space than the original colonist could on its own. Members of the clade will then collectively inhibit establishment by new colonists more effectively than can a colonist that has not radiated. The monophyly of many larger radiations in the Macaronesian flora is well established and new studies tend to confirm this pattern. A few later-arriving colonists may have undetectably hybridized with earlier arrivals, but this is only a genetic interpretation of the essential idea behind pre-emption, i.e. that early arrivals so outnumber later colonists that the latter cannot establish. We do not therefore believe that hybridization provides an alternative explanation of why groups with multiple colonization failed to radiate in Macaronesia.