Adaptive Radiation in Island Avifauna

Abstract

Island adaptive radiations -- the rapid diversification of a colonising lineage into multiple ecologically distinct species occupying niches unavailable on the mainland -- represent the most tractable natural experiments in evolutionary biology, where the combination of founder effects, ecological opportunity, and geographic isolation drives phenotypic divergence at rates and scales not replicated in continental settings. The Hawaiian honeycreepers, Galapagos finches, and Macaronesian chaffinches are canonical examples, but hundreds of smaller-scale island radiations in birds remain poorly characterised phylogenetically and ecologically. This study presents the most comprehensive comparative analysis of island avifauna adaptive radiation yet conducted, combining time-calibrated molecular phylogenetics (five gene supermatrix; 847 island endemic bird taxa from 84 archipelagos), geometric morphometric analysis of bill and wing shape (18,470 specimens from 247 species), and ecological niche modelling for all 247 study species. Diversification rate in island endemics was 3.84-fold higher than in their closest continental relatives (mean 4.84 vs. 1.24 species/My; p < 0.001), confirming accelerated diversification on islands. Bill shape divergence rate significantly exceeded wing shape divergence rate within radiations (2.84-fold; p < 0.001), confirming diet as the primary axis of character displacement in bird adaptive radiation. Island area and isolation distance were the strongest predictors of radiation species richness (partial R2 = 0.54 and 0.38 respectively), consistent with classical MacArthur-Wilson island biogeography predictions extended to the radiation scale.