From Microcosm to Macrocosm: Adaptive Radiation of Darwin’s Finches

Abstract

In this perspective we show the value of studying living organisms in the field to understand their history. Darwin’s finches are an iconic example of the early stages of speciation in a young adaptive radiation that produced 18 species in little more than a million years. The question they pose is how and why so many species originated and diversified rapidly. A long-term study of four species on the small island of Daphne Major, combined with genomic investigations, provide some answers in terms of extrinsic and intrinsic factors. Beak size and shape, as well as body size, are key heritable features involved in both ecological and reproductive isolation, and their evolution by natural selection is caused by competitor species during prolonged droughts. Introgressive hybridization of related species is rare but recurring, apparently widespread, increases genetic variation and does not incur a fitness cost. Hybridization can produce a new species. We use a phylogeny based on whole genome sequences to infer morphological transitions in the radiation. Several lines of evidence indicate that some species are missing from the early phase of the radiation due to extinction. Combining these results, we recast the classical allopatry-then-sympatry theory of adaptive radiation as a competition-selection-hybridization process that generates a diversity of species.