Whether Darwin’s mystery of mysteries is actually mysterious depends on the species concept: A bird’s eye review

Abstract Speciation has been termed Darwin’s mystery of mysteries. I suggest that whether speciation is deemed mysterious is species-concept dependent. I review speciation under the biological species concept (BSC) and the phylogenetic species concept (PSC) with a focus on the geography and evolutionary processes of avian speciation. In birds, the dominant geographic mode is allopatry. Under a BSC, speciation requires intrinsic reproductive isolation between taxa, an epiphenomenon, a process that has appeared mysterious because of the varied ways reproductive isolation can arise between sister taxa. Using intrinsic reproductive isolation to rank taxa as species can lead to paraphyletic species taxa, which are inappropriate for evolutionary and comparative studies. In the PSC, speciation involves the evolution of diagnostic characteristics between allopatric populations via genetic or phenotypic drift, natural selection, and sexual selection, which are not mysterious. Here I place pairs of New World warblers along a speciation continuum, which revealed that some species currently considered biological species are actually phylogenetic species that retain the ancestral ability to hybridize. A BSC and PSC classification of Galapagos finches reveals differences owing to hybridization. Any study of speciation should declare a species concept, criteria for recognizing species, and a research program to identify processes of lineage divergence or reproductive isolation.