The typical model of avian speciation proposes the division of an ancestral population into two allopatric populations and subsequent evolution via natural selection, sexual selection, or genetic drift of fixed genetic differences in the daughter populations. Whether these taxa are recognized as species depends on which species concept is followed, with reproductive isolation the requirement for biological species status (Mayr 1942), diagnosability for a lineage-based concept (Cracraft 1983),

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... d divergence of uniquely coadapted mitochondrial and nuclear genes for a mitonuclear compatibility concept (Hill 2017). Under any model of species and speciation (Wiens 2004), however, it is not a foregone conclusion that every isolated population will give rise to a new species. Mayr (1942:155) wrote 'Geographic variation is thinkable only if subspecies are incipient species. This, of course, does not mean that every subspecies will eventually develop into a good species. Far from it! All this statement implies is that every species that developed through geographic speciation had to pass through the subspecies stage.' Molecular work has confirmed Mayr's assertion. Numerous studies have documented that hybridization between differentiated taxa can lead to breakdown of accumulated differences between diverging populations