Abbott’s conundrum: why did half of all known recently formed hybrid species arise in the British Isles?

Background: Remarkably, half of the 10 plant hybrid species confirmed to have arisen naturally since 1500 arose within the British Isles (BI). The mechanisms included translocation of natural hybrids to a botanic garden, contact between native and a newly arrived species, and allopolyploidisation from a clonally spreading garden hybrid. Aims: To examine why the BI has so many described hybrid species. Methods: Data on the recording of hybrids from the BI and nine other regions are examined in detail. Results: Four factors played a part. First, Europe has more alien-to-native hybrids than elsewhere, probably due to large-scale habitat change plus phylogenetic proximity between aliens and natives. Second, the large number of botanists on the BI made the discovery of newly evolved hybrid species more likely. Third, the BI has many clonally spreading garden hybrids, which may generate hybrid neospecies. Finally, the eighteenth-century policies of Oxford Botanic Garden allowed the stabilisation of both S. squalidus and Platanus acerifolia, and hence eventually Senecio cambrensis and S. eboracensis. Conclusion: Investigations of these four factors will improve our understanding of the rate at which new species are forming in response to human activity across the world.