British swallowtail split from European cousins much earlier than thought, study finds
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- The British swallowtail butterfly (Papilio machaon britannicus) has been a genetically distinct subspecies for between 200,000 and 1.7 million years, according to a whole-genome sequencing study published in Insect Conservation and Diversity — far longer than the previously assumed 8,000 years since Doggerland flooded.
- Britannicus is smaller and darker than the continental swallowtail (Papilio machaon gorganus) and is the only swallowtail to regularly breed in Britain, confined to the Norfolk Broads wetlands of eastern England.
- The study found some evidence of inbreeding in britannicus but concluded its surviving populations are not suffering from damaging mutations.
- Mark Collins, president of the Swallowtail and Birdwing Butterfly Trust and a co-author, called britannicus "a relict population... not just a relict for Britain but a relict from a once much wider distribution in wetlands across Europe."
- Britannicus caterpillars rely almost exclusively on milk parsley, a rare wetland plant that salty water from rising seas is rapidly killing across the Norfolk Broads — England's largest freshwater wetland.
- Some lepidopterists want to introduce the continental gorganus subspecies — already breeding occasionally in Kent and Sussex as global heating expands its range — but Collins argued britannicus could "survive into the future" in protected wetlands if hybridization is contained to the fringes.
- Collins said the trust is eyeing translocation sites including Lakenheath in Suffolk, Shapwick in Somerset and sites in Yorkshire to establish new milk parsley-rich wetlands for britannicus.
Why it matters: The new genetic timeline reframes britannicus as a 200,000-year-old relict of a once pan-European wetland fauna, strengthening the scientific and legal case against introducing continental gorganus swallowtails. With salty water already rapidly killing its sole food plant, milk parsley, across the Norfolk Broads, the subspecies will require active translocation to protected higher-elevation sites like Lakenheath and Shapwick to survive.
