British swallowtail split from European cousins much earlier than thought, study finds
A new genetic study reveals that the endangered British swallowtail subspecies Papilio machaon britannicus has been distinct for at least 200,000 years, not 8,000 as previously believed.
British Swallowtail: An Ancient Wetland Specialist
The endangered swallowtail butterfly Papilio machaon britannicus, which regularly breeds only in the Norfolk Broads in Britain, has been a distinct subspecies for at least 200,000 years, according to a new study. Smaller, darker, and rarer than its continental relative, britannicus was previously thought to have developed its distinct form during confinement in East Anglian wetlands over the last 8,000 years after the flooding of Doggerland.
However, the genetic research suggests that britannicus is a wetland specialist that may have once been widespread across northern European wetlands, separating from its continental cousins between 200,000 and 1.7 million years ago. The whole-genome sequencing of swallowtail populations across Europe, published in Insect Conservation and Diversity, found some evidence of inbreeding but concluded that the surviving populations are not suffering from damaging mutations.
Conservation Implications
The study is likely to transform conservation approaches to the swallowtail in Britain. Some butterfly experts have recently argued that the continental swallowtail Papilio machaon gorganus, which is more common because its caterpillars feed on a variety of plants, could be introduced to Britain and hybridize the “less successful” britannicus out of existence.
But Mark Collins, president of the Swallowtail and Birdwing Butterfly Trust and a co-author of the paper, said the genetic distinctiveness of britannicus makes it worthy of renewed conservation efforts. “We’re looking at a relict population in the Norfolk Broads that’s not just a relict for Britain but a relict from a once much wider distribution in wetlands across Europe,” Collins said.
The subspecies is threatened by global heating, which causes rising sea levels that salinate the Norfolk Broads, Britain’s largest freshwater wetland. The caterpillars of britannicus feed exclusively on milk parsley, a rare wetland plant also threatened by salination. Most breeding populations live at or below sea level, so the butterfly will need to be translocated to new wetland sites protected from sea-level rise.
Meanwhile, global warming is enabling the continental swallowtail to reach Britain more regularly, with sightings in Kent and Sussex. Collins believes that britannicus can survive in wetlands alongside the continental subspecies if conservation measures are taken, and hybridisation may only occur on the fringes.
