The British swallowtail has turned out to be an ancient and unique species

We are talking about the British swallowtail – Papilio machaon britannicus. It is the largest native butterfly in Great Britain and the only native species of swallowtail in the country. Its breeding is now mainly confined to the Norfolk Broads, where it depends on its host plant – milk parsley.

Details

The British Swallowtail resembles other members of the Papilio machaon complex in appearance: it has pale yellow wings with black markings, blue and red spots, and characteristic ‘tails’ on the hindwings. But appearance does not always reflect the true evolutionary history.

To test how much the British population differs from its continental relatives, the researchers carried out whole-genome sequencing of three British butterflies and compared these data with the genomes of swallowtails from Europe and North Africa. This approach allows us to look not at individual sections of DNA, but at the entire genetic ‘profile’ of the population.

The result proved significant: the British swallowtail is indeed genetically isolated and differs markedly from other populations of Papilio machaon. This supports its status as a valid subspecies and strengthens the case for the separate conservation of the British lineage in particular.

Why this discovery changes the picture

Previously, it was thought that the British swallowtail had become distinct relatively recently – for example, following the submergence of Doggerland, the ancient landmass that once connected Britain to continental Europe. But new research points to a far more ancient history.

According to data cited by The Guardian, referencing the study, the British lineage may have diverged from its European relatives approximately 200,000 to 1.7 million years ago. In other words, this is not a recent island anomaly, but a much deeper evolutionary branch.

Put simply, the British swallowtail is not just a butterfly that became ‘stranded’ in Britain following changes to the coastline. It is a lineage with its own long history, having adapted to marshland habitats and developed a narrow ecological specialisation.

What about its genetic health?

The scientists also investigated whether life in a small, isolated range had led to genetic deterioration. This is an important question: small populations are more prone to inbreeding, that is, the mating of closely related individuals.

Analysis showed that the British swallowtail has around 20 per cent less genetic diversity than its continental relatives. It also has a higher level of inbreeding.

But there is some good news: the researchers found no evidence of a significant accumulation of harmful mutations. The authors therefore phrase their conclusion carefully: the British swallowtail is endemic, but not ‘genetically ruined’. Its status requires attention, but does not appear hopeless.

Why we cannot simply introduce other swallowtails

At first glance, the solution might seem simple: if the British swallowtail is rare, we could introduce more numerous European swallowtails. But genetics show that this is a risky idea.

Continental swallowtails may be more adaptable and feed on different plants. But if they start interbreeding with the British subspecies, this could dilute the unique britannicus lineage. As a result, Britain might retain a ‘similar butterfly’, but lose its own ancient subspecies.

Therefore, the key conclusion for nature conservation is this: we must protect not just a beautiful yellow-and-black butterfly, but the specific British genetic lineage and its habitats.

Why the butterfly is under threat

The British swallowtail is heavily dependent on wetland habitats. Its key breeding ground is the Norfolk Broads, and its life cycle is linked to milk-parsley. If the marshes dry up, become overgrown, become salinated or are altered due to climate change and human activity, the butterfly loses not just a ‘beautiful place’, but the very basis of its existence.

An additional problem is its small range. When a species or subspecies inhabits almost exclusively one type of habitat within a limited area, a single poor season, a disease affecting the host plant, a change in water levels or climatic stress can devastate the entire population at once.

This is precisely why genomic data is important not only for taxonomy. It helps us understand how unique a population is, how vulnerable it is, and which conservation measures might be harmful.

Why this matters

This story shows how easy it is to underestimate a rare species if one looks only at its appearance. The British swallowtail looks like a relative of the European swallowtail, but its DNA tells a different story — a story of isolation, adaptation and survival in the marshes of Britain.

The conclusion is simple: sometimes it is not just the ‘species in general’ that needs protecting, but a specific local lineage. Because if that lineage disappears, a unique part of evolutionary history disappears with it.

The British swallowtail must be conserved by protecting and restoring suitable wetland habitats, rather than replacing it with visually similar continental butterflies.

Background

Swallowtails are one of the most recognisable groups of butterflies in the world. But similar colouration can conceal different evolutionary lineages. That is why modern research increasingly uses genomic data to understand which populations are truly unique and require separate conservation measures.

In the case of the British swallowtail, genomics has helped to resolve a long-standing dispute. Older methods provided an incomplete or contradictory picture, whereas whole-genome analysis has shown that Papilio machaon britannicus is genetically distinct from continental populations.

This is particularly important for insects. Their genetic diversity is less well understood than that of many vertebrates, even though it is insects that are rapidly disappearing due to habitat loss, climate change and intensive agriculture.

Source

Study: Benoit Nabholz et al., “Endemic but not eroded: Genomic distinctiveness and conservation genomics of the British swallowtail butterfly (Papilio machaon britannicus)”, Insect Conservation and Diversity, 2026.