Papilio polytes, a butterfly so good at disguise, has been fooling not just predators but also scientists for over a century. What was once considered a single species with regional variations has now been revealed to be at least three distinct species. But the story doesn’t end there - these butterflies are helping researchers to answer one of evolution’s most fundamental mysteries: how new species form.
Using genomic tools and behavioural observations, a recent study at NCBS examined butterfly populations in Southeast Asia to reveal how species split and evolve when their populations become geographically separated.
Traditionally, evolutionary biologists studied traits like wing patterns or body size as adaptations to local environments. Meanwhile, others focused on geography, arguing that when populations are physically separated –allopatry-- they slowly accumulate genetic differences until they can no longer interbreed.
The most dominant theory for much of the 20th century stated that geographic isolation causes genetic drift (random fluctuations of genes across generations) to split a species apart. But this raised new questions: how much of this divergence could be chance events, and how much is shaped by natural selection?
The Papilio polytes species is a particularly good system to answer these questions. “These swallowtail butterflies are famous for their female-limited mimicry–while males look like typical swallowtails, many females have evolved to mimic poisonous species like the aposematic Pachliopta butterflies,” says Dr. Riddhi Deshmukh, lead author of the study. Scientists thought P. polytes was just one widespread species found from India to the Philippines, split into many subspecies that varied slightly in wing colour and tail shape. But something didn’t add up. Some “subspecies” didn’t mate successfully in lab experiments. Recent genetic studies began to hint that P. polytes might be more than one species.
Deshmukh and team from Prof. Krushnamegh Kunte’s group at NCBS sequenced 82 butterfly genomes and reconstructed their evolutionary relationships. They found that what had long been lumped together as P. polytes was three distinct and geographically separated species:
Papilio polytes, Papilio theseus (formerly known as P. javanus), and
Papilio alphenor–each with its subspecies scattered across mainland and island regions. “This divergence likely began around 4 million years ago, with the youngest split - between P. polytes and P. theseus - happening about 1.5 million years ago. That’s fairly recent in evolutionary terms,” says Deshmukh.
To confirm that these weren’t just arbitrary genetic lineages, the researchers also did behavioural experiments. They found strong reproductive barriers between the species: even when placed together, individuals of different species didn’t mate freely. The barriers were both prezygotic–before fertilization, like mate choice and postzygotic –after fertilization, such as reduced viability of offspring. These species had become reproductively isolated - an important criterion for defining species.
In one species, the team found that some genes across the butterflies' genomes showed signs of natural selection - these genes had swept through the population rapidly, suggesting they may help the species adapt to its local habitat and conditions. These genes were linked to traits like mimicry, mate choice, and how the butterflies sense the world, showing that natural selection played a key role in their evolution.
“For decades, research focused on speciation in sympatry - when populations diverge while living in the same area. Many examples of such divergence, like mimicry or mate preference, showed clear signs of selection. But allopatric speciation was often treated as slower by genetic drift. Our study shows that even when populations are geographically separated, selection can leave strong footprints on the genome -even over a relatively short period of time. This supports the idea that adaptive divergence isn’t just limited to sympatric situations. Sometimes, even allopatric species evolve under the dual pressures of isolation and selection,” says Prof.Krushnamegh Kunte, the principal investigator of the study.
0 Comments