For decades, Culex pipiens f. molestus was called the “London Underground Mosquito”. Most people, including scientists, believed molestus rapidly evolved from an existing species in northern European cities, particularly in London’s subway system during the 19th and 20th centuries. This idea, popularized during World War II when people took shelter in the subways and suffered relentless mosquito bites, suggested that molestus had adapted to the underground environment in record time.
But a new study suggests the mosquito is far older. A team of researchers analyzed the genomes of Culex pipiens mosquitoes from around the world, including historical samples from London dating back to the 1940s. They found that molestus is not a recent urban species. Instead, its origins trace back over 1,000 years to the Middle East.
Mosquito cousins
Culex pipiens f. molestus is a nasty biter that seems to enjoy subways and underground passages all around the world. It’s so fond of these spaces that the theory of it evolving underground seems entirely plausible.
But when Lindy McBride at Princeton University and her colleagues analyzed the DNA of 790 mosquitoes from 44 countries, they found something else.
The mosquito is genetically closer to populations from the Mediterranean basin than those in northern Europe. The Mediterranean ones are also more genetically diverse than the London and northern Europe ones.
The researchers conclude that molestus first emerged in an aboveground, human-associated environment, possibly in the Mediterranean basin. The mosquito likely adapted to human settlements, thriving in warm climates where water storage and irrigation systems created stable breeding grounds. Later, molestus spread northward and eventually occupied underground urban environments in colder regions, where it found suitable conditions.
But this doesn’t mean that it stopped evolving in the underground.
Developing a taste for humans
The initial form of the mosquito, Culex pipiens, is primarily a bird biter. But f. molestus seems to prefer humans over birds. The two forms are genetically distinct in colder northern regions, where molestus stays underground and pipiens remains aboveground. In southern climates, however, the two forms are more genetically similar and sometimes interbreed.
However, even in crowded urban environments, the two can interbreed. This is concerning because the hybrids can bite both birds and humans, possibly passing diseases from birds in the process.
The story of this mosquito highlights that cities are powerful forces shaping evolution. Far from being static environments, urban landscapes impose new challenges—extreme temperatures, pollution, artificial light, and novel predators—all of which drive rapid adaptations. Understanding these changes is not just a matter of scientific curiosity—it is crucial for public health, conservation, and predicting how species will survive in an increasingly urbanized world.
The study was published in bioRxiv.
