Regular riders of the London Underground may bemoan the iconic and occasionally fierce tube mice. But there is another, smaller animal living in the Underground that is perhaps even more at home in the subterranean network – since it actually evolved in the unique conditions of the tube environment.

The London Underground mosquito is a genetically distinct subspecies. It was first reported during the Blitz of World War Two, when the Tube's tunnels were used as overnight bomb shelters. Over the course of the war, almost 180,000 people sheltered in the Underground. They were ravaged by all sorts of insects.

"The Tube then was a very different place than it is now," says Steven Judd, Head of Environment for the London Underground. With standing water and different pest controls, flies, ticks, lice and fleas were a lot more common than they are now, he says.

After the war, other than the odd complaint of biting made by maintenance workers, the mosquito received scant attention. That was until almost 50 years later when a London-based doctoral student decided to study these subterranean biters.

Over the course of the war, almost 180,000 people sheltered in the Underground

Katharine Byrne collected mosquitoes from seven sites across the 180km (110 mile) network. She found they were fundamentally different from their surface-dwelling relatives. While the above-ground Culex pipiens bit only birds, the Culex pipiens molestus – named for their tendency to molest – had a taste for human blood.

"The Culex is a very common mosquito," says biologist Bruno Gomes from the Liverpool School of Tropical Medicine. "There are hundreds or thousands of types of them and they're not very harmful."

While they look the same as Culex pipiens, the molestus mosquitoes behave in a different way. Aside from being bird-biting, the above-ground midge-like flies hibernate in the winter, need blood to lay their eggs and require a lot of space in which to mate. The new subspecies does not require any of these.

"These differences can be interpreted in a straightforward way as adaptations to a subterranean life," wrote Byrne. Since there were no birds to feed on, they began feeding on mammals, mostly rats and humans. They mated in closed areas, because they had to – and they lost their tendency to hibernate in winter because there are no obvious seasons underground.

The tunnels were largely sealed off from the surface, and some of the mosquitoes found themselves trapped underground

Byrne also found the underground mosquitoes are now so distinct they can no longer interbreed with other mosquitoes.

"There are differences in both the mating behaviour and the reproductive biology," says David Reznick, a biologist at the University of California in Riverside.

While the surface mosquitoes form big swarms in order to pair off and breed, underground ones are not as abundant so it is just individuals who choose each other to mate.

After the London Underground system's construction, the tunnels were largely sealed off from the surface, and some of the mosquitoes found themselves trapped underground. It was this physical barrier, Byrne wrote, that caused the divergent evolution of the two populations.

It was "evolution by natural selection but in a speeded-up form," writes Tom Quinn in his book London's Strangest Tales.

It's unclear if it evolved there or was brought into the Underground system

With one species breeding above ground and the other below, for some 100 years, the stage was set for a new subspecies to evolve. Scientists say that it might have taken just a few hundred generations to do so.

"People usually think of speciation as being very slow and as something you can't see happening," says Reznick. "But in this example, you can sort of see it happening. It's a relatively recent phenomenon and you can see a clear start date" – when construction of the London Underground began in 1863.

Not everyone is convinced though.

"It's unclear if it evolved there or was brought into the Underground system… from the freight and fruit movement into the docks of London," says Judd. There is not enough research to give us an indication, he adds.

It is also found in metropolitan Tokyo and in the New York subway

This might come as a surprise, given that there is actually a diverse array of species available to study in the London Underground network. Mice, foxes and even tortoises have been found below ground, says Judd. They live amongst tunnel fluff, made up of human hair and clothes fibres, and tunnel dust: carbon that comes from the train's brake shoes.

"In 2015 we had no customer complaints related to biting incidents, with 1.3 billion passengers travelling on the system," he says.

What is clear is that the molestus mosquito is not unique to the London Underground, says Gomes.

It is actually found in all sorts of human underground constructions, from water systems to the basements of large houses. It has been found in similar enclosed environments, such as caves and sewers, across Western Europe, particularly temperate countries such as Spain and Portugal. It is also found in metropolitan Tokyo and in the New York subway.

Still, Reznick argues there are a number of genetic factors that suggest the underground mosquito first evolved in London.

Mice, foxes and even tortoises have been found below ground

For instance, Byrne's research involved sampling 12 surface mosquito populations near Underground sites. She compared the genetic makeup of overground and underground mosquitoes and found their alleles, or gene variants, to be incredibly similar.

"If they [the subterranean mosquitoes] had come from Spain, you would expect them to have distinct alleles to those above ground," says Reznick. The underground insects should then have been most closely related to Spain's above-ground mosquitoes. But this was not the case: London's underground and overground mosquitoes are each other's closest relative.

What's more, the Underground mosquitoes are all quite genetically similar.

"If you have a big population of humans and then 20 go and colonise an island somewhere, the colonisers will only have genes that are a small subset of the initial population," says Reznick.

A few hundred years in the right circumstances can form a new species

The underground mosquitoes are so similar to each other that it "suggests a small number of genetic individuals created this population", he says.

Reznick says finding the molestus mosquito in other countries only further demonstrates how incredible the process of speciation is. It "shows that the capacity of surface mosquitoes to invade the underground exists elsewhere with the tendency to evolve other species", he says.

It has also shown that speciation does not have to be a painstakingly slow process taking place over tens of thousands to hundreds of thousands of generations, as Darwin first speculated.

"A few hundred years in the right circumstances can form a new species," says Reznick.

He points to the example of the marine stickleback fish in Anchorage, Alaska, which are changing genetically to adapt to freshwater environments following an earthquake in the 1960s which created new lakes there. Given that a generation in these fish lasts about a year, Reznick says speciation is occurring within 50 generations.

And, unlike the London Underground mosquito, the marine and freshwater sticklebacks do not have such a distinct physical barrier between them. "This is all happening without geographic isolation," he says.

I just love that it happened on Darwin's own turf and began almost the same year he wrote his book

Still, with no one having examined the London Underground mosquito since Byrne in the late 1990s, he says work needs to be done to bring our understanding up to scratch. "The genetic tools that they were using aren't the ones we'd use today. The truth is – it would be nice if someone studied the mosquitoes more closely."

While Reznick has followed this speciation process in a number of organisms – from moths to flies to invasive plants – the London Underground mosquito has such a special spot in his heart that he devoted a chapter to the insects' evolution in a 2010 book he wrote on the subject of Darwin's 1859 classic Origin of Species.

"I just love that it happened on Darwin's own turf and began almost the same year he wrote his book."

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