The London Underground mosquitoes were first noticed during World War II, when tens of thousands of people took shelter in the subway system's tunnels while their city was bombed. The sunless, concrete passageways were different from the mosquitoes' natural habitat — and their normal food source, the blood of birds, was hard to come by. But there were plenty of humans to bite, and abundant standing water to breed in, so the bugs persisted. After seven decades isolated from the outside world, they developed their own feeding and breeding habits and distinct DNA; they can no longer breed with their aboveground kin. The subterranean mosquitoes, known to science as Culex pipiens molestusare an entirely new species. And their existence is wholly thanks to humans.

Culex pipiens molestus is just one of hundreds of creatures whose existence has been shaped by human influence, intentional or otherwise. They include new crops and domesticated animals, but also creatures that were accidentally introduced to new habitats, or that evolved to exploit habitats built by humans. That's according to a review in the journal Proceedings of the Royal Society B this week documenting how humans drive the evolution of new species, as well as the extinction of old ones.

For all our clumsy destructiveness — ecologists say that human activity could precipitate a mass extinction comparable to the one that killed the dinosaurs — Homo sapiens have also instigated the stunningly swift evolution of new species and strains. According to the study, humans have relocated almost 900 known species and domesticated more than 470 animals and close to 270 plants since the end of the last Ice Age 11,500 years ago.

This doesn't necessarily mean that there are some 1,500 new species running around the planet — it's very difficult to determine when exactly a population diverges enough from its relatives that it can no longer interbreed, especially on such a large scale. Even if it's clear that the creature is genetically distinct, it's not always obvious that the change is wholly thanks to human influence.

"There are few categorical examples of, 'This is a species created by man,'" said author Joseph Bull, an ecologist at the University of Copenhagen's Center for Macroecology, Evolution and Climate. "But what we have is a lot of different human pressures that are causing a lot of rapid change in terms of characteristics, and that is happening on a very large scale with a lot of different organisms."

Conservation scientists have spent decades wringing their hands about the extinction crisis — with good reason, Bull said. There have been 1,359 documented extinctions since the last Ice Age; half of those took place in the last 500 years.

"But the speciation side of things could be quite large, and perhaps quite daunting, as well. We need to get a better idea of that to know what our overall impact on the natural world is," Bull said.

Bull and his co-author, Martine Maron of the University of Queensland, identify four main forces of speciation — relocation, domestication, hunting and novel ecosystem creation — and offer examples of each. A 1996 study found that guppies, which were native to South America but now are found all over the world, mature at different rates depending on their habitat and the predators it harbors. The difference is drastic enough that fish from recently separated populations may no longer be able to mate with one another. Domestication has produced hundreds of new kind of plants and animals — each of which, depending on whom you ask, might be considered a new species. In the wheat family alone, farmers around the world grow more than dozen different crops, all of which are wildly different from their wild grass ancestors.

"Unnatural selection" by hunters — who may specifically target creatures with certain desirable traits, such as large size or elaborate antlers — can lead to undesirable results: game with reduced body size, deer with smaller antlers, fish that mature faster (in order to be able to reproduce before they die). If that pressure endures long enough, species might find themselves permanently changed, Bull said.

The construction of human infrastructure — say, the London Underground — can prompt new species to emerge in order to exploit ecological niches that didn't exist before; so can changes in habitat brought about by human activities, like deforestation and climate change. Scientists believe that the neotropical damselfly diverged into several distinct species in response to the fragmentation of its native forests in Central and South America.

The researchers also note that the rise of genetically modified organisms might add to the number of new species if those creatures are able to generate self-sustaining populations or hybridize with existing ones. The dubious prospect of "de-extinction" — the effort to revive long dead species through back-breeding or genetic engineering — and the even more improbable (but "non-zero," as the scientists put it) possibility of moving organisms onto other planets could also contribute to speciation. Meanwhile, we have almost no measure of the change that the microbial world is undergoing, but it's surely just as dramatic.

What's stunning about this human-driven evolution is not just the number of new species that have emerged but the pace at which they do so. One study found that plants in Australia changed dramatically just 150 years after their ancestors were introduced to the continent. It took decades, not millennia, for the London mosquito species to diverge. Some changes took much longer — humans probably spent thousands of years domesticating dogs — but compared to the vast time scales that are typically used to describe Earth history, that is fast.

The number of species influenced by human forces is "comparable" to the number of observed extinctions attributable to humans, the authors write, but the emergence of new kinds of creatures hardly make up for all the damage we've done. Artificial species diversity is not a substitute for natural biodiversity — the complex interplay of different life forms that exists in a healthy ecosystem. Newly created species don't fill the same ecological niches as the ones humans have wiped out; often, those species went extinct because humans made life in their ecological niche untenable. Domesticated dogs can't replace wolves hunted to extinction; an underground mosquito isn't going to provide the same ecosystem services as vanishing bumblebees. They don't fill the same role, and they don't provide the genetic diversity that makes an ecosystem resilient.

“Anthropogenic species represent a nanosecond of the evolutionary time that many ‘natural’ species have passed through,” Christopher Dick, an evolutionary biologist at the University of Michigan, pointed out to Wired. “In conservation, there is no comparing a 10-million-year-old tree or turtle species with a decades-old strain of insect or plant.”

What's more, the introduction or emergence of new species could have an unknown domino effect on other creatures in their ecosystems. It's already been demonstrated that invasive species can lead to extinction of others — introduction of predators, competitors and disease have ravaged vulnerable populations around the globe. New species may hybridize with existing ones, or evolve to displace them, or have any number of impacts on the ecosystem overall.

"When we talk about conservation and conversation trends, it's kind of a crisis discipline," Bull said. "We tend to think, 'the higher the number of species the better off we are.' But that's not necessarily the case."

"A world full of all these strange hybrid species we’ve created and high abundance of certain species we’ve created, like pigeons and rats — having a world full of these kinds of species is not necessarily a picture we’d like to paint," he continued. "People might find this man-made biodiversity just as daunting as man-made loss."

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