The tunnels of the London Underground are hot, dark, and damp. Crowds of warm-blooded humans congregate on the platforms. Stagnant pools of water collect beneath the tracks. The subterranean transit system is, in other words, a lovely home for a mosquito (or many thousands). At some point during the last 150 years or so, a small population of mosquitoes came to the same conclusion, settling into the interconnected tunnels of the world’s oldest underground railway. They became infamous pests, feasting on Londoners who took shelter in the tunnels during World War II and hassling modern-day maintenance crews.

Although the insects look just like their aboveground counterparts, the common house mosquito Culex pipiens, their behavior is remarkably different. Unlike the street-level mosquitoes, the subterranean insects feed on mammals, are capable of breeding in confined spaces, do not require a blood meal before laying eggs, and do not go dormant during the dreary London winters. The two populations also have significant genetic differences and cannot interbreed. Together, these observations suggest that the mosquitoes that first colonized the tunnels have evolved, developing traits that better suit their underground environments. The subway, it seems, has birthed a brand new species.

We are living, it is often said, in Anthropocene, an era in which humans are radically altering Earth’s ecosystems. As evidence of our impact, experts often point to the dismal extinction statistics: Nearly 800 plant and animal species are known to have gone extinct since 1500, a figure that is surely an underestimate, and current extinction rates may be 100 to 1,000 times higher than before Homo sapiens appeared on the evolutionary horizon. We may even be teetering on the precipice of a global mass extinction event—the sixth in the planet’s history. “The primary narrative that you’re hearing about biodiversity in the Anthropocene is a story about extinctions,” says Erle Ellis, an ecologist at the University of Maryland, Baltimore County.

But human-induced extinctions are only part of the story. Our lifestyles and activities—the way we use the Earth’s land and exploit its resources—are also driving the evolution of new traits in organisms across the globe, and even, in some cases, the emergence of entirely new species. “It’s a counter narrative,” Ellis says. “Human-induced speciation.” This oft-overlooked phenomenon, the focus of a recent review paper, paints a more nuanced picture of the ways in which humans shape life on this planet—and exposes our need to reconsider how we measure biodiversity and exactly why we value it.

“If our target is to prevent the loss of species and we are creating new species, then implicitly, we're saying that we're happy to end up in a world where we have the same number of species as now, but some of them are different species or ‘artificial’ species,” says Joseph Bull, a conservation biologist at the University of Copenhagen and one of the authors of the recent review. “Is that really the vision we have?”

Humans have been driving evolution for millennia, often intentionally: Over the past 11,000 years, we’ve domesticated more than 700 plants and animals, turning the gray wolf into the toy poodle and the bland, bitter watermelon into a sweet, summertime snack. Among the 95 species that make up the world’s most important crops, at least six are human creations.

But we also shape other species indirectly. Hunting, fishing, and harvesting, for instance, can lead to selection against the very traits that we prize. Consider a naturally occurring genetic variant that gives some red foxes iridescent, silver coats. Furs from these foxes fetched higher prices than the traditional rust-colored pelts, leading economically rational hunters to target the silver canids, removing them from the gene pool at disproportionate rates. In 1830 in Eastern Canada, silver foxes made up 16 percent of the population; a century later, that figure had declined to just 5 percent. Likewise, hunters’ hunger for trophy specimens has led to a decrease in horn size among Canadian bighorn sheep.

We continuously reshuffle the planet’s species, moving organisms to foreign countries and continents, where they might interbreed with the native flora and fauna or develop traits that better suit their new homes; given enough time and separation from their parent populations, they may eventually become different species.

Even when plants and animals stay put, we radically transform their habitats. We carve up landscapes, leaving isolated, fragmented populations to evolve away from each other in the islands of wilderness that remain. Thanks to deforestation in Central America, the giant helicopter damselfly appears to be splintering into multiple species . And we create entirely novel ecosystems, such as subway tunnels and light-drenched cities. Artificial lights can be hazardous to nocturnal insects, drawing them to their deaths. And so some urban moths are evolving, becoming less attracted to light than their more rural counterparts, according to research released earlier this year. Climate change, and our widespread deployment of pesticides and antibiotics, are also driving the development of new traits in plants, animals, and microbes.

Technology is now giving us new opportunities to alter evolutionary trajectories. Genetic engineering allows us to meddle in genomes directly, and even space exploration could ultimately play a role in speciation, Bull says. “It sounds ludicrous to bring space travel into a paper on speciation and conservation biology,” he acknowledges. But speciation is a long process, one that unfolds over centuries or millennia. “What’s going to happen in terms of space travel over the next two, three, four, five hundred years, given what's happened in the last forty?” If humans transport microorganisms to another planet—even accidentally—those microbes could ultimately evolve into a species unlike anything we’ve ever seen on Earth.

Of course, “species” itself is a fuzzy concept, and there’s no bright dividing line that marks when one damselfly species officially becomes two. The appearance of a novel trait does not necessarily lead to the emergence of an entirely new species—and even when it does, it can be difficult to pinpoint the cause. (Indeed, some scientists believe that the London Underground mosquitoes are migrants from southern climes, rather than descendants of street-level insects that evolved inside the tunnels.)

Still, there's reason to believe that our impact on speciation is significant. In a 2015 paper, biologist Chris Thomas reported that over the last 300 years, more new plant species, primarily hybrid flowers, have materialized in Britain alone than are known to have gone extinct in all of Europe. Overall, the current rate of plant speciation could be hundreds or thousands times higher the natural background rate, he estimated. Meanwhile, human hunting and harvesting may be accelerating the rate of evolutionary change by as much as 300 percent, according to a 2009 study. “Even if a tiny fraction of those species where evolution rates are being sped up resulted in new species emerging, that would still be a lot of new species,” Bull says.

Considered alongside the looming extinction crisis, the potential speciation boom suggests a provocative possibility. “To take the idea to its extreme,” Bull says, “what would happen if we were having a ‘no net loss’ impact on species? What if really we were creating so many that they equal the number of extinctions that we're seeing?”

Species richness is a classic measure of biodiversity, a yardstick for conservation success—or failure. The commonly understood goal: to prevent the total number of species on the planet from plummeting. But that calculus suggests that we can compensate for an extinction simply by creating a new species in its place. To Bull—and likely to most of us—that trade-off feels unacceptable. “But why is that? Why do we instinctively feel that that's not correct?” Bull says. “If it's not species numbers, which is the way it's often framed in global conservation, what is it that we actually care about?”

There are solid scientific answers to these questions. For one, the organisms that develop in response to human pressures may be far less diverse than those that disappear. Some vulnerable species, such as the dugong and the purple frog, are evolutionarily distinct, with unique histories and few close relatives. If they die out, so do entire branches of the evolutionary tree. Creating yet another lineage of sheep or wheat won’t restore the lost diversity. What’s more, novel species may fill different niches than those that become extinct; the birth of a new type of mosquito in cities can’t possibly offset the loss of great apes in rainforests.

Beyond these biological facts, however, the prospect of trading existing creatures for those of our own creation seems to run counter to our values and ideals, including the (not-always-rational) premium we tend to place on all things “natural.” One reason we treasure nature in the first place, Bull says is “because it’s not man-made, because there's a sense that it's what should have been there if we hadn’t interfered.” Stocking what remains of the wild with human-created species violates that entire premise.

Ellis puts it slightly differently. The organisms alive today are part of the planet’s heritage, he says. Even if we create new species, extinctions represent a loss of this shared heritage that we can’t get back. “It’s like a work of art,” he says. “You can’t make yourself a Leonardo da Vinci whenever you want one.” Extinction is a natural phenomenon, of course, and humans aren’t always to blame. “But from a sentimental point of view, a human point of view, I don't want to see those species lost,” Ellis says.  

None of this means that the species that we coax into being have no worth or that they can’t play a vital role in the planet’s ecosystems. But the phenomenon of human-driven speciation, and its likely scale, suggests that it’s time to clarify what biodiversity means to us and precisely what we’re trying to preserve. Climate experts have set concrete goals for the future, detailing clear temperature and emissions targets. Conservation biologists, Bull says, should take a similar approach. “What nature conservation needs to do is answer questions like the ones we raise in this paper,” Bull says. “Where is it we're trying to get to? What do we want the natural world to look like? And then we can work out how we're going to get there.”