Human-Driven Evolution Is a Hallmark of the Anthropocene
During World War II, Londoners often sought shelter from German bombs in the city’s subway tunnels. There, they encountered another type of enemy: hordes of voracious mosquitoes. These weren’t your typical above-ground mosquitoes. They were natives of the Underground, born in pools of standing water that pockmarked the underground passageways. And unlike their open-air cousins, London’s subterranean skeeters seemed to love biting humans.
Fifty years after the war ended, scientists at the University of London decided to investigate the subway population. They collected eggs and larvae from subway tunnels and garden ponds and reared both populations in the lab. The outdoor mosquitoes fed on birds, but the tunnel bugs preferred mammal blood. And when the scientists put males and females from the different populations into close quarters designed to encourage mating, not a single pairing produced offspring. That sealed the deal: the underground mosquitoes were a whole new species, adapted to life in the subway tunnels people had built.
It’s stories like this one that got Joseph Bull thinking. As a conservation scientist at the University of Copenhagen, he hears a lot about how humans are driving other species extinct. If the current rate stays steady, the planet is on its way to its sixth mass extinction, a severe event on par with the meteorite impact that killed the dinosaurs. But he wondered whether there might be a flip side. Certainly people’s planet-transforming activities had to be creating new species, too. But how, and how many? Bull decided to see whether he could count all the new species humans had created or were on their way to creating, in a sort of mirror-image of extinction rates and endangered species lists.
First, Bull had to come up with a list of human activities that could create new species. The most obvious one is domesti-cation. By picking out the traits in a wild population that are most beneficial to humans and breeding for them, people can “force evolution in different species,” Bull says. Wolves become dogs, nubby grass becomes maize, wild boars become pigs.
As people’s environmental reach has expanded and our corresponding sense of responsibility for the planet has grown, we’ve started applying the principles of domestication to helping wild species, too. For example, several environmental organizations—including The Nature Conservancy and SECORE International—recently joined forces to breed resilient corals, selecting the heartiest parents and raising their offspring in protected pens in the Caribbean. In pilot projects on the island of Curaçao and in Mexico, scientists are currently working on transplanting these born survivors into wild reefs suffering the effects of climate change and pollution. The idea is to provide a shot of genetic diversity to the reefs, ensuring they can withstand high temperatures and acidifying oceans without bleaching. Researchers, including biologist Ruth Gates of the Hawaii Institute of Marine Biology, say coral reefs are unlikely to survive much longer without such “assisted evolution.” (1, 2)
People’s efforts to help species we like can sometimes backfire. The livestock industry pumps antibiotics into animals to keep them healthy in overcrowded quarters and to encourage them to grow more quickly, but such widespread use allows low levels of those antibiotics to leak out into the environment. Such small amounts don’t stop the growth of bacteria, but some scientists wondered whether they could affect pathogens in other ways. In a recent issue of the Philosophical Transactions of the Royal Society B dedicated to human influences on evolution, researchers exposed colonies of the common bacterium Pseudomonas fluorescens to very low levels of the antibiotic streptomycin. Streptomycin kills bacteria by interfering with its ability to make proteins, but at nonlethal levels that same mechanism causes the bacteria to mutate. The researchers discovered that exposure to nonlethal amounts of streptomycin caused P. fluorescens to evolve resistance not only to the antibiotic, but also to a common phage—a virus that infects P. fluorescens and naturally keeps its population down. (3) Efforts to strengthen livestock via antibiotics, therefore, could actually be doing more to help bacteria.
Humans can also drive speciation in less direct and less purposeful ways. “It’s important to think about the creation of new species as a process,” Bull says. One of the most dramatic ways people put that process into motion is by moving members of an existing species from one place to another. Sometimes those individuals die in the new environment. Sometimes they hang on and interbreed with native species. And sometimes they take over, like kudzu in the American South or snakes on Guam. Over time, the new environment exerts different pressures on the invasive population, causing it to diverge from its ancestors. The invasive species might also change the game for native species, pushing them in new genetic directions (if, of course, it doesn’t just drive them extinct).
Although hunting is one good way to drive a species extinct (just ask the passenger pigeon), it can also spur evolution by removing certain types of individuals from a species gene pool—birds of an easy-to-see color, say, or fish large enough to be caught in a net. No new species is known to have been created through hunting alone, Bull says. But given enough time, it’s far from impossible.
Finally, we have the process that created the underground mosquito: people’s propensity to create entirely new ecosystems, especially those including cities. Populations of animals colonize these new environments and adapt to their demands—from mosquitoes developing a taste for mammal blood underground to city birds becoming better problem-solvers than their rural relatives. In the same human-directed-evolution issue of Philosophical Transactions of the Royal Society B, researchers found that urbanization affected even plankton. After taking samples from 84 ponds in Belgium, located in areas ranging from rural to suburban to very urban, the researchers found that in most cases, the more built-up the pond’s environment was, the smaller its zooplankton tended to be. They posit that change in size is due to the fact that cities generate more heat than rural areas, and hotter temperatures mean that oxygen becomes less soluble in water. Animals with small bodies and lower oxygen requirements are therefore better able to survive in urban aquatic
environments. That could cause problems, the researchers say, because zooplankton graze on microscopic plants; if the zooplankton shrink too much, the tinier plants could grow out of control. (4)
Keeping these mechanisms in mind, Bull tallied up humans’ impact on species in a paper published last year in Proceedings of the Royal Society B. Scientists have recorded 1,359 plant and animal extinctions in the past 12,000 years. Meanwhile, humans have relocated 891 plant and animal species and domesticated 743, for a total of 1,634 species. (5) It seems that human-driven speciation could be as much a mark of the Anthropocene as extinction is.
Of course, extinction, like speciation, is hard to document while it’s happening. Many species likely disappear before scientists even know they are there. That’s why extinction rates are calculated with extrapolations and models but even so give widely different numbers. That’s all to say that many more than 1,359 lifeforms have likely gone extinct in the past 12,000 years. One scientist even estimates that the world has lost 130,000 species of recorded invertebrates alone. (6) It’s also possible humans create species without detecting them. Just think of the wild world of antibiotic-resistant microbes, which evolve so fast in response to drugs that it’s dangerously difficult to keep up.
Number of species, however, is just one way to measure the effects humans have on nature—and maybe not the best way. Drive keystone predators such as wolves or sharks extinct, and entire ecosystems collapse—no matter how many new species pop up to replace them. What’s more, older species can carry millions of years of evolutionary history in their genes; if they go extinct, that diversity is lost. “Anthropogenic species represent a nanosecond of the evolutionary time that many ‘natural’ species have passed through,” says Christopher Dick, an evolutionary biologist at the University of Michigan. “In conservation, there is no comparing a 10-million-year-old tree or turtle species with a decades-old strain of insect or plant.” Species whose genomes have been shaped by mere decades or even years of human-driven evolution may have lost the rare genes that aren’t particularly helpful in current environments but may regain their usefulness as the world changes. Something like this is currently happening to maize, as the varieties people have bred and planted most intensively can’t keep up with the climbing temperatures and longer droughts brought on by climate change. Researchers at the International Maize and Wheat Improvement Center, a seed bank outside Mexico City, are racing to collect the older and rarer maize varieties still grown in remote villages in Mexico—
in hopes of finding a constellation of ancient genes that could save the world’s supply of corn.
Bull agrees that speciation and extinction don’t cancel each other out. “If we only use number of species as a way of measuring progress that someone makes on conservation, then we’re missing a load of other important considerations,” he says. “We cannot replace something lost with something gained when it comes to nature.” Human-driven speciation may turn out to be a calling card of the Anthropocene. But no matter how many species of underground mosquitoes we inadvertently create, they won’t make up for what we destroy.
van Oppen MJH. et al. Building coral reef resilience through assisted evolution. PNAS. 2015.
van Oppen MJH et al. Shifting paradigms in restoration of the world’s coral reefs. Global Change Biology. 2017.
Cairns J et al. Sublethal streptomycin concentrations and lytic bacteriophage together promote resistance evolution. Philosophical Transactions of the Royal Society B. 2017.
Brans KI et al. Eco-evolutionary dynamics in urbanized landscapes: Evolution, species sorting and the change in zooplankton body size along urbanization gradients. Philosophical Transactions of the Royal Society B. 2016.
Bull JW and M Maron. How humans drive speciation as well as extinction. Proceedings of the Royal Society B. 2016.
Régniera C. Mass extinction in poorly known taxa. PNAS. 2015.
Lizzie Wade writes about science from her home in Mexico City. She is a contributing correspondent for Science, focusing on archaeology, anthropology, and all things Latin America.