In the wild, even a small genetic change almost always incurs what scientists call a fitness cost: Either an engineered insect won’t be as hardy as its wild peers or it won’t be an attractive mate. (Simply changing the fur color of a fruit fly from brown to yellow, as Gantz did, for instance, reduces its chance of mating by 99 percent.) More radical changes, like creating a mosquito that produces only male offspring, are likely to face even more resistance. Nature is good at circumventing anything that thwarts procreation.

Delphine Thizy of Target Malaria told me that because of these factors, the foundation didn’t expect gene drives to actually eliminate malaria. “The goal is really just to deplete the mosquitoes from an area enough that the parasite-insect-human cycle collapses,” she added. “If you look at all the obstacles — the physical obstacles, like geography, as well as the evolutionary pressures — it’s more likely that even a really well-engineered gene drive won’t spread as well as we’d think.”

Current research suggests that the spread of gene drives is likely to vary from species to species, with some propagating slowly, if at all, and others more rapidly or widely. Research also suggests that gene drives stay confined to a single species rather than spreading into a related one through interbreeding. But it’s not clear whether that will be true in all species or under all conditions. (Researchers are also working on a variety of containment strategies, including drives that stop working after a few generations.) And it’s very hard to assess what the environmental impact of removing a species, or even altering one, might be. While ecosystems tend to be resilient — plenty of species have gone extinct already, and it hasn’t led to a systemic collapse — they’re also complicated and difficult to model. The only way to conclusively determine what happens when a species changes or vanishes may be to try it and see.

Ethan Bier, who has become deeply involved with the technology since his and Valentino Gantz’s breakthrough, emphasized that the many potential applications are likely to have extremely different benefits and risks. Malaria, he noted, is one of the strongest cases. Studies show that reducing or even eliminating the Anopheles mosquito is unlikely to have a significant environmental effect (few birds or animals rely on it as a food source), and as it is one of 3,500 mosquito species on the planet, its disappearance wouldn’t appreciably dent the insect’s overall diversity. And given that malaria kills hundreds of thousands of people a year, the argument for not using a gene drive would have to be unusually strong. Bier recalled one early conversation in which Gantz asked: “Imagine you could genetically engineer a mosquito that would prevent you from getting cancer. Would people still object to it?”

In conservation and agriculture, gene drives could also have a profound effect, with the potential both to save endangered species and to reduce the amount of pesticides currently in use. But these, too, carry risks. New Zealand has discussed using a gene drive to eradicate the Australian brushtail possum, which preys on the nests of native birds and is currently controlled with poison traps. But should a few dozen Australian possums with an all-male gene drive be carried from New Zealand back to Australia, they could devastate the native possum population. Agricultural uses are even more fraught. If a corporation wants to use a gene drive to “cancel out” the herbicide resistance that some weeds have now developed, would that really benefit the planet — or just the corporation that can now sell more of the herbicide that caused the problem in the first place?

In theory, figuring out how to answer these questions should be the province of the world’s regulatory agencies, and most scientists agree that gene drives will need to be evaluated on a case-by-case basis, akin to how the Food and Drug Administration evaluates the safety of a new treatment or pharmaceutical. But regulating a technology that doesn’t stop at the border of a country or a state is a new problem. Unlike a chemical pesticide, gene drives are inherently mobile — able to cross borders or potentially even oceans. And while some species, like the malaria-carrying Anopheles gambiae mosquito, exist only in sub-Saharan Africa, others, like the Norway rat, are virtually everywhere. As Kuiken put it: “How do you regulate a technology that’s undetectable, self-propagating and can fly? If one community doesn’t want it, does that mean that the other four or five communities around it aren’t allowed to move forward? How do you set up an international governance regime that enables you to make those kinds of decisions? So far, I haven’t seen any proposals that get us there.”

The United Nations and the International Union for Conservation of Nature have created working groups to study the problem and begin to hash out best practices around gene-drive use, though these may be difficult to enforce. A handful of countries have been more rigorous. In June 2018, the National Institute for Public Health and the Environment in the Netherlands passed legislation that included a detailed evaluation process for any gene drive to be used outside the lab.