What if you could get rid of the mosquitoes that carry Zika virus? All of them. An entire species. Would it be right to do? What if the gene-editing process known as “gene drives” could modify a species of mice on an island so that only male offspring are produced, effectively setting in motion an irreversible extinction and protecting a species of bird? What if a tool already existed, dubbed the CRISPR-CAS9, that would simply give “technicians the ability to intervene in evolution, to engineer the fate of an entire species, to dramatically modify ecosystems, and to unleash large-scale environmental...