The Amazon molly is an evolutionary enigma: an all-female fish that reproduces by cloning itself. Because it doesn’t mix its DNA with a mate’s, Darwinian logic holds that harmful mutations should pile up over time, eventually driving the species extinct.
Yet the molly (Poecilia formosa) has stubbornly refused to follow that script. That seems to be because the fish’s genome repeatedly rewrites itself, researchers report March 11 in Nature. It uses a copy-and-paste DNA repair mechanism called gene conversion, in which small stretches of genetic code are copied from one chromosome to another. These genetic handoffs can erase unfavorable mutations and sometimes spread beneficial ones, allowing natural selection to