This fish species survived 100,000 years without males. Scientists thought it should be long dead – but it's thriving

All-female species have been long thought to be evolutionary dead ends. So how has one remarkable fish survived for 100,000 years without males? The answer is revealing new insights into how nature keeps genomes healthy. In the rivers of Mexico and southern Texas swims a fish that shouldn't exist. In the warm, slow-moving waters, she drifts among her all-female shoal, her silver scales brushing against males of closely related species. It's here that she selects a mate. But in an unusual evolutionary twist, his genes play no part in her offspring. This is a biological heist known as gynogenesis, in which she uses the male's sperm only to trigger egg development, but quickly discards his DNA. She produces only daughters, each a clone of herself. This fish is the Amazon molly, named after the all-female warrior tribe in Greek mythology, and it has been puzzling scientists for nearly a century. Evolutionary theory suggests that asexual species should quickly die out, as without sex harmful mutations build up in their genomes over time. But this female-only species has persisted for around 100,000 years. By conventional thinking, it should have been a fleeting blip in the tree of life.