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. Yet, this small, unassuming creature endures.

How has the Amazon molly survived when theory suggests it should be long extinct? As new research starts to unravel this mystery, scientists are finding that asexual species may be more resilient than once thought – challenging the long-held idea that life without sex is doomed to fail. 

To understand why the Amazon molly's survival without sex is so remarkable, it helps to know: why does sex exist at all?

"Sexual reproduction is a pretty weird and complicated way to reproduce, right?" says Edward Ricemeyer, computational biologist at the Ludwig Maximilian University of Munich in Germany, and co-author of the new study on the Amazon molly. 

Sex is costly, Ricemeyer explains. Individuals must find and compete for a mate, and each parent contributes only half their DNA. Reproduction is often unequal, with females of many species investing far more energy than males in producing, birthing or incubating, and raising offspring.

Asexual reproduction, by contrast, sounds like a much better deal. No need to find (and deal with) a mate, and you can pass on 100% of your genes. Yet across the tree of life, sex – the mixing and recombination of genes from different individuals – is truly dominant.

"If you look at the overall picture, it's 99.9% sex," says Dave Speijer, an evolutionary biologist at the University of Amsterdam, in the Netherlands, specialising in the origins of sexual reproduction.

One such reason, Speijer argues, is that sex allows populations to explore the genetic "space of possibilities" more efficiently.

During sexual reproduction, the DNA of two parents is reshuffled through a process called recombination, giving each offspring a unique combination of genes. It's a little like shuffling and dealing out a deck of cards, each reshuffle creating a new hand for evolution to test out. This means there is usually more genetic variety within sexual species, as every individual has a different mix of genes – a unique hand of cards – which is typically beneficial to a species' survival.

Sex also offers protection. Without this genetic reshuffling, genomes face a slow, creeping threat called Muller's ratchet.

When DNA is copied, explains Speijer, "there are always errors". In sexual species, these mistakes can be shuffled out of the gene pool, but in clonal species, they are passed down over and over again. Over time, these harmful mutations are thought to build up like notches on a one-way ratchet – degrading the genome, click by click, until the species goes extinct.

According to this idea, asexual species should be short-lived, doomed to genetic decay. Yet some, like the Amazon molly, not only survive, but thrive.

Speijer thinks part of the confusion could stem from how the theory is interpreted. "Muller's ratchet........

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