The Amazon Molly (Poecilia formosa), an extraordinary all-female freshwater livebearing species from northeastern Mexico and the Texan border, has confounded scientists by continuing to develop and thrive whilst flouting all the usual evolutionary rules - reproducing only via hundreds of thousands of generations of clones. 

This fish reproduces by a method known as gynogenesis, a form of asexual reproduction related to parthenogenesis (the development of embryos without fertilisation). According to conventional evolutionary theories, the Amazon Molly should have long ago been a prime candidate for extinction due to a restricted gene pool; most species that reproduce asexually showing a marked lack of genetic variation and subsequent ability to adapt to and cope with ever-changing environmental conditions. 

The Amazon Molly - so named for the tribal warrior women of Greek mythology - is found in lazy river backwaters, quiet pools, streams, and ditches, usually over a muddy substrate and amongst dense marginal vegetation. This fish was thought to have evolved some 100,000-200,000 years ago, as a result of the hybridisation between the Atlantic Molly and the Sailfin Molly. In 1932, it was discovered that this fish reproduced asexually. This caused a sensation, as it was the first species with a backbone found to reproduce in such a way. Asexual reproduction is exceptionally rare amongst vertebrates.

Reaching sexual maturity within 1-6 months, gynogenesis in the Amazon Molly is triggered by copulation with and stimulation of sperm from the males of other species from the same generation, either P. latipinna, P. latipunctata, P. mexicana, or, rarely, P. sphenops. But it is here that the method of reproduction takes an unexpected course. Once the sperm has activated an egg, it takes no further part in the development of the embryo. Indeed, genetic material from the male is not incorporated into the already diploid eggs that the mother is carrying (which contain only maternal chromosomes). This results in a mass production of daughter fish, identical clones of the mother. A typical brood size is 80-100 fry born every 30-40 days, as long as 'host' males from the aforementioned species are present to trigger gynogenesis. 

Conventionally, scientists believe organisms that reproduce via this atypical method aren’t equipped to survive the relentless onslaught of new pathogens and other dangers that arise as environments evolve. As a rule, new generations of identical clones do not get to build upon the successful genetic material of previous generations, as would be observed in organisms reproducing sexually, with new material being injected into the gene pool on a regular basis. 

Yet, new research recently published in the journal Nature Ecology & Evolution, shows that, incredibly, the Amazon Molly has been able to circumvent the expected disastrous fate. Upon sequencing the genome of these remarkable all-female fish, and comparing the findings to that of their original parental fish species, they actually found little evidence of genetic degeneration and instead, a high degree of genetic variability in the Amazon Molly's immune-system genes. This somewhat surprising result indicates that despite the unconventional method of reproduction, the fish's immune-system genes are somehow evolving along with its surroundings, allowing the fish to adapt to changes and thrive. 

Such findings could be explained by a phenomenon known as heterosis or 'hybrid vigor', where the offspring resulting from the hybridisation of two different parent species, are bigger, stronger, more colourful, and more fecund than the purebred parents themselves, giving subsequent generations of cloned hybrid fish survival benefits. This fascinating discovery is now prompting scientists to reconsider the way in which they think about the extraordinary process of asexual reproduction amongst vertebrates.