The species will eventually have problems reproducing and can often fall victim to extinction.

Scientists at Edinburgh University have been studying complex mathematical models on a highly powerful computing system to look at the case of the Amazon Molly.

Researchers calculated the time to extinction for the fish based on modelling genetic changes over many thousands of generations.

They are now able to say conclusively, for the first time, the fish ought to have become extinct within the past 70,000 years, based on the current simple models.

Scientists believe the fish, which are still thriving in rivers in south-east Texas and north-east Mexico, are using special genetic survival "tricks" to help them stay alive.

One theory is that the fish may occasionally be taking some of the DNA from the males that trigger reproduction, in order to refresh their gene pool.