An all-female species of fish appears to have defied the normal rules of evolution by continuing to develop – despite thousands of generations of clones.
The Amazon molly is the only animal with a spine that is capable of reproducing without having sex and its existence has baffled scientists.
“According to established theories, this species should no longer exist. It should have long become extinct during the course of evolution,” said Dr Manfred Schartl of the University of Wurzburg in Germany.
Dr Schartl led a study which sequenced the Amazon molly’s genome and is published in the journal Nature Ecology and Evolution.
He said the fish appeared to be thriving.
Normally, harmful changes occur in a creature’s genome which would destroy such a species.
Dr Schartl explained: “In creatures whose offspring are pure clones, these defects would accumulate over generations until there are no more healthy individuals.”
Species that are direct clones are also not expected to evolve because new generations do not get to build upon the successful genetic material of previous generations.
“These species are usually not capable of adapting to environmental changes as quickly as their sexually producing counterparts,” said Dr Schartl.
But after sequencing its genome, the researchers found that the Amazon molly was different.
“We found little evidence of genetic degeneration in the Amazon molly, but rather a unique genetic variability and clear signs of an ongoing evolutionary process,” he said.
According to the research, the Amazon molly evolved approximately 100,000 years ago – and with a new generation born every three to four months, there have been approximately 500,000 identical molly species.
Although its name suggests a southern American habitat, the fish is actually native to the fresh waters in the border region of Texas and Mexico.
It gained its name as a reference to the Amazon warriors of Greek mythology.
Its offspring are all female, and all identical clones of the mother – a process known as gynogenesis.
In order to reproduce, the fish does require sperm from other closely related molly species, but this sperm is only used as a trigger for the cloning process.
The male’s DNA is not incorporated into the egg and instead destroys the male’s genes.