Even though there are only female Amazon molly fish, they can't actually survive without males.
It turns out that even though they reproduce asexually, the females first need to mate with males from closely related species of molly. The Amazon molly (Poecilia formosa) mates with the males from one of four different species, stealing their sperm to activate their eggs before destroying the sperm so that it doesn’t contribute to their genetics, a process known as gynogenesis.
We all know by now that much of what we got taught at school was not necessarily wrong, just simplified. Nowhere is this more evident than when it comes to what it means to be a species. Biology is messy, and after hundreds of millions of years of dead ends, peaks, and troughs, pretty much anything that could have evolved has. And that has given rise to some creatures that seem to defy what we thought we knew.
“According to established theories, this species should no longer exist. It should have long become extinct during the course of evolution,” explained Manfred Schartl, co-author of the paper published in Nature Ecology & Evolution. And yet here the Amazon molly is.
There are a number of reasons why the majority of animals – well, vertebrates at least – reproduce sexually. The first is related to a species' ability to adapt to changing environments. If all members of a population are clones of a single founding individual, then there is very little genetic variation.
These slight differences between one animal and the next are the bread and butter of evolution, and it means that if the environment was to change at least some would be able to survive. This is less likely to happen if they are all clones.
The second comes down to the genetic code in their cells. All DNA will mutate at some point as it is read, copied, and replicated again and again and again. Most of these mutations are caught by the inbuilt checking system, but occasionally a harmful one will slip through.
Because during sexual reproduction half the DNA comes from each parent, these mutations can be kept in check, with many being eliminated. In clones, this cannot occur and these mutations build up over time.
Despite having likely split from its closest ancestor some 100,000 years ago, the Amazon molly has few major genetic damages. After sifting through its genome, researchers were surprised to see that the fish were able to maintain a surprisingly high level of genetic variability. They found that the genes responsible for the immune system were particularly variable, which could hint at why the species is able to survive asexually, and not succumb to disease as many clones do.