According to the rules of evolution, it should not exist. The Amazon Molly (Poecilia formosa) is a small freshwater fish composed entirely of females that reproduces clonally, that is, by producing genetic copies of itself.
It is named after the fierce female warriors of Greek mythology, not because of the Amazon basin, where it does not actually live. The fish inhabits warm rivers and lagoons along the Mexico-Texas border.
For decades, scientists assumed that this reproductive strategy was an evolutionary dead end: Without genetic mixing, harmful mutations accumulate generation after generation until the species collapses. However, The Molly of the Amazon has been disproving that theory for more than 100,000 yearsand a new study published in the journal Nature finally explains why.
In evolutionary biology, sexual reproduction has a key function: mixing the DNA of two individuals to eliminate harmful mutations and combine advantageous characteristics. Darwinian logic indicates that, without this mixture, harmful mutations should accumulate over time and end up extinct the species.
Unlike most vertebrates, the Amazon Molly dispenses with paternal inheritance through a process called gynogenesisbased on a form of asexual reproduction known as apomixis. In this mechanism, the mother produces eggs that already contain a double and complete genetic set, so they do not require the male’s DNA to complete.
However, Females still need to mate with males of nearby species so that the sperm acts as a biological trigger to initiate embryo development..
In the end, the male genetic material is discarded and the offspring are born as identical clones of the motherperpetuating an exclusively female lineage that defies the rules of sexual evolution.
Each generation is, in essence, a clone of the previous one.. With new generations being born every three to four months, the species has accumulated about 500,000 generations since it first appeared, many more than standard theory predicts as the maximum time before extinction.
The new study, conducted using advanced genomic mapping techniques, revealed the mechanism behind this extraordinary survival. The Amazon Molly’s genome repeatedly rewrites itself using a DNA repair mechanism called gene conversion.in which small fragments of genetic code are copied from one chromosome to another.
Although the Amazon Molly accumulates mutations faster than any of its parent species, these mutations have not resulted in genetic degradation. Instead, the fish manages to purge some harmful mutations thanks to this DNA repair processin which one version of a gene overwrites the other.
In simple terms, it works as a self-correction system within the DNA itself: when a harmful mutation appears, the mechanism detects it and replaces it with a functional copy of the gene. These genetic exchanges can erase unfavorable mutations and sometimes spread beneficial ones, allowing natural selection to continue operating.
