Scientists at Cambridge have found "hotspots" in the butterflies' genes that they believe will explain the similarity.
Heliconius, or passion-vine butterflies, live in the Americas -- from the southern US to southern South America. Although they cannot interbreed, H. melpomene and H. erato have evolved to mimic one another perfectly.
These delicate butterflies have splashes of red and yellow on their black wings, signalling to birds that they contain toxins and are extremely unpalatable. They mimic one another's colour and pattern to reinforce these warning signals.
The Cambridge-led team of researchers from Britain and US universities, which has been breeding the butterflies in Panama for the past decade, has been searching for the genes responsible for the butterflies' wing patterns.
They also sought answers to the question of whether the same genes in two different species are responsible for the mimicry.
According to study co-author Chris Jiggins, zoologist at the University of Cambridge: "The mimicry is remarkable. The two species that we study -- erato and melpomene -- are quite distantly related, yet you can't tell them apart until you get them in your hand."
"The similarity is incredible -- even down to the spots on the body and the minute details of the wing pattern," said Jiggins.
That the two species have evolved to look exactly the same is due to predation by birds, said a Cambridge release.
According to Jiggins: "It's interesting because it tells us how flexible evolution is. If you get the same wing pattern evolving independently in different populations, do you expect the same genes to be involved?"
The new results show that the regions of the genome associated with the wing patterns are very small -- akin to genetic "hotspots."
The findings were published in two parallel papers in PLoS Genetics.