The toadflax plant, Linaria vulgaris, normally produces asymmetrical flowers, but occasionally a mutant form grows symmetrical flowers, with identical upper and lower petals. When the great 18th-century Swedish botanist Carolus Linnaeus discovered the mutant flower in 1742, he thought it must be a new species, even though the rest of the plant was identical to typical toadflax.
Linnaeus called the flower Peloria, Greek for "monster", not because it was ugly but because it had monstrous implications for the belief in the immutability of species.
"This is certainly no less remarkable than if a cow were to give birth to a calf with a wolf's head," he wrote.
Enrico Coen and his student Pilar Cubas, of the John Innes Centre in Norwich, have found the genetic explanation.
The Lcyc gene changes as a result of having extra organic substances called methyl groups added to its chemical structure, according to research published in the journal Nature. It results in structural changes in the way the flower develops.
Linnaeus, the founding father of the binomial system of biological classification, concluded that the mutant flower must have resulted from some sort of transformation of the common toadflax into a new species.
Professor Coen said: "This is the first time that a natural modification of this kind has been found to be inherited, suggesting that this kind of change may be more important for natural genetic variation and evolution than has previously been suspected."
"It provides a molecular understanding of what people have been doing for a long time about flowers, by taking them from nature and propagating them in their garden.
"It shows that many of the flowers grown in gardens result from specific changes to the genes in the wild," he added.