In parts of Italy, Mediterranean wall lizards are starting to look surprisingly alike. Scientists from the University of Tasmania are helping uncover why.
Common wall lizards provide a striking example of colour diversity. While their backs are typically an unremarkable brown, their throats display vivid colours: white, yellow, and orange.
This kind of colour variation, known as colour morphs, is common across the animal kingdom and is not just cosmetic. They are often linked to how individuals compete, choose mates and survive.
These colour types have coexisted for millions of years, but now many populations are losing that variety. Instead of three colours, only one remains.
New research published in Science , involving Associate Professor Geoff While from the University of Tasmania's School of Natural Sciences, shows the change is being driven by the rapid spread of a larger, more aggressive form of wall lizard. This newcomer is known as the 'Hulk' lizard.
The nickname comes from its bold green and black markings and its dominant behaviour. Compared to the brown wall lizard, the Hulk lizard is larger, stronger, and far more likely to win fights with rival males.
It behaves very differently despite being the same species, and, importantly, unlike its relatives, it has only one throat colour: white.
The Hulk form emerged recently in evolutionary terms, near what is now Rome. From there, it spread rapidly across Italy and can now be found almost as far north as the border with France.
When researchers examined throat colour patterns from over 150 wall lizard populations spanning the Hulk lizard's expanding range, they found a striking pattern.
"Wherever these green and black 'Hulk' lizards spread, the yellow and orange throat morphs disappeared," says lead author Professor Tobias Uller from Lund University in Sweden.
Entire populations that once had all three colour types are now white alone.
"What makes this so remarkable is how fast it has happened," said Associate Professor Geoff While.
"This colour system survived ice ages and major climate shifts, yet it has collapsed in a very short time because of the arrival of one new type of lizard."
To figure out why, the team looked closely at the lizards' DNA and found that as the Hulk lizard spreads, the yellow and orange throat genes steadily disappear.
This isn't because the genes for the white colour are genetically tied to the Hulk lizard and are automatically inherited with it. Rather, the loss appears to happen because lizards carrying yellow or orange colours do worse once the Hulk lizard arrives.
"In many animals, colour variation is associated with different ways of competing and reproducing," Associate Professor While said.
"For millions of years, systems like this can persist because no single approach is always best. Different colour types tend to do better under different conditions, allowing diversity to be maintained over long periods of time.
"The Hulk lizard is larger and more aggressive, and its arrival appears to change how competition plays out in this system. By dominating encounters and access to mates, it likely removes the balance that once allowed multiple colour types to coexist - which helps explain why yellow and orange forms are now disappearing."
The findings show how fragile even long-standing evolutionary systems can be. Diversity that appears stable over deep time can vanish quickly when new traits reshape competition and social behaviour.
Evolution does not always protect variety. Sometimes, it only takes one powerful newcomer to change everything.
