New lizard form alters the reproductive strategies of its fellow species
Profile picture of a male common wall lizard with the "Hulk" appearance.
© Roberto Garcia-Roa
To the Point
- Evolution story with a hook: A newly evolved, aggressive "Hulk" wall lizard is spreading out from the Rome region - and researchers are watching a million-year stable system unravel in real time.
- From three morphs to one: Across 220 populations, the pattern is consistent: where "Hulk" arrives, yellow and orange throat morphs vanish, leaving populations dominated by - or fixed for - white.
- Not just colour - a social shake-up: Genomic data suggest this isn't simple genetic hitchhiking. The spread of "Hulk" likely changes the social ecology (competition, dominance, mating), tipping the balance so long-standing alternative strategies can no longer coexist.
For millions of years, Mediterranean wall lizards have played by the same rules. Across populations, individuals show one of three vivid throat colours - white, yellow, or orange - and those colours are more than decoration. They are linked to differences in behaviour, social interactions, and reproduction, forming alternative strategies that coexist within the same species.
Now, researchers have found that this long-standing diversity is collapsing in large parts of Italy. A newly evolved form of wall lizard - larger, highly aggressive, and marked with bold green-and-black patterning - has been spreading outward from the region near present-day Rome. The team nicknamed it the "Hulk" form for its striking appearance and dominant behaviour. But it comes with a crucial twist: it shows only one throat colour - white.
"Hulk" lizards spread
A male common wall lizard showing the nigriventris phenotype in its natural habitat.
© Roberto Garcia-Roa
By collecting colour data from 220 wall lizard populations, the researchers uncovered a clear pattern: "Wherever these green-and-black "Hulk" lizards spread, the yellow and orange throat morphs disappeared," says lead author, Professor Tobias Uller from the Lund University in Sweden. In several regions, populations that once hosted all three morphs now show only white.
Diversity collapses
One of the rare individuals showing both the "Hulk" appearance and an orange throat - the "Hulk" appearance usually goes with a white throat color.
© Roberto Garcia-Roa
The loss is not confined to the "Hulk" lizard's own lineage. In nearby lineages that have acquired the green-and-black appearance through hybridisation, the same collapse of throat-colour diversity is seen - suggesting a broader shift in the evolutionary dynamics of these lizards.
In other words, a colour polymorphism that persisted through million years of evolution and major environmental changes, such as ice ages or the emergence of modern humans, appears to be disappearing rapidly following the spread of a single new evolutionary variant.
What's driving the collapse?
Comparisons of common wall lizard heads showing the nigriventris white, ancestral white, yellow and orange throat coloration (from top to bottom)
© Roberto Garcia-Roa
To understand what was happening, the team analysed detailed genomic data. The results show that as the green-and-black form spreads, genetic variants associated with yellow and orange throat colours are being lost. "Only because we had detailed knowledge about how the green-and-black body pattern and the throat colours are genetically encoded we were able to connect the dots" says Nathalie Feiner of the Max Planck Institute for Evolutionary Biology in Germany, who co-authored the study. They found that it is not a simple case of genetic "hitchhiking" - where throat-colour genes are physically locked to the genes controlling the green-and-black body pattern. Instead, the findings point to a more surprising possibility: a change in social ecology.
"As the "Hulk" lizard spreads across the landscape, it appears to fundamentally alter how lizards interact." says senior author Geoff While from the University of Tasmania. Heightened aggression and dominance may disrupt the social balance that previously allowed multiple colour strategies to persist side by side. "When the rules of the game changed, the game itself collapsed."
Why it matters
Colour polymorphisms are often used as textbook examples of how evolution can maintain diversity over long time spans - through frequency-dependent selection, alternative mating strategies, or shifting environmental conditions. This study presents an unexpected discovery: even ancient evolutionary equilibria can be fragile.
Diversity that lasted millions of years may be erased when new traits - especially those affecting behaviour and social competition - reshape the landscape of who wins, who mates, and which strategies survive. Evolution does not always preserve balance - and sometimes, a single powerful newcomer is enough to break it.
