The Mediterranean region is home to 28 different species of wall lizards of the genus Podarcis, whose remarkable diversity has fascinated – and puzzled – naturalists for over a century. In some groups of organisms, such as birds, individuals can usually be assigned clearly to distinct species. Mediterranean wall lizards, however, often defy such neat classification. This is because the variation within a single species can be so extensive that it obscures the differences between species.
The dazzling variation of these lizards has been a challenge for taxonomists, but it turns out to be the perfect setting for studying the process of evolution in action. Together with a European network of collaborators, our group aims to understand what makes wall lizards such remarkably evolvable organisms. Our research combines fieldwork to map the geographic distribution of variation with genomic and developmental analyses aimed at uncovering the mechanisms behind the striking color patterns on the lizards’ backs.
Three morphs of the common wall lizard.
The Guardian recently featured some findings arising from more than a decade of our research on the common wall lizard, Podarcis muralis, published in the journal Science. In this species, three alternative ventral colorations (known as white, yellow and orange ‘morphs’) have co-existed for millions of years, maintained by a balance of ecological and social interactions.
This equilibrium, however, appears to have been disrupted by the emergence of a novel phenotype known as ‘nigriventris’. This phenotype has evolved in ancient times close to today’s Rome and has since spread along the coasts of Tuscany and Liguria. Lizards with this novel phenotype are characterized by bulky heads with prominent jaws and a pitch-black base coloration, fluorescent blue spots on their flanks and vivid green markings on their backs – an appearance that inspired their nickname, the ‘Hulk lizard’. Males with the nigriventris phenotype outcompete males with the ancestral phenotype and therefore drive the geographic expansion of this new type.
Comparison between the nigriventris phenotype (top) and the ancestral form (bottom).
The striking finding that our team made is that wherever the new phenotype of lizard appears, the three-morph-system collapses and only a single ventral color prevails. This outcome is unexpected, as it is not caused by genetic incompatibility between the new phenotype and the existing morphs. Instead, the presence of the novel form appears to disrupt the ecological and social interactions that had maintained the three-morph equilibrium for millions of years.
As The Guardian highlights, common wall lizards provide a clear example of an often overlooked dimension of biodiversity: variation within species. As Nathalie Feiner explains: “Without such variation, evolution would stall, and the capacity for adaptation would be lost.”
A strong capacity for adaptation is urgently needed by the Ibiza wall lizard Podarcis pityusensis. As described in a recent article in National Geographic, these unique island endemics are acutely threatened by a snake invasion. Following the accidental introduction of a small number of individuals around 20 years ago, the horseshoe whip snake has now established itself on Ibiza and is spreading rapidly. Wherever this highly effective predator appears, wall lizards disappear. The situation is evolving quickly, and the once widespread Ibiza wall lizard has now been eradicated from most of the main island.
Ibiza wall lizards as found on 6 different islets.
“We are witnessing the extinction of a species’ life—and not only a species, but probably one of the most striking cases of color biodiversity on Earth,” says Roberto García-Roa, an evolutionary biologist and photographer at the University of Valencia, Spain, who also studies the lizards. Much of this striking diversity is not found on Ibiza itself, but on small surrounding islets. At least 40 of these islets are home to Podarcis pityusensis, many of which display unique color patterns not seen anywhere else. Understanding the evolutionary processes that generated this mosaic of variation is a central research focus in our group. As Tobias Uller puts it: “Understanding why wall lizards have this extraordinary capacity to generate diversity is fascinating and brings us closer to understanding why nature works the way it does.“
The fact that much of the unique variation found within this species is located on isolated islands may give the idea that they are safeguarded from the snake invasion. Unfortunately, this hope is undermined by the fact that horseshoe whip snakes are capable swimmers. In fact, a handful of island populations have already been extirpated.
Is this species doomed to extinction? Not necessarily. Ongoing efforts to control snake populations, combined with captive breeding programs aimed at safeguarding Ibiza wall lizards, are well underway. The National Geographic article is also helping to draw attention to the urgent conservation challenges this species faces.
The Guardian article was published online on Feb 28th 2026. The National Geographic article was published on May 5th 2026 as the online version and a smaller article was published in the 06/2026 issue of the printed version.