For millions of years, common wall lizards in the Mediterranean region have existed with three distinct throat color forms: white, yellow, and orange. Each color is linked to a different competitive strategy, and together they form a stable example of evolutionary coexistence. Recent research has found that this balance is being disrupted by the spread of a larger, more aggressive green-and-black lizard, which is driving the decline of the other color types.
The study, led by Professor Tobias Uller at Lund University and published in Science, analyzed color data from about 240 populations and over 10,000 individual lizards. The results show that a single dominant trait can rapidly alter a system that has remained stable for millions of years.
Three Morphs, One Species
In Podarcis muralis, the common wall lizard, throat color is genetically determined and linked to specific behavioral and competitive strategies. The three color variants have historically maintained a color polymorphism, where each color type is more successful when it is rare, and faces increased competition as lizards of the same color become more common. This process, called negative frequency-dependent selection, has allowed all three morphs to persist over many generations.
This system stayed stable because no single morph could take over. When one color became common, the others gained an advantage, helping maintain balance among the three forms.
Enter the Hulk
This equilibrium changed with the appearance of what researchers call “Hulk” lizards. These are larger, more aggressive individuals with green-and-black coloration that distinguishes them from the original three morphs. As these lizards move into new populations, the yellow and orange morph populations fade. In many cases, only the white morph survives alongside the Hulk variant.
This pattern extends beyond the areas where the Hulk trait first appeared. In nearby lineages, the same loss of throat color diversity occurs. This means the new traits appear to consistently change how the lizards compete, not just a localized fluke.
“The aggressive behavior disrupts the finely tuned social systems that previously enabled several color strategies to coexist,” Uller added.
Breaking the Balance
The importance of this case extends beyond just these lizards. Researchers describe the Hulk variant as a sexually selected syndrome, in which inherited traits like color, size, and behavior disrupt the social interactions that once maintained diversity.
Scientists commonly observe color polymorphisms, like those in Podarcis muralis, across many animal species and usually consider them stable features of populations. This stability is based on the idea that each morph has an advantage when it is rare. However, the study shows that new factors can disrupt this stability. A new phenotype that changes the competitive balance can quickly eliminate the other morphs.
“We are seeing how the coexistence of several different color morphs, something that has been stable for millions of years, is being lost over a very short evolutionary time scale,” Uller said.
Faster Evolution Than Expected
These findings challenge the idea that evolutionary change always occurs slowly over geological timescales. In this case, evolution has acted quickly enough to disrupt the diversity that developed over millions of years.
“By showing how color variants that have coexisted for millions of years are wiped out, we now better understand how the emergence of new traits changes competition in nature,” Uller concluded.
Austin Burgess is a writer and researcher with a background in sales, marketing, and data analytics. He holds an MBA, a Bachelor of Science in Business Administration, and a data analytics certification. His work focuses on breaking scientific developments, with an emphasis on emerging biology, cognitive neuroscience, and archaeological discoveries.