Enlarge (credit: Kristian Bell)

The lizard Podarcis muralis nigriventris might not grow to a freakish size and smash everything in sight, but evolution has turned this lizard into the Incredible Hulk of sorts—green skin included. P. nigriventris is something like the imposing Marvel superhero when compared to other strains of common wall lizards (Podarcis muralis). While the common version tends to be relatively small and brownish to greenish-brown, the nigriventris subspecies, which is found in central Italy, is visually impressive because of its green(er) skin with black markings, larger size, and heightened aggression.

A team of evolutionary biologists led by Nathalie Feiner of Lund University in Sweden decided to find out which genes contributed to making P. nigriventris so Hulkish. Like many fictional humans with superpowers (but unlike the mutant Hulk), this lizard is a hybrid.

Hulking hybrids

Though common wall lizards are found from the Iberian peninsula all the way to Asia Minor, the researchers focused on lizards from populations in central Italy (IT lineage) and the southern Alps (SA lineage). These lineages most likely diverged from a common ancestor between 5–6 million years ago and then began to hybridize—individuals from the different lineages mated with each other to produce hybrid offspring.