The lands around the western Mediterranean are crawling with wall lizards, many and myriad wall lizards. These colorful, zippy reptiles are, zoologists aver, the most successful reptile in the region, at least from the perspective of diversity and abundance.
Why they proliferated and diversified so much in that part of the world has been a mystery for over two centuries. Past efforts to construct a family tree for the wall lizards in Europe, North America and the Mediterranean islands yielded unstable topologies, the researchers explain.
Now a paper in Nature Communications suggests why wall lizards became so successful. Simply, different wall lizard species seized the opportunity to mate with one another when they had a chance, especially after the Mediterranean Sea dried up for 700,000 years. The result was hybrid vigor and a tweak to the theory of evolution.
A great flood
The genus Podarcis, aka wall lizards, seems to have arisen 20 million years ago, say Weizhao Yang of Sweden’s Lund University and colleagues. By now there are 26 recognized species of this lizard, plus a bunch of subspecies, quite the feat of diversification. Its evolutionary history is “remarkably entangled,” Yang and the team write.
How did that come to pass? Well, Yang and the researchers sequenced the genomes of them all: 34 types of Podarcis – 26 species and eight subspecies. Their goal: to deduce the lizard’s evolutionary history. Among other things, they deduced that the Messinian salinity crisis – more on that later – was followed by rapid speciation in this reptilian set, but it wasn’t the only event in their lizardly history.
Somewhere over 11 million years ago, the wall lizards became separated into groups living in Iberia, the Balkans, and the Mediterranean islands. Then about 6 million years ago, climate change led the Mediterranean Sea to all but dry up completely, an episode known as the Messinian salinity crisis. Roughly 5.3 million years ago the Atlantic breached the Strait of Gibraltar and refilled the salt-encrusted Mediterranean valley in the putatively cataclysmic event known as the Zanclean Flood. (Not all agree that the Zanclean Flood was catastrophic; some believe the Mediterranean basin refilled more gradually and gently.)
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During the Messinian salinity crisis, the lizards within these groups diversified madly. Their genomic sequencing reveals mosaics: evidence of genetic exchanges.
Indeed, when the Mediterranean vanished, the lizards encountered new expansion opportunities. It seems these lizard species made contact and mated whenever they had a chance. “Genetic exchange between lineages has been a pervasive feature throughout the entire history of wall lizards,” the team writes.
Thus today’s immensely diverse lizards are the offspring of ancient mosaics, viable lizards with major contributions from more than one parental direction.
Spanish lizard? Not exactly
Take the lovely lizard scuttling on the walls of the Spanish island of Ibiza. It’s a hybrid of wall lizards that today reside on the Iberian Peninsula, with lizards living in the Balkans and on Greek islands.
Yes, the Ibiza lizard benefits from hybrid vigor – a new combination of old genes resulting in evolutionary opportunity. The hybrid can be “more robust” than the parents.
This explains the variation in color among the Ibiza island lizards. “Despite close relationships and geographic proximity, they are a single color on one island, but a variety of colors on the next,” the authors write.
How drastic is this? Quite: green lizards with brown/black spots, blue lizards ditto, green on top and sandy colored on the tummy, yellow and browns – these wee reptiles run the gamut.
The Ibiza lizard is just one example. Others also show introgression from other geographically adjacent but sometimes rather distantly related lizardly lines – in fact, “rampant introgression,” the team writes.
So millions of years before our early hominin ancestors even began to evolve, let alone become humans, the Sicilian-Maltese lizard met the Iberian lizard and the Italian lizard and they begat the lizard seen in the Mediterranean’s Western islands. The Italian lizard meanwhile can thank the Balkan and Sicilian-Maltese lizard for around a quarter of its genes. The wall lizard of the P. peloponnesiacus got over a quarter of its genome from an extinct wall lizard, the researchers add.
The discovery tweaks evolutionary theory to a degree. Wall Lizard species A and Wall Lizard species B can make merry and produce hybrid Wall Lizard C, which then becomes a successful species in its own right, evolving and further diversifying. It turns out that such hybridization (between lineages that had been reproductively isolated for a long time) can be a persistent feature of animal groups with a common ancestor over millions of years.
“Our results show that the dramatic changes at the time probably contributed to the emergence of new species. They also shed light on why biodiversity looks the way it does today,” says Tobias Uller, professor of evolutionary ecology at Lund University who led the international study.
So what do we have? A lot of speedy wall lizards that differentiate vastly in color, size and so on. Fussy in their mating habits, evidently they were not, but judge not. Neither were we, according to the evidence of introgression from Neanderthals, Denisovans and even other human species.