Icons of adaptation: tracing the evolutionary journey of West Texas' blanched lizards
Its bland name is misleading. The common lesser earless lizard — Holbrookia maculata — is indeed common in our region's plains and deserts. It's humble in stature — 3 to 5 inches long, and typically grey-brown. But this ordinary creature has extraordinary capabilities.
In 2016, amidst the white dunes of Guadalupe Mountains National Park, in West Texas, a biologist discovered lesser earless lizards as white as the sands themselves. And as scientists look more closely at these creatures, they're gaining insight into the mechanisms that drive the diversity of life on Earth.
The Salt Basin Dunes, on the west side of Guadalupe Mountains National Park, are a secret of the Southwest. Their geological story is similar to that of the famous dunes at White Sands National Park. But they're just a fraction the size of that New Mexico dunefield — about 3 square-miles, compared to 200 square-miles. These dunes remain largely unexplored by park visitors and scientists.
But in 2016, herpetologist Drew Dittmer took a detour there, after a conference near Marfa. He was drawn by a hunch — and it paid off.
“I slept in a bed roll in the campground in the park,” Dittmer said, “and then went around really early the next morning – and I found one. You see it, and you're like, it looks really white, and the sand's pretty white. I took a picture of it. From that point it just kind of snowballed.”
Many lizards can change hues to match terrain. But Dittmer's find wasn't a matter of such “plasticity.” The population of “blanched” lizards he'd discovered were genetically adapted to this specific niche.
Dittmer had reason to suspect they might exist. There are blanched versions of three lizard species at New Mexico's White Sands — whiptail and fence lizards, as well as lesser earless lizards. And with his discovery, Dittmer reached out to the Rosenblum Lab, at the University of California-Berkeley, which has done pathbreaking research on those white lizards.
Dr. Telma Laurentino is a geneticist who has worked at the Rosenblum Lab.
“This is a very rare and incredibly beautiful system to tell you about adaptation,” Laurentino said, “how it happens, where it happens. Is nature using the same tools to produce similar things? Or does it arrive at similar things through different genetic avenues? So that was my fire about it from the beginning.”
One focus of her research is “convergent evolution” – the independent evolution of similar features in creatures separated in space or time. The two blanched lizard populations suggested this phenomenon.
The two dunefields are a hundred miles apart, and it seemed unlikely blanched lizards from one location had colonized the other. But to better understand the Texas lizards' evolutionary story, Laurentino looked at their DNA.
An organism's full genome, Laurentino said, is like an IKEA manual – the genetic code provides the “instructions” for “building” the entire organism. Analyzing a full genome is daunting. So out of the full “manual” for the Salt Basin Dunes lizard, Laurentino looked at the equivalent of just two “phrases,” she said.
The first was a set of markers in the lizard's mitochondrial DNA. These markers allowed Laurentino to place the lizard within a “phylogenetic tree” – a diagram capturing lines of evolutionary descent. The Salt Basin lizards, she found, are more closely related to the “normal,” brown lizards found in the surrounding desert than they are to the blanched lizards at White Sands.
It's compelling evidence for convergent evolution.
“So the mitochondrial marker showed us that what we're seeing at the mitochondrial DNA level is that there's a geographic signature of the similarities of this population,” Laurentino said, “and it also hints at the fact that the populations in the Salt Basin Dunes have an independent origin.”
The second “phrase” was a marker called the melanocortin 1 receptor. The blanched whiptail and fence lizards at White Sands have a mutation in this gene – as do other pale creatures, from yellow mice to red-haired humans. But Laurentino found no such mutation among the blanched earless lizards, either at the Salt Basin Dunes or White Sands.
“So basically it remains unknown what is genetic basis of the blanching in the earless lizards,” she said. “We know it in the fence lizards and the whiptail. But we still don't know it in the holbrookia. It still remains to answer if they did it twice in the same way, or did it in different ways.”
It's easy to imagine how pale coloration might serve as camouflage for a creature living on glinting dunes. But the Salt Basin lizards appear to have other distinctive traits.
Laurentino studied the creatures from photos Dittmer provided, and noticed that in many of the images, the lizards were perched – sometimes spread-eagle – in low shrubs.
“I only had one seasons of White Sands work,” she said, “but I was like, I've never seen them doing this there. So I went back and asked all the veterans from the White Sands, and everyone was like, 'I've ever never seen that, or I've seen that once or twice in 20 years of cumulative observations of those lizards.' And then I asked Drew, and he was like, 'No, they do that a lot.'”
Earless lizards aren't known to perch in other environments. Elsewhere, different lizard species claim that vertical niche. But as the only resident lizards in these dunes, the earless lizards have the run of the place. They may perch to evade potential predators, or to find relief from the heat at ground level. Whatever the benefit, they've adapted to take advantage of it: the Salt Basin lizards have longer legs, feet and toes than their kin elsewhere.
These West Texas creatures have evolved relatively rapidly — the Salt Basin Dunes are thought to be 5,000 years old. And with their distinctive adaptations, they're “all in” on a surprisingly small locale, Dittmer said.
“That's the interesting thing,” he said. “They get there, and they commit, like, 'No, we're in it.' And they're in it for something that is smaller than most college campuses, in the case of the Salt Basin Dunes. They're like, 'This is enough, this 3 square-miles.'”
It was famously a trip to the Galapagos Islands that sparked Charles Darwin's transformative insight into natural selection. From island to island, he observed related birds, with distinctive features – their beaks were uniquely suited to local food sources. Amidst our region's “sea” of desert, dunefields are likewise “islands” of habitat. And like Darwin's finches, lesser earless lizards are emblems of evolution, icons of adaptation.