The Strange Tale of a New Species of Lizard

Each year, scientists publish roughly 17,000 detailed descriptions of newly discovered animals. Recently, in the journal Breviora, researchers described yet another, a new species of lizard called Aspidoscelis neavesi.

At first glance, this seems to be a run-of-the mill lizard: a small, slender creature with spots along its back and a bluish tail. In fact, Aspidoscelis neavesi is quite exceptional. The lizard was produced in the laboratory by mating two other species, and its creation defies conventional ideas about how new species evolve.

The evolution of a new animal species is usually a drawn-out affair.

Typically, an existing animal population is somehow divided, and the newly isolated populations reproduce only among themselves. Over thousands of generations, the animals may become genetically distinct and can no longer interbreed.

Of course, scientists have long known that some related species sometimes interbreed. But the hybrid progeny generally were thought to be evolutionary dead-ends — sterile mules, for instance. In recent decades, however, researchers have learned that these hybrids may represent new species.

Some of the most striking examples occur among whiptail lizards, which live in the southwestern United States. In the 1960s, scientists noticed that some whiptail lizard species had a strange genetic makeup. They have two copies of each chromosome, just as we do, but each copy is very different from its counterpart. The genes look as if they come from different species.

Perhaps stranger, many species produce no males. The eggs of the females hatch healthy female clones, a process known as parthenogenesis.

Normally, unfertilized animal eggs have only one set of chromosomes. The second set is derived from a male’s sperm following fertilization. But parthenogenic female whiptail lizards can duplicate the chromosomes in their offspring without males.

These findings led scientists to a hypothesis for how these strange species came about: Sometimes individuals from two different species of whiptail lizards interbreed, and their hybrid offspring carry two different sets of chromosomes.

Somehow, this triggers a switch to parthenogenesis. The female hybrids start to produce clones distinct from either parental species. In other words, they instantly become a new species of their own.

But it gets even more bizarre. Some species of whiptail lizards carry three sets of genes, rather than two.

How can that be? Scientists hypothesized that male lizards from sexually reproducing species sometimes mated with parthenogenic females. Sometimes, their sperm succeeded in fertilizing a female’s eggs, which already contained two sets of chromosomes. The egg now had three sets, and voilà: yet another a new species.

The strangeness doesn’t end there. In 1967, a Harvard graduate student named William B. Neaves was searching for whiptails around Alamogordo, N.M., when he found one with four sets of chromosomes.

Dr. Neaves concluded that the lizard was a hybrid. Three sets of chromosomes appeared to have come from a species called Aspidoscelis exsanguis. The fourth set hailed from a species called Aspidoscelis inornata. Both species live around Alamogordo.

Dr. Neaves didn’t follow up on this finding, instead pursuing a career researching fertility and stem cells. But at a dinner in 2002, he mentioned the whiptail lizards to Peter Baumann, a molecular biologist at Stowers Institute for Medical Research, where Dr. Neaves served as president.

Dr. Baumann decided it was high time to use new scientific tools to study whiptail lizards, and he and Dr. Neaves started making road trips to New Mexico to catch them and take them back to Stowers. As they came to understand the biology of the lizards better, they and their colleagues began to bring different species together to see if they could hybridize. Most of the time, their experiments failed.

In 2008, the scientists tried to recreate the hybrid with four sets of chromosomes. They put female Aspidoscelis exsanguis (the parthenogenic species with three sets of chromosomes) and male Aspidoscelis inornata in the same containers. In short order, the lizards started mating, and the females laid eggs. When the eggs hatched, the scientists examined the genes of the baby lizards and found four sets of chromosomes.

Four of the new hybrids were females. To the delight of the scientists, the females could clone themselves — and the offspring could produce clones of their own. Today, the scientists have a colony of 200 of these lizards.

Eventually the scientists became convinced they had produced a new species. They ran the idea past Charles J. Cole, a herpetologist at the American Museum of Natural History, who has studied whiptail lizards since the 1960s.

“As soon as they told me what they had done, I knew it was a species,” Dr. Cole said. The lizard’s body was clearly distinct from its parental species, and the fact that the species emerged in a lab seems irrelevant to Dr. Cole.

“It’s not a Frankenstein genome manipulation,” he said. “It’s lizards in cages doing their thing.”

Dr. Cole agreed to help Dr. Baumann and his colleagues formally describe the new species. Dr. Cole carefully cataloged the many features, both striking and subtle, that set the new lizard apart from other known whiptail species. They named it Aspidoscelis neavesi to honor Dr. Neaves.

“These lizards should have their own species,” said Laurence M. Hardy, a biologist at Louisiana State University Shreveport, who was not involved in the study. He said that they fulfilled the formal requirements.

But David Hillis, an evolutionary biologist at the University of Texas, questioned whether any lineage of hybrid whiptail lizards should be considered true species. “It is widely practiced, but often questioned,” he said.

Traditionally, scientists have looked at how animals reproduce to decide if they truly represent a species or just a subspecies. Animals within a real species produce offspring mostly with one another. As a result, their genes mix together into a single gene pool.

Are these new lab lizards really a species? Aspidoscelis neavesi doesn’t need to mate at all. It doesn’t maintain a single gene pool. The mutations that one lizard acquires will be passed down only to her offspring, not to others.

Aspidoscelis neavesi also raises a special puzzle, Dr. Hillis noted, because it emerged over and over again. Dr. Baumann and his colleagues have now successfully produced fertile hybrids of Aspidoscelis inornata and Aspidoscelis exsanguis dozens of times from different parents. Since each lineage comes from different parents, they could arguably be considered separate species, not just a new one.

Dr. Hillis says he thinks biology needs a different way to describe these lizards. “I think they are better termed ‘hybrid clones,’ ” he said. “That would represent a more accurate reflection of their relationship to the tree of life.”

Dr. Baumann argues that giving the lizards a species name is important simply to help scientists communicate. “It allows them to know what they’re talking about,” he said.

He and his colleagues have a lot to talk about. They are investigating how Aspidoscelis neavesi copes with having four sets of chromosomes. In humans, extra chromosomes can cause dramatic changes. An extra copy of chromosome 21, for example, leads to Down syndrome.

Yet Aspidoscelis neavesi appears to be a perfectly healthy, normal group of lizards. “If anything, we see a slight advantage,” Dr. Baumann said.

The good health of Aspidoscelis neavesi raises yet another puzzling question: If the parental species of Aspidoscelis neavesi live near one another, shouldn’t Aspidoscelis neavesi exist in the wild, too?

It’s possible that healthy hybrids do emerge from time to time, but that they have been the victims of bad luck. “If you’re rare, you’re more likely to go extinct by chance,” said James Mallet, a Harvard biologist who was not involved in the study.

On the other hand, it may be that Aspidoscelis neavesi is thriving unseen around Alamogordo. Dr. Cole hopes that the new paper, with its detailed description of the new species’ appearance, will help people to identify them.

“They may be out there somewhere, and we just don’t know it yet,” said Dr. Cole.