In a lab in Kansas, Aracely Lutes has created a new species of all-female lizard that reproduces by cloning itself. There wasn’t any genetic engineering involved; Lutes did it with just a single round of breeding.
This feat stands in stark contrast to the slow pace at which species usually arise. Here’s the typical story: different populations become separated in some way, whether by space, time, predators, sexual preferences, or an inability to understand one another. Differences gradually build up between them, until they can no longer produce fit and fertile offspring. Voila – where there was once one species, there are now two.
There are exceptions to this recurring tale of slow divergence. Different species sometimes mate to create hybrids, whose genomes are a mash-up of those of their parents. These individuals are often infertile (think mules) or weak. But in rare cases, they survive and prosper. For example, there is a hybrid bat from the Caribbean that combines the genomes of three separate species, one of which is now extinct. Other mammals including the red wolf and stump-tailed macaque might also be hybrids. More and more, scientists are discovering that these fusion animals are a surprisingly common part of life’s history.
Some groups are exceptionally good at making hybrids. The North American whiptail lizards (Aspidoscelis) are grandmasters at it. Of the twelve species in New Mexico, seven of them are hybrids. For example, the New Mexico whiptail (Aspidocelis neomexicana) was born when the Western whiptail (A. inornata) mated with the little striped whiptail (A. tigris).
In these unions, the origin of a new species happens in an instant. The hybrid lizards have genomes that are distinct from their parents. These distinct genomes stay distinct for the new hybrids don’t mate with their parental species. They have the ability to reproduce without sex – they are always female and their daughters are all clones of their mothers.
The genomes of the living lizards testify to their weird origins, but it’s much harder to actually witness these beginnings. When scientists find first-generation hybrid whiptails in the wild, they’re almost always sterile. One group even spent 29 years trying to breed a new hybrid species in the lab, a project that involved 230 lizards, nine species, five sterile hybrids, and zero success.
There has been one tantalising exception to this catalogue of failure. In 1967, William Neaves (who was also involved in this new study) described two eggs that had been laid by a hybrid lizard, the offspring of a Western whiptail (A.inornata) and a Chihuahuan spotted whiptail (A.exsanguis; itself a hybrid). The lizard seemed fertile, but its eggs dried out before they could hatch. No one could prove that the hybrid would truly have raised viable young.
But that was enough for Lutes. She set about trying to breed a hybrid whiptail, using the same combination of parent species that Neaves described.
She succeeded.
Lutes recovered six eggs that had all been produced by the same A.exsanguis female and the same A.inornata male. All six hatched. The young lizards were similar in size and pattern to A.exsanguis, but from A.inornata, they had inherited a subtle blue tint in their tails. They have four copies of every gene rather than the standard two – three inherited from their mother and one from their father. And they are most definitely fertile.
Four of them, all female, managed to clone themselves. So did their daughters and granddaughters. These four lab-raised dynasties are now in their third generation, with 68 members in their family. It’s dramatic proof of the process that supposedly gave rise to many of New Mexico’s other whiptails.
Do these lizards count as a separate species? Peter Baumann, who led the study, thinks so, but he says it “depends on who you ask”. This is why he hasn’t given the hybrids a formal name. “The whole topic of naming new species can be controversial and we did not want to distract from the scientific finding by drawing attention to the issue of naming. Under the assumptions that these lineages will continue to thrive, we will at some point need a name for them,” he says.
In the meantime, the lab-reared hybrids raise a new suite of questions. Would they thrive in the wild, in competition with their parents? Certainly, in the lab, the hybrids can capture live crickets and beetles as effectively as their parents, even in direct competition. Maybe Lutes’ lizards are “the prototype of a species that might eventually emerge in the deserts of the southwestern US or northern Mexico.”
Or perhaps their wild counterparts exist and haven’t been found. “Either these animals have been misidentified as A.exsanguis or there is an ecological reason that they cannot persist in nature,” says Baumann. “We are presently pursuing both ideas.”
Reference: Lutes, Baumann, Neaves & Baumann. 2011. Laboratory synthesis of an independently reproducing vertebrate species. PNAS http://dx.doi.org/doi/10.1073/pnas.1102811108
More on hybrids:
- Holy hybrids Batman! Caribbean fruit bat is a mash-up of three species
- Extra chromosomes allow all-female lizards to reproduce without males
- Drought drives toads to mate with other species
- New plant species arise from conflicts between immune system genes
- When bacteria merge – two species are turning into one
- And one that isn’t… The pink Galapagos iguana that Darwin never saw
May 5th, 2011 at 12:16 pm
I love that New Mexico’s been getting some good attention lately, between these lizards and the “daemon” dinosaur discovered at Ghost Ranch.
I noticed that the example hybrid in the picture is missing the extremely long toes of her parents. Is that true of all members of the new species, and if so, could they be at a disadvantage?
May 5th, 2011 at 12:27 pm
Well spotted. The other image in the paper, which I didn’t use, shows the 2nd and 3rd generation forms, some of which do have the long toes. So perhaps that’s just a feature of that one individual, or maybe one lineage.
May 5th, 2011 at 1:09 pm
Very cool study and post.
These common names and binomials are switched. [error recurs throughout]
Some have suggested that all of these parthenogenic hybrid clone-species of whiptails are of recent origin–hundreds of years–and perhaps induced by widespread habitat change due in turn to overgrazing.
May 5th, 2011 at 2:12 pm
Given the recent discoveries concerning Neanderthal DNA in modern Homo Sapiens, wouldn’t we ourselves be an example of a hybrid species? I suppose that depends whether Neanderthals are a separate species or a sub-species, which I understand there is some debate about.
May 5th, 2011 at 2:39 pm
It thrills me to read your post. Excellent writing!
Some 250 years ago, Carlous Linnaeus claimed to have made a new species. In an experiment, he crossed two species of Tragopogon. The hybrid looked different from the parents, and Linnaeus thought he had some solid evidence.
During most of the 20th century, it has been more or less tabu to talk about a hybrid origin of species. partly because of what Charles Darwin wrote, partly because Ernst Mayr was so heavily against it. Today, the picture is different and most systematists agree that most flowering plants have some kind of hybrid origin in their past, and loads of animals do as well.
I think its interesting how the history of ideas can make these steep turns. Its also interesting to see that hybridization never has been excluded out of evolution by the hard-core of evolutionary theory, it has rather been ad-hoc like arguments that have been put forward against it. Now, it seems hybridization is truly recognized a important part of the speciation process. It is kind of ironic that Linnaeus findings never really took of into the biology hall-of -fame. Still, parts of Gregor Mendels pea-experiments a hundred years later could be seen as following in Linnaeus (but mainly Koelreuters) tradition.
And what’s the contemporary science standpoint when it comes to the complex genetic evidence when it comes to human evolution..? E.g. http://www.nature.com/nature/journal/v441/n7097/abs/nature04789.html
May 5th, 2011 at 3:26 pm
What a shame we can’t use the chromosome-doubling trick that works so well with plants on animals. One of the disadvantages of having a complex biochemistry: any large-scale genetic alterations tend to be highly lethal.
May 5th, 2011 at 3:34 pm
Plants are less complex than animals? Really?
… Noob
May 5th, 2011 at 5:54 pm
A bit of a quibble about your red wolf reference… within the context you’ve placed it, you imply that the red wolf originated as a hybrid between two different species. This is a common misinterpretation, but also not an accurate one. In the past decade, many scientists have largely rejected a hybrid origin for red wolves based on multiple lines of evidence including morphology, systematics and genetics. (1) As Ronald Nowak so elegantly wrote in 2002, “… such hybridization generally was considered a modern phenomenon that contributed to the demise, not the origin, of the red wolf.” {Nowak, R. M. 2002. The original status of wolves in eastern North America. Southeastern Naturalist, 1(2), 95-130.} {1. See: Wilson, P. J. et al. 2000. DNA profiles of the eastern Canadian wolf and the red wolf provide evidence for a common evolutionary history independent of the gray wolf Canadian Journal of Zoology, 78, 2156–2166.}
May 5th, 2011 at 10:37 pm
Why are hybrid whiptails all parthenogenetic, and might their asexual reproduction have something to do with the apparent lack of A. inornata x A. tigris hybrids in the wild?