by Greg Mayer
Since I first read about them when I was about 12 years old, I’ve been intrigued by the somewhat mysterious Vegas Valley leopard frogs. Known from a handful of springs, all in what is now more or less metropolitan Las Vegas, they had disappeared before the middle of the 20th century, and had been poorly known before apparently slipping out of existence. But now they’re back– sort of.
Evon Hekkala of Fordham University and her colleagues have a paper in press in Conservation Genetics in which, using DNA extracted from specimens of Vegas Valley frogs in the collection of the California Academy of Sciences, they find that Vegas frogs are closely related to the Chiricahua leopard frog of Arizona and nearby areas. In fact, the Vegas frogs are nested within the many samples of Chiricahua frogs they studied. From this they conclude that the Vegas frog is conspecific with the Chiricahua frog, and thus the Vegas Valley leopard frog is not extinct.
That the Vegas frogs (Rana fisheri) are conspecific with the Chiricahua frogs (Rana chiricahuensis) is a not unreasonable inference. North American leopard frogs, once thought to be a single widespread species, have proven to be a complex of several biological species, and although the situation is modestly clear in the eastern United States, there is much work yet to be done in the southwestern US and especially Mexico to figure out what’s going on. The Vegas frogs are not genetically identical to the Chiricahua frogs (contrary to some media reports), and, with only about 1200 base pairs examined at three loci, more differences are sure to be found.
So is the Vegas frog not extinct anymore? This isn’t just a biological species vs. amount-of-difference species concept question. One could accept, under various species concepts (and we know which one is right!), that the Vegas and Chiricahua frogs are conspecific, but still ask, is the Vegas Valley frog still extant because similar, but not identical, frogs continue to exist 100’s of kms away? I’m not sure. The US Endangered Species Act recognizes that population segments, even if they lack nomenclatural recognition or distinction, can be endangered, and be worthy of protection. And if a segment can be endangered, it can, of course, go extinct. So, while I’m glad we’ve learned who the Vegas Valley frogs’ closest relatives are, and that they’re quite similar, and that, due to the rules of nomenclature they will bear the name fisheri, I’m afraid the Vegas Valley frogs are still extinct.
The great herpetologists Albert & Anna Wright and Robert Stebbins summarized most of what will ever be known about the Vegas Valley frogs, and wrote, movingly yet scientifically, of their unsuccessful searches in the Vegas area in the 1940’s (they were last collected in January 1942, and last reported seen in the summer of ’42). The Wrights concluded:
Our R. fisheri may go with the old springs gone, the creek a mess.
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Fisher, J., N. Simon, and J. Vincent. 1969. Wildlife in Danger. Viking, New York
Hekkala, E.R., R.A. Saumure, J.R. Jaeger, H.-W. Herrmann, M.J. Sredl, D.F. Bradford, D. Drabeck, and M.J. Blum. 2011. Resurrecting an extinct species: archival DNA, taxonomy, and conservation of the Vegas Valley leopard frog. Conservation Genetics in press. pdf
Moore, J.A. 1944. Geographic variation in Rana pipiens Schreber in eastern North America. Bulletin of the American Museum of Natural History 82:345-370. pdf
Pace, A.E. 1974. Systematic and biological studies of the leopard frogs (Rana pipiens complex) of the United States. Miscellaneous Publications, Museum of Zoology, University of Michigan 148, pp. 140. pdf
Stebbins, R.C. 1951. Amphibians of Western North America. University of California Press, Berkeley.
Wright, A.H. & A.A. Wright. 1949. Handbook of Frogs and Toads of the United Sates and Canada. Comstock, Ithaca, New York.
David Hillis of the University of Texas (who Jerry just mentioned) has done much of the work that has more recently both clarified, and pointed out the lacunae in, our understanding of leopard frogs. Many of his papers (on leopard frogs and many other subjects) are available here.
Update. David Hillis has kindly commented below, and quotes one of his papers (Hillis and Wilcox, 2005); here’s a link to the pdf of that paper.
I am pleased to see this confirmation of the conspecific status of Rana fisheri and the Mogollon Rim populations that had been called Rana chiricahuensis. This is exactly what had been suggested by the morphology (e.g., here is what we said in Hillis and Wilcox, 2005. Molecular Phylogenetics and Evolution, 34: 299-314):
“[t]he populations of leopard frogs from the Mogollon Rim in Arizona that are currently recognized as R. chiricahuensis are morphologically distinct from R. chiricahuensis in southern Arizona, New Mexico, and Mexico, and may be referable
to R. fisheri (a species described from southern Nevada, and considered extinct by many authors). Rana fisheri appears to have been closely related to the Mogollon Rim populations of “R. chiricahuensis” based on
morphological similarity, and the name R. fisheri may be applicable to these Mogollon Rim leopard frogs. Our sample of R. chiricahuensis from Arizona is from the Mogollon Rim, and therefore perhaps should be referred to R. fisheri. However, we have followed the current taxonomic practice
of referring to these frogs as R. chiricahuensis, pending detailed analysis of the problem.”
This current study appears to be exactly what was needed to clarify and confirm this taxonomic resolution. It perhaps should be noted that if these two lineages are not recognized as distinct species, then the name Rana fisheri of course has priority.
The Northern Studfish is widely distributed and abundant, except that only a few were known from six localities in Indiana. We studied the situation, found them numerous at a number of localities, but they were not present at any of the historical localities, undoubtedly as a result of human habitat alteration. We addressed the question, “If they were extirpated in Indiana, which other population is most similar and the best candidate for reintroduction.” A colleague working on Fundulus isozymes found that the Ozark population was most similar. Link here.
http://ichthyology.usm.edu/sfc/proceedings/sfcpro20.pdf
Happy to see they’re back. They’re very green compared to local leopard frogs. It’s hard times for amphibians around here in SE Texas with the drought. It’s gonna take a couple of tropical storms or hurricanes to get us back to our amphibian friendly 50-60 inches rainfall. One Tropical Storm Allison would do it, 40in in a few days, with twenty in one horrible night. They did not bring in the National Guard for Ike in 2008, but the Guard came for Allison. It’s the only Atlantic storm name that has been retired without having been a hurricane. It was that bad. It even went NE and soaked Philly.
But I love that the Vegas frogs are back. Maybe the Houston Toad will come back to Houston. But not this year, their peak breeding season is late winter, and we had a pretty dry winter.
http://en.wikipedia.org/wiki/Houston_toad
I think it would help to bring the term “sub-species” into more common usage. And perhaps there needs to be two or more levels of taxonomy below “species”. “Sub-sub-species”? “Breed”?
Then it would make perfect sense to talk about the extinction of this or that sub-species or breed.
Not sure it would be appropriate for “extinction” to apply to local populations which are not significantly distinct, genetically speaking. But perhaps we can refer to “local extinctions” and “regional extinctions”, with “extinction” referring by default to “global extinctions”.
Heaven forfend!!! (Pardon the expression; I just like the sound of it.)
We do not need more nomenclature! What we need is more careful study and description of variation within and among species, as in the studies linked in this thread. Even botanists are tending away from the bulky species-subspecies-variety-forma nomenclature with which that field is still formally burdened. We have enough nomeclature by recoginizing the populations that can be designated as species, and the rest should be accounted for by good natural history, comparative morphology, etc.
Isn’t “race” still an OK term to use with non-human spp?
I am generally not a fan of subspecies, not in the field of herpetology. There are cases where to me it clearly is valid and should be retained. The Mountain Gartersnake (Thamnophis elegans elegans) and the Wandering Gartersnake (T. e. vagrans) have different behavior and ecological niches. The former is often found near water but rarely enters water to escape, it is not that good of a swimmer. While they will take fish, they are not good at catching them. The latter frequently retreats to water and is excellent at catching fish, fish making up a large part of its diet.
Yet the contact zone between the two forms results in gene flow between them, a broad range of intermediary forms (the formerly recognized Klammath Gartersnake is hybrid form) so they are not reproductively isolated and are the same species. But the differences go beyond just pattern and to their natural history and behavior, that warrants subspecies level designation in my opinion.
The Coast Gartersnake (T. e. terrestris) on the other hand I think is invalid subspecies, I consider it to just be a color morph of the Mountain Gartersnake. Behavior and natural history is identical, and the red color is not present at birth and occasionally not even as adults.
I do see value in naming morphs of a population and the Coast Gartersnake would certainly be a valid morph, but I don’t personally see enough evidence to warrant a taxonomic difference from the Mountain Gartersnake.
However, I usually just go with the flow and use what SSAR follows. Easier that way because it is a standard everyone has access to that has been discussed by people with far more education than I’ll ever have.
But I generally do not like subspecies, I think they often are invented for invalid reasons.
Wait a minute. Why does the nesting of one population inside another mean they’re conspecific? Under the biological species concept (which I am assured is the best one), all that counts is propensity to interbreed, and nesting tells us nothing about that. There are in fact known good species that are nested inside others — polar bears inside brown bears being the most familiar. Even a lack of private alleles doesn’t tell us about species status, unless we have assayed all the genes that might be important in speciation.
We can agree that mutual monophyly is partial evidence of separate species status; it’s even excellent evidence if the populations are sympatric. But what does absence of mutual monophyly tell us? Not much, I think, unless we have some hypothesis that incorporates an old date of divergence..
Or am I missing something?
In this case, there is mutual monophyly of two species: Rana chiricahuensis (which is the name still applied to the species that occurs to the south and east of the Mogollon Rim), and Rana fisheri (which occurs across the Mogollon Rim, and at one point occurred west to the Las Vegas valley (where it has been extinct since 1942). The two species appear to occur sympatrically in New Mexico, where they remain phylogenetically distinct. The phylogenetic evidence shows that the Las Vegas populations were genetically a part of the species that still exists across the Mogollon Rim.
Although reproductive isolation is critical for considering how and why species arise and remain distinct from one another, the primary evidence for the recognition of species is (these days) almost always phylogenetic in nature. I think Jerry is correct for caring almost exclusively about reproductive isolation when he studies speciation, because he is interested in how sexual lineages become and remain genetically isolated from one another. Reproductive isolation results in lineages remaining distinct from one another, but the evidence that biologists use to discover and delimit species these days is mostly phylogenetic information. I think calling these different aspects and interests about speciation different “species concepts” is distracting and unhelpful. Of course reproductive isolation is important for understanding speciation (of sexual species); but of course phylogenetic information is important for understanding the history and status of species. These are just different emphases and different aspects of the nature of species, often studied by different biologists. I don’t feel any need to choose a “correct” species concept; to me, the various concepts simply emphasize different aspects of species. Which one is “correct” simply depends on what questions we wish to ask about species.
I don’t believe you have answered the question. Do the genetic data show that the Las Vegas frogs are conspecific with R. fisheri? The evidence adduced is nesting. Is that good evidence? We agree that it’s good evidence that the Las Vegas frogs are not (were not) R. chiricahuensis.
Genetic data can tell us that populations X and Y belong to different phylogenetic species. I don’t see how it can tell us they’re different biological species. And I don’t see how it can tell us they are not different biological species either.
There was never any doubt that the Las Vegas frogs were Rana fisheri…Las Vegas was the type locality for Rana fisheri, so that population was Rana fisheri by definition.
There was already evidence (as I discussed in my first post) that the species formerly called Rana chiricahuensis actually consisted of two distinct species. Several people (myself included) had suggested that one of these two species (the one found across the Mogollon Rim) was conspecific with the frogs that once occurred in the Las Vegas valley (hence, were Rana fisheri). The data that were presented in this paper confirm that the Mogollon Rim populations of leopard frogs represent the same species that once occurred in Las Vegas, and hence are also Rana fisheri. They are morphologically the same, and genetically the Las Vegas populations contained a subset of the haplotypes present in the Mogollon populations. I can’t think of any more straightforward evidence that all the Mogollon Rim populations are conspecific with the populations that once occurred in Las Vegas. Every line of evidence supports that conclusion.
I must be very unclear. Though I really should have read the actual paper before spouting off, I still don’t think my point has been addressed. Apparently there is a claim that two populations are conspecific based on sharing of alleles and/or nesting. Shared alleles suggest several possiblities: current gene flow, gene flow in the not too distant past, or mantenance by selection. We can probably dismiss the last, but can we distinguish the first two? I’m thinking particularly of a few good bird species among which there is no apparent mtDNA haplotype structure.
Well, there is obviously no current gene flow, since the Las Vegas populations are extinct. Perhaps it makes sense to turn this around: what is the evidence that the extinct Las Vegas populations and the extant Mogollon Rim populations are NOT the same species? They are indistinguishable morphologically, and they share some of the same genotypes, without even showing geographic subdivision in the few genes that can be examined from preserved specimens. The populations of Rana chiricahuensis (to the east and south) are distinct morphologically and are reciprocally monophyletic and divergent in mitochondrial DNA (and also nuclear genes, if we include other studies). If you insist on direct tests of reproductive compatibility, then a determination of the status of Rana fisheri will not be possible (since the Las Vegas populations, which represent the type locality, are extinct). But since all the evidence is compatible with the conclusions of this paper (e.g., both Las Vegas and Mogollon Rim populations represent Rana fisheri, both of which are distinct from Rana chiricahuensis to the east and south), I can’t see where the problem lies. There is now better evidence for which populations of leopard frogs belong within Rana fisheri than we have for assigning populations of 99.9% of all species on Earth, and no evidence to the contrary. So, I don’t think I understand your concern. Direct tests of reproductive compatibility are relatively rare; the best available evidence usually comes from morphological and phylogenetic analyses, both of which support the conclusion that Rana fisheri populations are extant along the Mogollon Rim of Arizona.
Put that way, I grant your point. If we have no reason for suspecting two populations are separate species, one species is the way to bet. In birds, we very often do have more or less direct behavioral evidence, in the form of responses to live or recorded calls of different populations. And of course I’m fixated on my own study group.
Call responses are great data when you can get them! Unlike birds, female choice with respect to mating calls are only known for a few species of frogs, I’m afraid, and those experiments are very hard to conduct compared to birds. I’m not even sure that anyone has yet tried to compare and distinguish the calls of Rana fisheri and Rana chiricahuensis. Of course, that is not even possible for the extinct Las Vegas populations.
An interesting aside: Some Rana chiricahuensis call while completely submerged underwater (rather unusual for a frog). That was actually the basis for a supposedly new species, Rana subaquavocalis, which is now subsumed under Rana chiricahuensis. It appears to be a behavioral polymorphism…one that someone should study. It should make a big difference if a male frog is calling in air or submerged in water, one would think.
Presumably we only have to go back to about 10,000 years ago at the end of the last glaciation to find that these frogs were (probably) far more widely distributed? I assume it is the drying out of large areas since then that has left these isolated populations?
You are quite correct. Unfortunately, it is quite fashionable for researchers to look for any morphological and/or genetic differences among different isolated populations and use these differences to split existing species. The Arizona population of Hyla eximia, for example, is being recognized by some as a different species, Hyla wrightorum, simply because the spectrogram of its mating call is different from that of the Mexican populations of this species. The difference is being touted as evidence of premating isolation, even though these calls were not actually played back to real female frogs to see if they actually consider the calls different.
It is quite clear that the many “species” of leopard frogs share a common ancestor, unless one is a creationist, or unless one can show that they are the products of convergent evolution. The question, then, is whether and why some of these species should be considered distinct from Rana pipiens. Are they reproductively incompatible with one another? Are they so different ecologically from one another that a hybrid between these species will be negatively selected? If not, then naming different species on the basis of their genetic differences (which are often the result of geographic isolation and neutral genetic changes due to genetic drift) would seem to me to be a useless exercise and a retrogressive return to the typological species concept. Unfortunately, it would appear to me that the many species are in fact typological, as one can see from the arguments put forth by the investigators. They are classifying Rana fisheri and Rana chiricahuensis as different species, merely because they are genetically and morphologically different, which is something one would expect to find for two populations that are isolated from one another by inhospitable terrain, not because there are noticeable ecological differences between them. Ecological differences would likely result in the evolution of premating isolation should two populations meet again. Without these differences, they should be considered the same species. Sadly, it appears that the typological species is alive and thriving, even if many amphibians are on the decline for various reasons.
As a former creationist who was even at one point preparing for seminary, I must state that many creationists believe that evolution takes place. Taxonomic species are defined by man, and nothing about there being an initial creation process says that the species created by that process would necessarily fit the same taxonomic definitions 5,000 years later.
I no longer hold to creationism, too much overwhelming evidence against it AND predictions that are mutually exclusive that have been found true, but when I was a creationist, it was rather frustrating how many atheistic evolutionists assumed that we rejected any and all evolution. We actually embraced quite a bit of it.
Back on topic, with respect to how we define a species, which concept is best really does depend upon what you are investigating.
The Common Sagebrush Lizards living in the Sutter Buttes are geographically isolated from the Common Sagebrush Lizards living in the Sierra Nevada mountains. I have not seen a genetic study but they are clearly currently reproductively isolated, but that isolation is by geography. Morphologically, natural history, etc. all point to them being the same lizard. Assuming they remain isolated, perhaps they will be a different species in the future but there is no valid reason I am aware of to classify them as different now.
The same applies to the Vegas Valley Leopard Frogs. The Nevada populations may have been geographically isolated from the Arizona populations but that does not mean speciation had taken place. The evidence we have to examine suggests that speciation had not taken place at the Nevada populations went extinct.