This just in: there aren’t enough lizards in the world!
As of February 2014, the TIGR Reptile Database reports the existence of 5,914 lizard species (9,372 if you include the highly specialized lizards we call snakes). Well, TIGR, as of May 2015, tally up one more! My collaborator, Rob Bryson Jr., and I have recently described a new species of Mexican lizard (genus Sceloporus) based on extensive DNA and morphological analyses. The title of this post is in response to an article from the Irish Times referencing a new species of horned lizard my advisor Dr. Adam Leaché (and collaborators) recently described, “As if there weren’t enough lizards in the world…“
I began my Master’s thesis in 2008 at San Diego State University under the guidance of Dr. Tod Reeder. As academic incest would have it, I am now a PhD candidate in the lab of Dr. Adam Leaché, who also got his MSc from Dr. Reeder, but ~10 years before me. Anyways, my Master’s project was initially focused on studying genetics amongst various populations in Arizona and New Mexico of the bunchgrass lizard Sceloporus slevini, a member of the Sceloporus scalaris species group. Ten species existed in this group before I began my research, all of which are distributed across mid- to high-elevation mountains in Mexico and the U.S. southwest. This group of lizards is cool, despite their drab grey-brown appearance, because one species has evolved viviparity, or the act of giving live birth. This attribute, along with a unique montane distribution, attracted the attention of many biologists in the 1990s to figure out the relationships and “species limits” amongst members of this group. Since the 1990s, sequencing and analytical techniques have substantially progressed. Most importantly, for population geneticists and systematists, researchers have embraced the use of multi-locus nuclear DNA when inferring the species-level history of organisms. Similarly, multi-locus species delimitation has become a popular subject in the past 5-10 years. Many methods and programs that delimit species have been devised by a variety of researchers, each with their own strengths and weaknesses. Myself, under the guidance of my Master’s thesis advisor, along with collaborator Rob Bryson Jr., have developed another method for delimiting species. In doing so, we also discovered and described a new species of Sceloporus in Mexico.
Our new method of species delimitation takes advantage of the fact that many species tree inference programs require the user to assign individuals to lineages (a.k.a. “species”) before analysis. In this sense, the researcher can determine the best configuration of populations into species, given a particular dataset. For instance, based on the most current data, we know that the closest living relative to humans is the chimpanzee, and that humans and chimps together are closest related to the gorilla, and that the three of these are all equally related to orangutans. To a phylogeneticist, this “tree”, or set of evolutionary relationships, can be expressed as (Orangutan, (Gorilla, (Human, Chimp))).Now let’s say that we sequence individuals from multiple populations of each “species” of great ape, so that we have DNA from multiple individuals for each species. Using our method, our first hypothesis would assign every chimp individual to the species “chimp”, every human individual to the species “human”, etc. A phylogenetic tree would be constructed with these assignments, generating a likelihood score that we can now use to rank this model of species limits against other such models. In our toy example, a second model of species limits we could create would have all human individuals assigned to the “chimp” species, just to ruffle those creationist feathers. After inferring the species-level phylogeny with these species limits, we would see that the score of this model is worse because the data do not support this relationship (the creationists can now rejoice, we are indeed a different species from the chimpanzee). Although the outcome is obvious in this example, identifying species limits with drab tropical lizards is not quite so simple.In this exact same way, my co-authors and I tested many models of species limits amongst 12+ populations of lizards in the Sceloporus scalaris group using multi-locus nuclear DNA. This method identified and validated a “cryptic” species within the group, meaning that it is morphologically indistinguishable from other close relatives (except for some males who brandish an orange belly during the breeding season). Although not overly sophisticated, our method was published in the journal Systematic Biology at the end of 2013. Our article has been viewed over 1,200 times and used by 4 other research groups so far in their scientific publications.Our research highlights the need for in-depth genetic analyses of widely distributed species, especially those in the tropics. Central America in particular is a geologically complex region that has generated a large number of species, many of which are endemic to very isolated regions. The new species of lizard we discovered, Scleoporus aurantius (aurantius owing to the orange sides discovered in breeding males), exists in southwestern Mexico in the states of Jalisco and Aguascalientes, but we are currently uncertain of its exact geographic extent.
We are excited to discover a new species of vertebrate amongst an era marked by habitat destruction and global species declines. But maybe more importantly, we are very excited to present a method to the scientific community that we hope many researchers will use in their studies and aid in the discovery of new species in the future.
If you would like a copy of any of these articles, please email me at grummer@uw.edu.
Methods paper: http://sysbio.oxfordjournals.org/content/early/2013/12/25/sysbio.syt069.abstract
New species description (preview): http://www.mapress.com/zootaxa/2014/f/z03790p450f.pdf
UW Daily article: http://dailyuw.com/archive/2014/05/21/science/new-species-lizard-discovered-recent-research#.U36Say_ezAS
