Abstract
The Mohave Rattlesnake (Crotalus scutulatus) is a highly venomous pitviper inhabiting the arid interior deserts, grasslands, and savannas of western North America. Currently two subspecies are recognized: the Northern Mohave Rattlesnake (C. s. scutulatus) ranging from southern California to the southern Central Mexican Plateau, and the Huamantla Rattlesnake (C. s. salvini) from the region of Tlaxcala, Veracruz, and Puebla in south-central Mexico. Although recent studies have demonstrated extensive geographic variation in venom composition and cryptic genetic diversity in this species, no modern studies have focused on geographic variation in morphology. Here we analyzed a series of qualitative, meristic, and morphometric traits from 347 specimens of C. scutulatus and show that this species is phenotypically cohesive without discrete subgroups, and that morphology follows a continuous cline in primarily color pattern and meristic traits across the major axis of its expansive distribution. Interpreted in the context of previously published molecular evidence, our morphological analyses suggest that multiple episodes of isolation and secondary contact among metapopulations during the Pleistocene were sufficient to produce distinctive genetic populations, which have since experienced gene flow to produce clinal variation in phenotypes without discrete or diagnosable distinctions among these original populations. For taxonomic purposes, we recommend that C. scutulatus be retained as a single species, although it is possible that C. s. salvini, which is morphologically the most distinctive population, could represent a peripheral isolate in the initial stages of speciation.
References
Baird, S.F. & Girard, C. (1853) Catalogue of North American reptiles in the Museum of the Smithsonian Institution. Part I. Serpents. GPO, Washington, D.C., 172 pp.
https://doi.org/10.5962/bhl.title.5513
Cadena, C.D., Zapata, F. & Jiménez, I. (2018) Issues and perspectives in species delimitation using phenotypic data: Atlantean evolution in Darwin’s finches. Systematic Biology, 67 (2), 181–194.
https://doi.org/10.1093/sysbio/syx071
Campbell, J.A. & Lamar, W.W. (2004) The Venomous Reptiles of the Western Hemisphere. Cornell University Press, Ithaca, 870 pp.
Cardwell, M.D. (2016) Mohave rattlesnake, Crotalus scutulatus (Kennicott 1861). In: Schuett, G.W., Feldner, M.J., Smith, C.F. & Reiserer, R.S. (Eds.), Rattlesnakes of Arizona. Vol. 1. ECO Publishing, Rodeo, pp. 563–606.
Coyne, J.A. & Orr, H.A. (2004) Speciation. Sinauer Associates, Sunderland, 545 pp.
de Queiroz, K. (1998) The general lineage concept of species, species criteria, and the process of speciation: A conceptual unification and terminological recommendations. In: Howard, D.J. & Berlocher, S.H. (Eds.), Endless Forms: Species and Speciation. Oxford University Press, New York City, pp. 57–75.
de Queiroz, K. (2007) Species concept and species delimitation. Systematic Biology, 56 (6), 879–886.
https://doi.org/10.1080/10635150701701083
D’Orazio, M. (2015) Integration and imputation of survey data in R: the StatMatch package. Romanian Statistical Review, 2, 57–68.
Douglas, M.E., Douglas, M.R., Schuett, G.W. & Porras, L.W. (2006) Evolution of rattlesnakes (Viperidae; Crotalus) in the warm deserts of western North America shaped by Neogene vicariance and Quaternary climate change. Molecular Ecology, 15 (11), 3353–3374.
https://doi.org/10.1111/j.1365-294X.2006.03007.x
Dray, S. & Dufour, A. (2007) The ade4 package: Implementing the duality diagram for ecologists. Journal of Statistical Software, 22 (4), 1–20.
https://doi.org/10.18637/jss.v022.i04
Feder, J.L., Egan, S.P. & Nosil, P. (2012) The genomics of speciation-with-gene-flow. Trends in Genetics, 28 (7), 342–350.
https://doi.org/10.1016/j.tig.2012.03.009
Fisher, R.A. (1918) The correlation between relatives on the supposition of Mendelian inheritance. Transactions of the Royal Society of Edinburgh, 52, 399–433.
https://doi.org/10.1017/S0080456800012163
Fraley, C. & Raftery, A.E. (2007) Model-based methods of classification: using the mclust software in chemometrics. Journal of Statistical Software, 18 (6), 1–13.
https://doi.org/10.18637/jss.v018.i06
Gloyd, H.K. (1940) The Rattlesnakes, Genera Sistrurus and Crotalus: A Study in Zoogeography and Evolution. Chicago Academy of Sciences, Chicago, 266 pp.
https://doi.org/10.2307/1437993
Gutberlet, R.L. Jr. & Harvey, M.B. (2002) Phylogenetic relationships of new world pitvipers as inferred from anatomical evidence. In: Schuett, G.W., Höggren, M., Douglas, M.E. & Greene, H.W. (Eds.), Biology of the Vipers. Eagle Mountain Publishing, LC, Eagle Mountain, pp. 51–68.
Kennicott, R. (1861) On three new forms of rattlesnakes. Proceedings of the Academy of Natural Sciences of Philadelphia 13, 204–208.
Klauber, L.M. (1930) New and renamed subspecies of Crotalus confluentus Say, with remarks on related species. Transactions of the San Diego Society of Natural History, 6, 95–144.
https://doi.org/10.5962/bhl.part.11694
Klauber, L.M. (1972) Rattlesnakes: Their habits, Life Histories, and Influence on Mankind. 2 Vols. University of California Press, Berkeley, 1533 pp.
Mantooth, S.J., Hafner, D.J., Bryson, R.W. Jr. & Riddle, B.R. (2013) Phylogenetic diversification of antelope squirrels (Ammospermophilus) across North American deserts. Biological Journal of the Linnean Society, 109, 949–967.
https://doi.org/10.1111/bij.12084
Meik, J.M., Streicher, J.W., Lawing, A.M., Flores-Villela, O. & Fujita, M.K. (2015) Limitations of climatic data for inferring species boundaries: insights from speckled rattlesnakes. PLoS ONE, 1, e0131435.
https://doi.org/10.1371/journal.pone.0131435
Meik, J.M., Schaack, S., Flores-Villela, O. & Streicher, J.W. (2018) Integrative taxonomy at the nexus of population divergence and speciation in insular speckled rattlesnakes. Journal of Natural History, 52 (13–16), 989–1016.
https://doi.org/10.1080/00222933.2018.1429689
Myers, E.A., Hickerson, M.J. & Burbrink, F.T. (2016) Asynchronous diversification of snakes in the North American warm deserts. Journal of Biogeography, 44, 461–474.
https://doi.org/10.1111/jbi.12873
Nosil, P. & Feder, J.L. (2012) Genomic divergence during speciation: causes and consequences. Philosophical Transactions of the Royal Society B, 367, 332–342.
https://doi.org/10.1098/rstb.2011.0263
R Core Team. (2017) R: A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna. Available from: https://www.R-project.org/ (accessed 4 June 2019)
Rafinesque, C.S. (1818) Natural history of the Scytalus Cupreus, or copper-head snake. American Journal of Science, 1 (1), 84–88.
Riddle, B.R. & Hafner, D.J. (2006) A step-wise approach to integrating phylogeographic and phylogenetic biogeographic perspectives on the history of a core North American warm deserts biota. Journal of Arid Environments, 66 (3), 435–461.
https://doi.org/10.1016/j.jaridenv.2006.01.014
Sabaj, M.H. (2016) Standard symbolic codes for institutional resource collections in herpetology and ichthyology: an online reference. Version 6.5. American Society of Ichthyologists and Herpetologists. Available from: http://www.asih.org/ (accessed 3 June 2019)
Schield, D.R., Card, D.C., Adams, R.H., Jezkova, T., Reyes-Velasco, J., Proctor, F.N., Spencer, C.L., Herrmann, H.W., Mackessy, S.P. & Castoe, T.A. (2015) Incipient speciation with biased gene flow between two lineages of the Western Diamondback Rattlesnake (Crotalus atrox). Molecular Phylogenetics and Evolution, 83, 213–223.
https://doi.org/10.1016/j.ympev.2014.12.006
Schield, D.R., Adams, R.H., Card, D.C., Corbin, A.B., Jezkova, T., Hales, N.R., Meik, J.M., Perry, B.W., Spencer, C.L., Smith, L.L., García, G.C., Bouzid, N.M., Strickland, J.L., Parkinson, C.L., Borja, M., Castañeda-Gaytán, G., Bryson, R.W.Jr., Flores-Villela, O.A., Mackessy, S.P. & Castoe, T.A. (2018) Cryptic genetic diversity, population structure, and gene flow in the Mojave rattlesnake (Crotalus scutulatus). Molecular Phylogenetics and Evolution, 127, 669–681.
https://doi.org/10.1016/j.ympev.2018.06.013
Schield, D.R., Perry, B.W., Adams, R.H., Card, D.C., Jezkova, T., Pasquesi, G.I.M., Nikolakis, Z.L., Row, K., Meik, J.M., Smith, C.F., Mackessy, S.P. & Castoe, T.A. (2019) Allopatric divergence and secondary contact with gene flow – a recurring theme in rattlesnake speciation. Biological Journal of the Linnean Society, 128, 149–169.
https://doi.org/10.1093/biolinnean/blz077
Scrucca, L., Fop, M., Murphy, T.B. & Raftery, A.E. (2016) mClust 5: clustering, classification and density estimation using Gaussian finite mixture models. The R Journal, 8, 205–233.
https://doi.org/10.32614/RJ-2016-021
Strickland, J.L., Mason, A.J., Rokyta, D.R. & Parkinson, C.L. (2018a) Phenotypic variation in Mojave rattlesnake (Crotalus scutulatus) venom is driven by four toxin families. Toxins, 10 (4), 1–23.
https://doi.org/10.3390/toxins10040135
Strickland, J.L., Smith, C.F., Mason, A.J., Schield, D.R., Borja, M., Castañeda-Gaytán, G., Spencer, C.L., Smith, L.L., Trápanga, A., Bouzid, N.M., Campillo-García, G., Flores-Villela, O.A., Antonio-Rangel, D., Mackessy, S.P., Castoe, T.A., Rokyta, D.R. & Parkinson, C.L. (2018b) Evidence for divergent patterns of local selection driving venom variation in Mojave Rattlesnakes (Crotalus scutulatus). Scientific Reports, 8, 1–15.
https://doi.org/10.1038/s41598-018-35810-9
Sullivan, J., Demboski, J.R., Bell, K.C., Hird, S., Sarver, B., Reid, N. & Good, J.M. (2014) Divergence with gene flow within the recent chipmunk radiation (Tamias). Heredity, 113 (3), 185–194.
https://doi.org/10.1038/hdy.2014.27
Templeton, A.R. (2006) Population Genetics and Microevolutionary Theory. John Wiley and Sons, Hoboken, 718 pp.
https://doi.org/10.1002/0470047356
Venables, W.N. & Ripley, B.D. (2002) Modern Applied Statistics with S 4th edition. Springer, New York, 498 pp.
https://doi.org/10.1007/978-0-387-21706-2
Wood, D.A., Meik, J.M., Holycross, A.T., Fisher, R.N. & Vandergast, A.G. (2008) Molecular and phenotypic diversity in Chionactis occipitalis (Western shovel-nosed snake), with emphasis on the status of C. o. klauberi (Tucson shovel-nosed snake). Conservation Genetics, 9 (6), 1489–1507.
https://doi.org/10.1007/s10592-007-9482-0
Wood, D.A., Vandergast, A.G., Barr, K.R., Inman, R.D., Esque, T.C., Nussear, K.E. & Fisher, R.N. (2013) Comparative phylogeography reveals deep lineages and regional evolutionary hotspots in the Mojave and Sonoran Deserts. Diversity and Distributions, 19, 722–737.
https://doi.org/10.1111/ddi.12022
Wu, C.I. & Ting, C.T. (2004) Genes and speciation. Nature, 5, 114–122.
https://doi.org/10.1038/nrg1269
Zancolli, G., Calvete, J.J., Cardwell, M.D., Greene, H.W., Hayes, W.K., Hegarty, M.J., Herrmann, H.W., Holycross, A.T., Lannutti, D.I., Mulley, J.F., Sanz, L., Travis, Z.D., Whorley, J.R., Wüster, C.E. & Wüster, W. (2019) When one phenotype is not enough: divergent evolutionary trajectories govern venom variation in a widespread rattlesnake species. Proceedings of the Royal Society B: Biological Sciences, 286, 20182735.