|Core Manuscript: Comparative Population Cytogenetics, Speciation and Evolution of the Crevice Using Species of Sceloporus (Sauria, Iguanidae)|
|Manuscript: Hybridization of Karyotypically Differentiated Populations In the Sceloporus grammicus complex (Iguanldae). [with R.K. Selander] Evolution 27 (In press),||17 pages|
|Manuscript: Three Probable Cases of Parthenogenesis In Lizards (Agamidae, Chamaeleontidae, Gekkonidae), Experientia 26: 1271-1273.||3 pages|
|Miscellaneous Material||24 pages|
The two major and one minor manuscripts which are the body of this thesis all deal with the cytogenetics of speciation in lizards. Evolution has two major aspects: change through time and the origin of new species from old through the development of reproductive isolation. Of the two aspects, the mechanisms of speciation are the less well understood. With regard to speciation, the greatest debates have concerned the relative roles that allopatric separation and intrinsic changes in the species' genetic systems have played in the development of reproductive isolation (see Mayr, 1963, 1970; Dobzhansky, 1970; Stebbins, 1950; Goldschmidt, 1955; Lewis, 1973; White, in press, etc., for discussions of these issues). Unfortunately, speciation is a process which is not readily susceptible to study in the laboratory, and probably the most effective way to understand the roles of the genetic system in speciation is through the comparative study of speciation in suitable natural radiations which show important differences between their branches in aspects of their genetic system. Unfortunately, few such studies have been made; and, in fact, few radiations are well enough known systematically or cytogenetically to be suitable for such studies.
However, as will be outlined below, I had the good fortune to be introduced to the sceloporine radiation of the lizard family Iguanidae, which provides an exceptionally ideal system within which intrinsic aspects of speciation may be studied by the comparative approach. This informally designated subfamily (Savage, 1958; Etheridge, 1964; Presch, 1969) contains nine genera and is undoubtedly the systematically best known radiation of comparable size in all of the reptilia (Smith, 1936, 1939, etc.; Savage, 1958; Norris, 1958; Axtell, 1958; Etheridge, 1964; Presch, 1969; Ballinger and Tinkle, 1972). As can be determined from the cited monographs and systematic treatments, the radiation of the sceloporines is contemporaneous with the evolution of the North American deserts and is therefore comparatively recent. And, although this is, of course, difficult to prove, this radiation seems remarkably intact in that there seem to be few gaps to be accounted for by the extinctions of major lineages. Also, as will be documented in the two major manuscripts of this thesis, different branches of the sceloporine radiation show striking differences in their adaptive and evolutionary plasticities, as measured by their comparative abilities to proliferate species. These differences are closely correlated with equally striking differences in the genetic systems of the different branches.
On the other hand, the sceloporine radiation as a whole, or even just that of Sceloporus, is far too vast to be encompassed in one thesis; so here I present only the analysis of one radiation within Sceloporus, that of the crevice-using species, which still allows a fruitful comparison to be made between a cytogenetically conservative line speciating allopatrically (the torquatus group) and a karyotypically diverse line which has speciated extensively without any obvious allopatric isolation (the grammicus complex).
The manuscripts included in this thesis are presented in the reverse order to which they were written, because only in the last written has the nature of the speciation been well enough understood to be adequately described as a theory to be tested. The theory is then given first in the thesis and followed by the studies required to test it. The third manuscript is trivial by comparison, but I include it since it provides examples of instantaneous speciation in lizards through parthenogenesis and polyploidy. I then conclude the thesis with a short summary of work I have in progress (along with relevant illustrations and tables which I have completed) which has materially contributed to the principal manuscripts.
Before I present the body of the thesis, I wish to review the history of my involvement with speciation in the sceloporines, because this provides the most appropriate way of acknowledging the many people who have contributed so much to the inception and progress of the study.
The central problem was first outlined while I was an undergraduate at San Diego State College (now California State University, San Diego). As a fugitive from a disastrous attempt to major in physics at Occidental College and UCLA, Richard Etheridge introduced me to the studies of comparative biology, evolution, herpetology, Sceloporus, and Don Hunsaker, more or less in that order. Hunsaker then introduced me to the deserts and much more fully to the local species of Sceloporus; and it was Hunsaker that convinced me that far more interesting work could be done with these common, attractive, and easy-to-study lizards than could be done with intertidal fish living in the cold, wet Japanese Current, Hunsaker was also the first to ask what special feature(s) of their biology had enabled the Sceloporus to become the dominant and by far most speciose lizard genus of North America. However, his greatest contribution to the study was when he suggested that I try the new tissue culture procedures for chromosome preparation on Sceloporus as my cytogenetics course project. The project was a total failure, at least in terms of providing original results, but I was able to compile enough evidence on iguanid karyology (Cavazos, 1951; Hunsaker, unpub. data; Matthey, 1931; Painter, 1921; Schroeder, 1962; and Zeff, 1962); which, although much of it was wrong in detail, accurately indicated a concentration of karyotypic variability in Sceloporus as compared to the remaining iguanids. Thanks to the background on the Iguanidae provided by Etheridge and Hunsaker, this immediately suggested the possibility that some form of chromosomal speciation might account for the great evolutionary proliferation of Sceloporus (Hall, 1963, 1965).
On graduation from SDSC in 1964, because of my poor record as a physics major, I was unable to obtain support from them to continue in their graduate program, so I spent the next 15 months working as a field biologist in the Nevada desert at the UCLA Rock Valley Field Station under F. B. Turner, While there, on the Nevada Test Site, I worked with Joseph R. Lannom, Jr., an experienced collector, and joined him on several extensive herpetological collecting trips into the center of the Sceloporus radiation along the west coast of Mexico, which provided the beginnings of my knowledge of the field biologies of many of the species. Joe also showed me where and how to night collect the various sceloporines that sleep buried in the sand. Additionally, while I worked on the Test Site, I was able to maintain in large indoor pens many of the species Lannom and I had collected, as well as several additional species kindly provided by various correspondents. I saw much behavior in these pens that I would not have under other conditions. Finally, while at the Test Site, although I was still using inferior squash techniques, I was able to obtain enough additional information on the karyotypic variability of Sceloporus to further document the karyotypic diversity of Sceloporus relative to the other iguanids, and to verify the validity of the problem.
As a result of my report of these initial findings (Hall, 1965), Ralph W. Axtell invited me to work under him at Southern Illinois University, Edwardsville, and to pioneer their then non-existent graduate program. Since I still had no other options for graduate work because of my undergraduate record, I was glad to take the chance. Axtell taught me much of what I know about biogeography, and with his extensive knowledge of the sceloporine lizards and Mexico, he provided an invaluable sounding board with whom I could argue my developing ideas regarding the evolution of Sceloporus. Also, during the summer of 1966, I was able to accompany him on an extensive collecting trip through the southwestern United States and northwestern Mexico, during which I was able to karyotype almost 150 lizards and successfully developed the karyotyping procedures used for the remainder of the study. Also, while a student at SIU.E, I was able to study cytogenetics and evolution under Hampton Carson at Washington University (St. Louis), which vastly improved my comprehension of comparative cytogenetics.
From the time the nature of the problem became clear, I had been searching for recent cases of karyotypic differentiation or polymorphism in Sceloporus, since analyses of these would provide the clearest tests of the chromosomal speciation idea. Due to an erroneous report of a 2n = 30 for Sceloporus undulatus by Painter (1921), I had expected to find this variation in the undulatus group, and the 1966 field work was largely planned to test this possibility. It confirmed that there was no variation in chromosome number in the undulatus. However, a Christmas, 1966, field trip by Axtell to southern Texas provided some Sceloporus grammicus specimens from Kingsville that proved to be karyotypically quite different from some individuals that I had obtained from El Salto, Durango, while working on the Nevada Test Site. As can be seen in the body of the thesis, the variability of grammicus proved to exceed my wildest expectations. However, this was not confirmed until the summer of 1968, after I had been accepted into the doctoral program at Harvard.
Again, thanks to my undergraduate record, most schools would still not consider supporting my further graduate work. Harvard, as encouraged by Ernest E, Williams and George C. German, was the major exception. My work here has benefited greatly from my contacts with Williams and Ernst Mayr, and from my contacts with the many students working on the other major radiation of the Iguanidae, Anolis. While a student at Harvard, I was able to spend full summers in Mexico during the years of 1968, 1970 and 1971 and to work on the Sceloporus study, and I made other long trips during the summer of 1969 and the spring of 1971. The many people who helped my work during the Harvard years are acknowledged in the appropriate manuscripts.
A final acknowledgment must be made to Hobart M. Smith, whose monographs on Sceloporus provide the foundation without which the present study would have been impossible. Although, as he knows, I will eventually make considerable revisions in species he has defined and in his conceptions of the evolution of the genus, he has provided a constant source of encouragement and moral support for the continuation of the work, a support which began while I was academically isolated on the Nevada Test Site. This has helped to keep me at the study under some rather trying circumstances when I might have otherwise decided to give it up as a bad deal.
Axtell, R. W. 1958. A monographic revision of the iguanid genus Holbrookia. Ph.D. Thesis, University of Texas, Austin, 222 pp.
Ballinger, R. E. and D. W. Tinkle. 1972. Systematics and evolution of the genus Uta (Sauria: Iguanidae). Misc. Publ. Mus. Zool., Univ. Michigan no. 145: 1-83.
Cavazos, L. F. 1951. Spermatogenesis of the horned lizard Phrynosomacomutum. Amer. Natur. 85: 373-379.
Dobzhansky, T. 1970. Genetics of the Evolutionary Process. New York, Columbia Univ. Press. 505 pp.
Etheridge, R. 1964. The skeletal morphology and systematic relationships of sceloporine lizards. Copeia 1964: 610-631.
Goldschmidt, R. B. 1955. Theoretical Genetics. Berkeley, Univ.California Press.
Hall, W. P. 1963. Cytogenetic studies in the family Iguanidae, Unpub. MS, San Diego State College.
Hall, W. P. 1965. Preliminary chromosome studies of some Nevada Test Site lizards. Paper delivered to the annual meeting of the American Society of Ichthyologists and Herpetologists, Lawrence, Kansas.
Lewis, H. 1973. The origin of diploid neospecies in Clarkia, Amer.Natur. 107: 161-170.
Matthey, R. 1931. Chromosomes de reptiles. Sauriens, Ophidiens,Cheloniens, L'evolution de formule chromosomiale chez les Sauriens. Rev. Suisse Zool. 38: 117-186.
Mayr, E. 1963. Animal Species and Evolution. Cambridge, Harvard Univ. Press. 797 pp.
Mayr, 1970. Populations, Species, and Evolution, Cambridge, Harvard Univ. Press. 453 pp.
Norris, K. S. 1958. The evolution and systematics of the iguanid genus Uma and its relation to the evolution of other North American desert reptiles. Bull. Amer. Mus. Nat. Hist. 114: 247-326.
Painter, T. S, 1921. Studies in reptilian spermatogenesis. I. The spermatogenesis of lizards. J. Expt. Zool. 34: 281-327.
Presch, W. 1969. Evolutionary osteology and relationships of the horned lizard genus Phrynosoma (family Iguanidae). Copeia 1969:250-275.
Savage, J. M. 1958. The iguanid lizard genera Urosaurus and Uta, withremarks on related groups. Zoologica 43: 41-54.
Schroeder, G. 1962. Chromosome studies in the genus Sceloporus. Unpub. MS, San Diego State College.
Smith, H. M. 1936 . The lizards of the torquatus group of the genus Sceloporus Wiegmann, 1828. Univ. Kansas Sci. Bull. 24:539-693.
Smith, H. M. 1939, The Mexican and Central American lizards of the genus Sceloporus. Zool. Ser. Field Mus. Nat. Hist. 26: 1-397.
Stebbins, G. L., Jr. 1950. Variation and Evolution in Plants. NewYork, Columbia Univ. Press 643 pp.
White, M.J.D. (in press ). Animal Cytology and Evolution, 3rd Ed. [Cambridge Univ. Press. 961 pp. (1973)]
Zeff, E. W. 1962. A technique for delineation of chromosomal constitution of reptilian leucocytes grown in culture. Unpub. Ms, Univ. California, Los Angeles.