Scope
Sceloporus as a resource for the study of evolution
Karyotype variation in Sceloporus
In this series of reports I will attempt to trace both the evolutionarily important functions of and the evolutionary changes in different aspects of the cytogenetic systems of the various species of the iguanid lizard genus Sceloporus. Concomitantly, it will be necessary to review, and in some cases revise, the accepted species groupings and ideas regarding phylogenetic relationships within Sceloporus (Smith, 1939; Smith and Taylor, 1950). An analysis of karyotypic variation and its probable significance in speciation is central to my phylogenetic treatment, but other sets of characters will be considered when needed. In the present work I examine the radiation of the live-bearing Sceloporus which have specialized for the use of crevices in one form or another for escape and sleeping cover. This radiation includes Smith's (1939) species groups grammicus and torquatus plus megalepidurus and pictus of the megalepidurus group and asper of the formosus group. Also, to help interpret some of the karyotypic differences observed among these crevice-using Sceloporus, I include data for S. clarkii and melanorhinus, a natural group which I believe shares a close common ancestry with the crevice-users.
Sceloporus is the dominant lizard genus of continental North America, and one or more of its species can be found in virtually every non-fossorial lizard habitat from western Panama to the northern limit of lizard distribution. According to present taxonomy (Smith and Taylor, 1950; Smith and Bumzahem, 1953; Webb and Hensley, 1959; Langbartel, 1959; Cole, 1963; Lynch and Smith, 1965; Webb, 1967; Smith and Lynch, 1967; Stuart, 1970, 1971; Dixon et al., 1972) Sceloporus contains 64 species, and my planned revisions will raise this to above 70, which makes Sceloporus one of the more speciose lizard genera in the world. In the Iguanidae, only the tropical Anolis radiation of some 200 species and possibly the comparatively poorly known Liolaemus radiation of southern South America have more species. The magnitude of the radiation of Sceloporus is impressive even without other considerations, but osteology (Etheridge, 1964; Presch, 1969) and biogeography (Savage, 1960, 1966) indicate that it may be one of the more recently differentiated genera within the Iguanidae. Not only has the proliferation of Sceloporus been remarkably extensive, but it appears to have been even more remarkably rapid. Important evolutionary questions implicit in this list of superlatives are: Why and how has Sceloporus evolved so many more species than have been produced by other genera of related and older origins? Might understanding these questions provide a more general insight into problems of species formation in other groups? Although we cannot subject such questions to controlled experiments in the laboratory, subdivisions within Sceloporus and the existence of eight other closely related genera in the sceloporine branch of the family (Savage, 1958; Etheridge, 1964; Presch, 1969) provide a series of natural experiments which may be studied by the comparative approach to determine possible relationships between various biological characteristics of the subdivisions and their present evolutionary successes.
Since the pioneering investigations of T. S. Painter (1921) and Robert Matthey (1931, 1933, 1949), lizards have been comparatively well studied karyotypically; and among them the family Iguanidae is presently best known. This is mainly due to recent work by Gorman, Cole, and Pennock, plus extensive but still unpublished work from the MCZ cytogenetics lab by T. P. Webster, R. B. Stamm, and myself. Gorman (in press) reviews most of the karyotypic work on lizards published through 1971, Table 1 abstracts data from his article plus additional unpublished information from the MCZ cytogenetics lab to summarize what is known about chromosomal variation in lizards.
Of 54 iguanid genera the 29 karyotyped represent all major radiations in the family. Figure 1, based on a tentative phylogeny of the Iguanidae by Etheridge and Estes (unpub.; see also, Etheridge, 1964), summarizes the known karyotypic variability of each genus in the family. In the sceloporine branch, half or more of the species in each of its nine genera have been karyotyped. And within Sceloporus itself, more than 2000 individuals from 56 of the 64 recognized species have been karyotyped, and these karyotyped species represent all 15 of Smith's (1939) species groups. Some interesting conclusions can be drawn from these data.
In general, iguanids are karyotypically conservative. Of the 29 karyotyped genera, 27 have species with either a pattern believed by Gorman (in press; Gorman et al., 1967; Gorman et al., 1969; Bury et al., 1969) and myself to be primitive in lizards; i.e., a 2n=36, 12 meta-centric macrochromosome, 24 microchromosome karyotype, or a 2n=34 pattern which differs from the primitive karyotype only by the absence of a pair of micro chromosomes. Only four genera show striking deviations from the general karyotpic conservatism of the family, with these deviations mainly due to fixation of Robertsonian rearrangements (i.e., centric fusions and/or centric fissions--Hsu and Mead, 1969) between species. These four strikingly variable genera include the three largest in the family: Sceloporus, with 64+ species and 2n's ranging from 22 to 46; Anolis, with 200+ species and 2n's ranging from 26 to 48; and Liolaemus, with 50+ species and 2nls ranging from 32 to 40 in a small sample of species (unpub. data from R. D. Sage). The fourth notably variable genus is Polychrus, with five species and 2n's ranging from 20 to 30 without obvious Robertsonian relationships between the present karyotypes (Gorman, in press; Peccinini, 1969; Becak et al., 1972).
Excepting the three genera with 50+ species, each of which shows remarkable karyotypic diversity, no other iguanid genus contains more than about 15 species (excluding the insular radiation of the Caribbean genus Leiocephalus with 16 species and the approximately eight Tropidurus species in the Galapagos); and, excepting Polychrus and Plica (the latter with two species and a 2n=40 in the one karyotyped), none of the other 24 genera karyotyped show any significant deviation from the 2n=36 or 2n=34 patterns. The concentration of interspecific karyotypic variation in the remarkably speciose genera such as Sceloporus is notable and immediately suggests that there may be some functional relationship between the establishment of these interspecific differences and the^rol iteration of species in these genera. Furthermore, Sceloporus and the related sceloporine genera show more than enough different patterns of speciation and karyotype variation to provide an ideal system of natural experiments which may be compared for studying this possible relationship. Even an analysis of the crevice-using radiation within Sceloporus, which will be described in the present report, will suggest some answers. However, before I turn to this analysis it will be appropriate to review, and in part to develop theory regarding the roles interspecific chromosomal differences may have played in species formation.