In comparative biology, much hinges on the radiation chosen for the study. Lizards, besides being relatively typical vertebrates in most aspects of their genetics and evolution, are ideal for comparative studies. They offer large, taxonomically well known, and evolutionarily diverse lineages; and all levels from molecular to populational of their genetic systems may be easily studied in natural populations. In no other vertebrate group can such a variety of levels of organization be so easily sampled.
In lizard radiations large enough to be suited for the kind of comparative analysis outlined above, studies of systematics and a diversity of biological parameters involved in genetic systems are most complete in the family Iguanidae. The phylogeny of the family is well understood and it offers a variety of closely related lineages demonstrating remarkably different patterns of evolution and speciation (Paull, et al. 1976). It is large enough that most of these patterns are replicated several times. In the absence of a general review, I mention a few of the studies at various levels of genetic system organization to document the kind of information available: Work at the molecular level is represented by Gorman and Kim (1976), Gorman, et al. (1971), Hall and Selander (1973), Soule and Yang (1973), Tinkle and Selander (1973), Webster and Burns (1973) and Webster, et al. (1973). Cytological level studies are represented by my own work (Hall, 1973; Hall and Selander, 1973; Webster, et al., 1973; and Paull, et al., 1976) and a diversity of papers summarized in Gorman's (1973) review. Andrews and Rand (1974), Crews (1975), Cuellar (1966), and Jones, et al. (1976) are typical of information available on iguanid reproductive biology. Behavioral and population level works relevant to iguanid genetic systems are represented by Blair (I960), Evans (1961), Philobosian (1975), Rand (1967), and Tinkle (1967).
When these data for the Iguanidae are examined from the comparative approach we see the same relationships discussed above: an association of interspecific chromosomal variation, subdivided populations, rapid speciation, and rapid phyletic evolution vs an association of chromosomal and evolutionary conservatism (Hall, 1973; Paull, et al., 1976). Although major review papers are anticipated, I will summarize my work with the sceloporine branch of the family (Hall, 1973; Hall and Selander, 1973) to document the relationships between specific evolutionary and genetic system variables and to provide a basis for formulating specific predictive models for the processes involved in the non-allopatric speciation seen in the sceloporines.
As presently defined, the sceloporines include 9 genera (Savage, 1958; Etheridge, 1964; Presch, 1969). Eight of these 9 genera, totaling 44 generally well differentiated species, plus another 5 or 6 species in the closely allied Crotaphytus and Gambelia (Montanucci, et al., 1975) form a conservatively evolving basal radiation; while Sceloporus itself, the phylogenetically most recent branch of the radiation, contains by my count something on the order of 73 species, many of them relatively cryptic and still unnamed (Hall, 1973; Paull, et al., 1976). In terms of its utility for comparative studies, note that this sceloporine radiation alone, with ±122 species, is larger than most mammalian families, and even many orders. Over half of the species in each of these 11 genera have been karyotyped (Gorman, 1973; Paull, et al., 1976). All examined species in the 8 conservative sceloporine genera have identical 2n=34 karyotype, close to the 2n=36 pattern of Crotaphytus and Gambelia, which is probably primitive for all lizards (Gorman, 1973; Paull, et al., 1976); while no more than 20 of the ±73 Sceloporus are likely to have the 2n=34 pattern. Five of these 20 species have not been karyotyped and 2 of the 15 that have been differ in non-numerical ways from the ancestral pattern: S. pyrocephalus differs by a conspicuous pericentric inversion of its largest chromosome pair (Cole, 1971b; Hall, 1973); and S. maculosus is reported by Cole (1971a) to have a 2n=33X X-Y male, 2n=34 female (based on a total sample of 3 individuals), while Carol Axtell (pers. comm.) found one individual from near the type locality for the species to have a 2n of about 40, which I confirm from her slides. On the other hand, more than 50 species of Sceloporus are known or expected to deviate from the primitive 2n=34 pattern of the other sceloporines by at least two rearrangements, almost all of which are centric fissions or fusions (Table 1).
Smith (1939) separated Sceloporus into two major divisions: a small-sized, small-scaled branch and a large-sized, large-scaled branch; each of which contains several species groups. Table 1 lists the currently recognized species according to Smith's scheme and their karyotypes. Following my systematic interpretations (Hall, 1973; in prep.), summarized in Fig. 1, the small-scaled branch includes 18 species and is phylogenetically older (Purdue and Carpenter, 1972a, 1971b; Larsen and Tanner, 1974, 1975);2 where the more recent, large-scaled branch has about 55 species. As shown in Fig. 1, the small-scaled Sceloporus include two independent sequences of karyotypic derivation away from the primitive 2n=34; while the large-scaled radiation includes 3 major lines deriving away from an extinct, 2n=32 common ancestry. A brief summary of my phylogenetic interpretations of the 5 lineages follows:
In the small-scaled radiation (Fig. 1), what is probably the oldest sequence of fissioning leads to the 2n=46 S. merriami Fig. 1A, in which all macrochromosomes have fissioned (all subspecies of Olson 1973) have been sampled: Cole, 1971a, Hall, 1973). Based on my own work and the published record, there are no species with plausibly intermediate karyotypes, at least in a phylogenetic sense; although Cole (1971a) did suggest that the 2n=46 pattern might be primitive in Sceloporus, and that the 2n=40 patterns found in clarki and melanorhinus might be early intermediates in a sequence of derivation to lower numbers. Of course this assumption is untenable given morphological relationships and the data on karyology from the rest of the genus and family (Paull, et al., 1976). On the other hand, it is at least possible that the 2n±40 maculosus (C. Axtell, pers, comm.) might represent an intermediate; although even with the limited evidence available, there are already some biogeographic and/or taxonomic inconsistencies. In any event, merriami is phylogenetically one of the most primitive and distinctly separated species in Sceloporus (Purdue and Carpenter, 1972a, 1972b; Larsen and Tanner, 1974, 1975; Hall, in prep.). However, in spite of the phylogenetic isolation of merriami, it is quite successful in its highly specialized niche on the face of rock cliffs. It occurs in most areas of major rock outcropping along the midline of the Mexican Plateau from the Big Bend area of Texas south to a limit at the sands of the Bolson de Mapimi in southern Chihuahua and Coahuila.
The second sequence of derivation away from the primitive sceloporine karyotype seen in the small-scaled Sceloporus is shown by scalaris and aeneus, both of which have 2n=24 karyotypes, which presumably evolved by microchromosomal centric fusions (11 pairs have been reduced to 6 - Fig. 1B). Karyotypes have been lightly sampled from throughout the range of the radiation (Lowe, et al., 1966; Hall, 1973, unpub.). These two species are closely related to the karyotypically unknown S. goldmani, which now appears to be extinct at all known localities due to habitat degredation and/or competition with scalaris (Smith and Hall, 1974; Thomas and Dixon, 1976; Hall, pers. obs.3). As is the case for merriami, except for the possibility of goldmani, there are no plausible karyotypic intermediates between the 2n=34 pattern and the 2n= 24 pattern of scalaris and aeneus. Although goldmani, scalaris, and aeneus probably trace their ancestry from a more modern small-scaled species than does merriami, these 3 species form a tight group which does not seem closely related to any other Sceloporus (Larsen and Tanner , 1974, 1975; Thomas and Dixon, 1976; Hall, unpub.). Ecologically scalaris and aeneus are quite successful in their highly specialized nich as commensals with bunch- and other tall grasses. Scalaris has even managed to shift to cultivated grasses in some areas, and may be extending its range along with agricultural practices (Thomas and Dixon, 1976). Geographically scalaris and aeneus range over mesic (grassy and forested grassy) areas of the Mexican Plateau from the mountains of southern Arizona and New Mexico to the southern slopes of the Sierra Volcanica Transversal.
In the large-scaled branch of Sceloporus, despite interpretations of Larsen and Tanner (1974, 1975--see note 2), I find all of the species to be quite closely related and to show clear patterns of phylogenetic derivation which are in some cases quite different from what they [Larsen and Tanner] have suggested. Except for a compact group of 4 species apparently separated by the opening of the southern part of the Gulf of California (nelsoni, pyrocephalus, [orcutti] licki, and a still undescribed sympatric sibling of licki) and the somewhat more distantly related orcutti proper (Hall, 1973, in prep.), all of the large-scale species which have been karyotyped by Cole or myself (Table 1) are chromosomally derived. Morphological and biogeographic relationships are consistent with the idea that all derived species trace their ancestry from a now extinct 2n=32 progenitor which had a Pliocene or early Pleistocene distribution in xeric areas around the upper end of the Gulf of California (Hall, 1973, in prep.).
Two of the three major lineages deriving from this ancestral 2n=32 species evolved from a population which must have been polymorphic for an enlargement4 of one of the microchromosomes ("Em"--Fig. 2, Fig 4): the two 2n=40 species, clarki and melanorhinus form one lineage and the second has given rise to a large group of crevice-using species which still have 2n=32 females, but have male karyotypes of 2n=31X1X2Y. Both of these lineages either retain the Em chromosome as a polymorphism (clarki and melanorhinus--Cole, 1970; Hall, 1973) or have fixed it in some species and lost it from others (the crevice-users--Hall, 1973--Fig. 5, Fig. 6). The third lineage (Fig. 3) offers no such clues to the details of its heritage.
Sceloporus clarki and melanorhinus are at the end of one lineage deriving from the 2n=32, polymorphic Em population and retain the Em mutation as a polymorphism in widely distributed populations (Cole 1970 and Hall 1973 have karyotyped all subspecies in this radiation except for the recently described clarki uriquensis). The evolution of the clarki karyctype required the fixation of four macrochromosomal fissions, and melanorhinus clearly derives from clarki by fusion of one of the acrocentric fission products to the large microchromosomal Y chromosome to form an X1X2Y sex chromosomal system in the males (Hall, 1973--Fig. 5, Fig 6). Cole (1970) noted the heteromorphism in male S. melanorhinus, but did not offer an interpretation for it because he lacked the meiotic material that makes the interpretation clear. In this lineage there are no surviving intermediates between the extinct 2n=32, polymorphic Em population and clarki.
Ecologically clarki dominates the tree trunk niche and ranges onto rocky areas which may also be dominated locally depending on congeneric competition. In the northern part of its range melanorhinus dominates tree trunks; while in the south, competition with the 2n=22 horridus and edwardtaylori seems to have pushed it higher up in the trees.
Geographically the two species form a continuous population from southern New Mexico and Arizona southward along the Pacific slopes of Mexico to the Isthmus of Tehuantepec, with a disjunct population in the central lowlands of Chiapas and adjacent Guatemala. The species meet in what is probably a parapatric contact at or near the Rio Grande de Santiago in Nayarit. I have no information on the details of this contact.
The chromosomally conservative crevice users. The other lineage deriving from the 2n=32, polymorphic Em population is a group of live-bearing, generally chromosomally conservative "crevice users," which exclusively use crevices and cracks in rocks or wood for escape and sleeping cover rather than showing a preference for burrows or shimmy burial (Axtell, 1956) in loose sand, as do all other sceloporines (Hall, 1973--Fig. 2). A All recognized species in this radiation have 2n=32♀,31♂ karyotypes, which are presumably identical to that of the 2n=32 common ancestor except for the formation of a microchromosomal X1X2Y sex chromosome system in the males. Only grammicus shows any intraspecific chromosomal variation, but this is quite spectacular: six karyotypically distinctive derived populations are included along with the conservative population in this one morphological species (Fig. 7). This situation will be discussed in more detail below.
In the early evolution of the crevice using radiation (Fig. 2), it appears that the primitive stock was pushed to high elevations in the mountains of the Mexican Pacific Slope, until it adapted to these montane and steep slope conditions with the evolution of ovoviviparity and specialization for the use of crevices in an environment where loose soil suitable for burrowing would be at a premium. Early in this adaptation, the stock split into two main groups: one primarily specialized to use rock crevices and the other primarily specialized to use wood crevices. Four species groups (basically following Smith, 1939) can be recognized in this radiation: asper, with one still very generalized tree-trunk species which is fixed for the Em chromosome (see Fig. 4, Fig 5, Fig 6), which may be a conservative pre-crevice using derivative of this lineage (see Table 1, footnote 12) ; the torquatus group of rock crevice users (11 species); and two groups of "wood" crevice users. The wood crevice users include the grammicus group, with three recognized species (see Table 1, footnote 13), which use cracks and crevices in "trees" and logs; and megalepidurus, with only one species (see Table 1, footnotes 15 and 16), which is fixed for the Em chromosome and which uses the crevices between the woody dry leaves of Yucca and Agave, a habitat also frequented by grammicus where megalepidurus is absent. It should be noted that torquatus do on occasion also use wood crevices, and that the wood crevice users may also be locally common in rocky areas.
Except for the chromosomal variation within the taxonomic species grammicus and the presence or absence of the Em chromosome, there is no interspecific karyotypic variation between any of the crevice using species. Asper and megalepidurus appear to be old species (probably pre-Pleistocene) which differentiated in opposite ends of the Sierra Volcanica Transversal, and which are now being restricted by competition with more effective crevice users. Although they are now morphologically quite different due to the selective pressures of their rather different habitats, it is possible that their common fixation of the Em chromosome indicates a common ancestry within the crevice using radiation. The richness of species in the rock-crevice using torquatus group is quite understandable given their ecological restriction to "islands" of rock outcrops in a "sea" of alluvium; a situation which offers abundant opportunity for allopatric speciation. In actuality there may be a hundred or more allopatric populations which could be distinguished at some taxonomic level. Only where two or more forms have secondarily come into sympatry is it clear where the species definitions are valid biologically. On most areas of the Mexican Plateau there are 2 rock crevice users living syntopically (a large and a small species), and in a few areas 3 may be found. Geographically the torquatus group is found above 1000 meters in the west (where congeneric competition is severe) and sea-level in the east from Arizona, New Mexico, and Texas south to Yucatan and Guatemala. Formation of the three morphological species in the grammicus group seems to be associated with the Rio Lerma-Lake Chapala-Rio Grande de Santiago drainage, which forms a major physiographic and climatic barrier between the Sierra Madre Occidental and the Sierra Volcanica Transversal to separate heterolepis and shannorum; and Pleistocene glaciation and climates, which probably resulted in a barrier across the crest of the Sierra Madre Occidental to separate shannorum. from from grammicus.
When the proto-grammicus once crossed the crest of the Sierra Madre Occidental, the chromosomally conservative "S" [Standard] population was able to spread continuously across the Mexican Plateau: north to the Rio Grande Valley, south to the Isthmus of Tehuantepec (S populations are found in these areas today-see distribution maps in Hall and Selander, 1973), and probably down to low elevations on the Gulf of Mexico coast. Since no other lizard so successfully exploits the wood crevice niche, this species is found everywhere within this range where large "woody" plants may be found. Climatically S. grammicus range from Upper Chihuahuan Desert, where they live in Yucca, Agave, and Opuntia, to mountain rain forest, where they use crevices in trees, stumps, and fallen logs. In areas where these climatic extremes are closely juxtaposed, such as in Oaxaca, Veracruz, and Nuevo Leon, S. grammicus populations range continuously between them, with no indication of barriers to gene flow.
The second major sequence of chromosomal derivation in the large-scaled Sceloporus is contained entirely within the morphological species grammicus (Fig 3 D). Based on karyotypes from about 1300 individuals (Hall and Selander, 1973; Hall, 1973, unpub.), there are 6 karyotypically distinctive populations besides the S. population (Fig. 7). Four of the six form one linear sequence of karyotypic derivation which appears to have begun possibly in the last Pleistocene pluvial.
The first derived population, "F6," became fixed for a fission of chromosome pair 6 (see Fig. 7 C) and spread through the most humid montane forests from the western parts of the Sierra Volcanica Transversal, at least to the Valley of Mexico, and then northward from there along the Sierra Madre Oriental to its northern limits in Nuevo Leon at the Lower Rio Grande Valley. Where in this range the mutation originated cannot be determined at this late date. In the much drier conditions of the present, the Sierra Madre Oriental populations are restricted to relict areas of mesic woodlands.
The next derivation in the sequence, "F5+6," involved fixation of a fission of pair 5 in an F6 population (Fig. 7 E) . Based on the present distribution of F5+6 populatons, I think that this population originated somewhere in the Sierra Madre Oriental of San Luis Potosi, and spread outward from there into climatically intermediate areas between the humid forests occupied by F6 and the deserts occupied by S. Presently F5+6 is found in oaks or in large acacia or mesquite type trees on the coastal plane north to the Rio Grande delta; and on the Plateau, it has spread to the southwest into northern parts of the states of Guanajuato, Queretaro, and Hidalgo, where it is most frequently associated with usually cultivated tree-like Opuntia.
Deriving from F5+6, the "FMl" population is found in central Hidalgo and extreme northern Mexico [the state], where it appears to follow rain shadows southward from the F5+6 distribution. Based on karyotypes from only 10 individuals, FMl is fixed for fissions of macrochronosome pairs 2,4,5, and 6, and polymorphic for fissions of pairs 1 and 3 (Fig. 7 F).
Based on karyotypes from 110 individuals, "FM2" is fixed for fissions of pairs 2, 4,5,6, and 15 (a microchromosome), it may be fixed for a fission of pair 1,5 and it is polymorphic for a fission of 3 (the fission has a much higher frequency than in FMl - Fig. 7 G). Individuals with the FM2 karyotypes were collected in an area about 55 x 20 km in the dry northeastern quadrant of the Valley of Mexico, where it lives in edificarian [buildings and houses] habitats and on cultivated Agave and Opuntia.
The "F5" population, fixed for a fission of pair 5 (Fig. 7 D), is found in the northern parts of the Sierra Madre Occidental of Chihuahua and derived independently from the standard grammicus. It is separated from F5+6 populations by S populations and the uninhabitable barrier of the sandy deserts of the Bolson de Mapimi.
The last grammicus population, "P1," is polymorphic for a fission of pair 1 (Fig. 7 B) in an otherwise standard karyotype. P1's range is limited to the high mountains of the east side of the Valley of Mexico. Although it has been studied intensively, its derivation is unclear. Chromosomally it should derive directly from an S ancestry. However, morphology and biogeography both suggest that it more probably derives from F6 by a back-mutation (the re-fusion of the acrocentric elements of the original pair 6).
Geographical and ecological relationships among the karyotypically differentiated populations of grammicus are interesting. Excepting areas which have neither trees nor the large woody succulents (Agave, Yucca and Opuntia), there seem to be no geographic barriers to migration of grammicus populations; and within the area occupied by a given chromosomal form, the lizards can be found over whatever climaatic range it offers, so long as "wood" crevices are reasonably abundant. Hence, it is almost certain that all of the different chromosomal types come into ecological and behavioral contact where their ranges meet. In several northern areas I have located contacts to within 50 km (S and F5+6, S and F6, F6 and F5+6). Similarly, F6+6 and FM1, and FM1 and FM2 collections are separated by no more than about 70 km. The intervening areas between all of these species pairs are undoubtedly inhabited by grammicus, but there simply has not been the opportunity to sample them to determine what the contact relationships are between the populations.
In the Valley of Mexico I have precisely located contacts between S and F6, F6 and Pi, and S and FM2. In all of these cases narrow zones of hybridization are found (Hall and Selander, 1973; Hall, Stamm, and Reichlin, unpub), but in no case where detailed transects have been run (S x FM2 and Pi x F6) are the hybrid zones more than 200-400 m wide (a not unreasonable dispersal distance for a single individual); and in the case of the P1 X F6 contact, the zone is clearly still less than the 1 km which separates the population of "pure" individuals of the two types.
In the P1 x F6 contact (Hall and Selander, 1973), hybridization is free with no indication of premating isolation, even though there are morphological and behavioral differences which could presumably be shaped by selection into premating isolating mechanisms (Moody, 1971). Hybrids are fairly fertile and backcross freely with both parental types. Statistical analyses of the genotype distributions of backcross individuals (Hall and Selander, 1973) suggest that some backcross phenotypes may survive poorly, but even here there is no direct evidence of a complete block to introgressive gene flow. On the other hand, the evidence from 2 chromosomal and 3 electrophoretic markers, genetic distances, and average heterozygosities is equally clear that there is no introgression beyond the 200-400 meter wide hybrid zone itself (except for one probable F1 hybrid which crossed a highway--probably through a culvert-to reach the midst of an otherwise pure F6 population).
In the S x FM2 contact which was studied in the ancient metropolis of Teotihuacan (Millon, 1970--Millon provided 1:6000 scale base maps for this study), the five chromosomal markers fixed between the populations provided a clear picture of the hybridization. F1 hybrids were infrequent, and only 3 to 7 backcross karyptypes were found (see footnote 5), and one of these was a triploid backcross. Whether the low level of hybridization here in comparison to that in the F6 x PI hybrid zone indicates some degree of premating isolation and hybrid infertility and/or backcross inviability, or the constraints of migration along rectilinear grid-lines formed by trees and weed succulents growing along ancient walls is unclear.
Unfortunately the contact relationships between F5+6, FM1, and FM2 are unknown. That FM1 is a population in its own right, rather than an "intergrade" between F5+6 and FM2 is suggested by the fact that two FM1 populations separated by approximately 40 km on a N-S line show the same polymorphisms and still differ from FM2, probably by the fission of pair 1 (fixed in FM2 and about 50% frequency in FM1) and definitely by the fission of microchromosomal pair 14. The closest F5+6 and FM1 karyotype samples are separated by about 70 km, as are the closest FM1 and FM2 karyotypes, so nothing can be said about the conditions of either contact. However, in developing a theoretical model for the contact zone interactions it will be assumed that the narrow Pi x F6, S x F6, and S x FM2 contacts are typical for the parapatric contacts throughout the entire grammicus radiation.
To summarize the events in the grammicus radiation, I believe that the S to FM2 sequence began in the last Pleistocene pluvial period and may have terminated in the FM1 and FM2 populations after native Americans began their cultivation of Agave and Opuntia in the area of the Valley of Mexico. Neither the F5 nor the P1 derivations can be dated from the present evidence, although I have no data to indicate that P1 and F6 would have ever been out of geographic contact since P1 formed as a discrete entity (Hall and Selander, 1973). Two scenarios are plausible for the origin of P1:
The last sequence of chromosomal derivation in the large-scaled Sceloporus (Fig. 3) seems to have occurred in or near the deserts at the head of the Gulf of California. Judging by the patterns of speciation in and the present distributions of its most highly derived products, the chromosomal derivation must have been completed by late Pliocene or early Pleistocene. As mentioned previously the 2n=32 ancestral stock common to all 3 large-scaled radiations presumably had its distribution in this area also. Two 2n=30 stocks survive: the magister subspecies from zosteromus to rufidorsum of the Baja California Peninsula (Hall, 1973, in prep.), and graciosus, which is a montane species in the northern part of the Peninsula and Southern California that spreads to lower elevations and generalizes as it loses competitors to the north.
Geographically graciosus ranges northward almost to the Canadian Border and from the Pacific Coast to the Continental Divide, with disjunct populations on the sand dunes of eastern New Mexico and West Texas. Sceloporus graciosus is clearly a member of the large-scaled division, but beyond that its relationships with any of the other large-scaled species are unclear. If the two 2n=30 stocks derive from a single mutational event, they have been separated for a long time relative to many other species pairs in the sequence of derivation. This is, of course, consistent with the supposed overall time scale of the derivation and the amount of differentiation seen within each 2n=30 stock.
Sceloporus magister proper, centered in the extreme deserts of the southwestern US and northwestern Mexico, with a 2n=26, is not an exact intermediate in the derivation, because it differs karyotypically from the hypothetical line by a conspicuous inversion of pair 1 (Lowe, et al., 1967; Cole, 1970; Hall, 1973), which might have caused problems in meiosis of heterozygous individuals. This species burrows for cover and uses anything from "trees" and rocks to aeolian sand dunes for perches.
The karyotypically terminal species in this last sequence of derivation all have 2n=22 karyotypes. To derive the 5 pairs of "microchromosomes of these species from the 2n=34 ancestor, which has 11 pairs of acrocentric microchromosomes (at least the large-scaled orcutti has this karyotype) requires more than just centric fusions, especially since at least one microchromosome pair in many of the 2n=22 species is definitely acrocentric (Cole, 1970, 1972; Hall, 1973). In any event this 22 chromosome stock forms the most successful and dominant lizard radiation on the North American continent. Its mostly parapatric species range from western Panama to the northern limits of lizard distribution, and include two groups of species: a northern oviparous group I term the horridus group (Smith and Taylor's 1950 undulatus group plus the 2n=22 species from their spinosus group), which reaches the Isthmus of Tehuantepec, with a disjunct species in northern Yucatan; and a southern, originally montane group of ovoviparous species (essentially Smith and Taylor's 1950 formosus group with the modifications suggested in Table 1), which extends from the eastern Sierra Volcanica Transversal south to Panama. North of the Isthmus, the formosus species remain montane lizards, but south of the Isthmus, they extend to sea level.
Major sympatry of species in this 2n=22 radiation is relatively uncommon, at least in its northern representatives belonging to my horridus grouping. Smith's (1938) undulatus group, represented by undulatus, and the 2n=22 section of his spinosus group, represented by olivaceus are closely sympatric over much of Texas; but on the other hand, I have evidence of probable intergradiation between olivaceus and cautus (a close relative of undulatus) in southern Nuevo Leon (Hall, unpub.). The only other major sympatry in the horridus group involves horridus and spinosus. I have found these to be syntopically sympatric in areas of the Sierra Madre del Sur of Guerrero, where I could see no obvious ecological differences between them.
The situation within the formosus group is greatly confused by the many taxonomic uncertainties and newly described cryptic sibling species (see footnotes 9, 10, 11 and 12 to Table 1). It appears that the formosus radiation is still actively speciating at the present, but far too little is known about the cytogenetics, biology, or distribution of any of the species to warrent speculation about what is happening. The ranges of the horridus and formosus groups overlap broadly north of the Isthmus of Tehuantepec, but they are usually separated altitudinally and/or climatically; with formosus species being found in higher, cooler and more humid areas. The overall pattern of this distribution strongly suggests spread of one ancestral stock outward from the xeric areas of the Gulf of California drainage, which was broken up by the major climatic fluctuations of the Pleistocene to form the present species in the north. Similarly, cool humid periods of the Pleistocene probably also selectively favored the evolution of ovovivipaity in the formosus stock, which is still rapidly evolving and speciating in a new adaptive zone.6
Most species of the horridus group have parapatric or allopatric distributions. In Mexico, no particular effort has been made to accurately plot the ranges of the species, so the details of their contact relationships are unclear. Data for the US are much better. The two most widely distributed species, occidentalis and undulatus are largely, if not completely allopatric. The ranges of virgatus and undulatus are closely juxtaposed and interdigitated over large geographic areas of southwestern New Mexico, southeastern Arizona, and the adjacent mountains to the south, but their mutual restrictions to different types of habitats seem to keep their populations separated by uninhabited areas several miles wide (Cole, 1963).
Much more interestingly, in Florida, woodi and undulatus meet in narrow zones of parapatric hybridization very similar to those described above for grammicus (Jackson, 1973a). Although my interpretation differs slightly from his, according to data presented by Jackson (1973b), the proto-woodi may have been isolated geographically on islands of the central Florida peninsula during late Pliocene and Pleistocene periods of high sea level. Presumably this would have provided enough time for then to differentiate genetically from the proto-undulatus stock. In any event, where the species presently hybridize and the hybrid zone has not been overly confused by human interference it appears to be no more than 1/4 mile wide (or about 400 meters, as observed in grammicus) (Jackson, 1973b). Although no genic markers were used, phonetic evidence from morphological characters of the woodi x undulatus hybrids strongly suggests that the hybrids are fertile and backcross successfully; but similarly to the cases in grammicus, there is no evidence for introgression of genes outside of the narrow hybrid zone. Jackson (1973a) suggests that this situation has persisted for at least 100,000 years (or at least 50,000 generations) without any evidence for the evolution of premating isolating mechanisms, except for the use of different habitats by the two species; which does not prevent them from hybridizing where the habitats adjoin. As is the case in the hybridization of P1 with F6 in the grammicus complex, populations of woodi are completely surrounded by undulatus, in such a way that appreciable fractions of the woodi populations must risk hybridization in any generation. As in grammicus, the failure to evolve effective means of preventing gametic wastage in the formation of unfit hybrids seems to be most paradoxical.
These [chromosomally derived] radiations in Sceloporus, contrasted with more conservatively evolving radiations in Sceloporus and in the other sceloporines, suggest several correlations between genetic system parameters and speciation which can be used to constrain attempts to model them.