Karyotypic phylogeny
Alternative phylogenies
the Smith phylogeny
problems with the Smith phylogeny
the Larsen phylogeny
problems with the Larsen phylogeny
Reconciliation and phyletic synthesis
the biology of crevice using
reproductive biology
biogeography
synthesis
The karyotypic results suggest certain phyletic relationships among the species treated in the present work. These are summarized by Fig. 11. Of all these species, S. clarkii has the most primitive micro- chromosomal pattern. S. clarkii and melanorhinus are clearly more closely related to one another than either is to any other Sceloporus, as evidenced by their common possession of the same four macrochromosomal fissions (i.e., FIS-1, FIS-3, FIS-4, FIS-5), which appear to have been fixed independently of fissions in any other Sceloporus. Based on their karyotypic similarities, these two species form a natural group (i.e., they have a common ancestry, and the group includes all of the species deriving from this common ancestry), which is hereby designated the clarkii group. Subsequent to the fixation of the fissions in the primitive clarkii group stock, a derived sex chromosomal condition became fixed in melanorhinus (i.e., the macrochromosomal X1X2Y) .
It also seems evident from the karyotypic data that both the clarkii group species and the crevice-using species with the standard and Em karyotypes were derived from an ancestral species with standard macrochromosomes and a 20 microchromosome pattern identical to that of clarkii (i.e., heteromorphic XY and polymorphic Em). This primitive stock is probably now extinct. The apparently identical X1X2Y heteromorphisms of all crevice-users, which are derived with respect to the XY condition of clarkii, indicate that the crevice-users are phylogenetically closer to one another than they are to either the clarkii group species or any other Sceloporus. This is in spite of the great morphological divergence (see below) of the torquatus group on one hand, which Is specialized for the use of rock crevices, and the grammicus group species, megalepidurus and pictus of the megalepidurus group, and asper of the formosus group, which are all specialized for the use of one form or another of plant crevices. Then, to turn this around, the great morphological divergence between the rock crevice-users and the plant crevice-users indicates that their divergence, both from the clarkii group and from one another, must have taken place comparatively early in the phylogeny of the genus, or at least in that of the large-sized, large-scaled branch (see below) to which they all belong.
Based on karyotypic data to be presented in a later paper of this series, S. cryptus is hereby transferred from the megalepidurus group to the formosus group. Although S. subpictus has not yet been karyotyped, it is morphologically sufficiently close enough to cryptus to indicate that it also should be transferred to the formosus group. Similarly, asper's karyotype clearly indicates that it should be removed from the formosus group. However, asper is sufficiently distinctive that it will be treated as a monotypic group, at least in the present analysis. The grammicus group remains unchanged. Karyotypic data provide no information on the phylogenetic relationships between these four crevice-using species groups.
In contrast to the apparently early karyological divergence of the clarkii and crevice-using stocks, the very small degree of morphological divergence among the cryptic species of the grammicus complex suggests that the chromosomal differentiation within this radiation is quite recent.
However certain these karyotypic relationships may seem, at this early stage in our understanding of the chromosomal differentiation of species, it is especially important that the relationships suggested by the karyotypic similarities should be tested by independent lines of evidence. Independent confirmation is even more important if we wish to use the karyotypic data to test theories of chromosomal speciation.
Some workers familiar with Sceloporus (e.g., R. W. Axtell, pers. comm.; Larsen, 1972 and pers. conun.) would dispute my contention that the karyotypically similar crevice-using species form a natural grouping within Sceloporus. They base their arguments on the above-mentioned great morphological disparity between the rock-crevice-using species and the plant-crevice-users. The rock-crevice-users are considerably larger and larger-scaled than are all but asper among the plant-crevice-using species. The rock-crevice-users are generally robust and dorsoventrally quite flattened, their scales tend to be strongly mucronate, and most have bold black nuchal collars highlighted with white borders. By contrast, the plant-crevice-using species, excepting the intermediate asper, are smaller than the smallest rock-crevice-users and generally have smaller and smoother scales--which in some of the grammicus become almost granular. Furthermore, the plant-crevice-users are generally more gracile and are less flattened, Their nuchal collars, if they can be considered that, are much less developed and tend to blend into the generally cryptic dorsal patterns typical of these species. To determine the phylogenetic significances of these morphological differences, it will be useful to review Smith's (1936, 1939) phylogenetic scheme for the genus, as well as other forms of evidence and other phylogenetic interpretations.
Figure 12 summarize Smith's (1939) ideas regarding the grouping of species in Sceloporus (as emended by subsequent taxonomy) and the phylogenetic relationships of these groups. When these ideas were being developed, Smith (pers. comm.) believed that the ancestral stock from which Sceloporus derived was to be found in the tropical South American tropidurine radiation--a not unreasonable idea based on what was then known about iguanid evolution. According to this concept, Sceloporus diverged early into generally large-sized, large-scaled (Fig. 12 left) and small-sized, small- scaled branches (Fig. 12 right), with the latter eventually giving rise to the well differentiated variabilis group. In turn. Smith (1936) believed that the variabilis group gave rise to the xeric adapted but otherwise generalized sceloporine genera Uta, Urosaurus, and Petrosaurus (all then included in Uta). He also thought (Smith, 1939) that the tropical formosus group was primitive relative to the large-sized, large-scaled branch and that the groups radiating out to the north and/or into more xeric areas were derived from it and more advanced in their ecology.
Smith (1939) placed the grammicus group origin within the formosus group, and derived the megalepidurus group from within the grammicus group. On the other hand, torquatus were thought to derive from within the spinosus radiation, which, in turn, were derived from another part of the formosus radiation. According to this concept, the crevice-using assemblage could not be natural. If the compositions of Smith's species groups are accepted and it is also accepted that the primitive Sceloporus was tropical, then this treatment is not unreason- able. And, in this regard, it should also be noted that, before the present work and Larsen's (1972) preliminary report, no one has directly challenged Smith's groups or their phylogenetic treatment (e.g., see Cole, 1970, 1971a, 1971b, 1972; and Carpenter, 1972). However, recent work outside of the genus Sceloporus suggests that some of Smith's phylogenetic assumptions are untenable.
Comparative osteology (Etheridge, 1964; Presch, 1969) clearly shows that Sceloporus is derived with respect to other sceloporine genera, and that the closest relationship between the sceloporines and tropidurines is actually through the xeric adapted genus Petrosaurus. Even a casual inspection of other suites of characters among the Iguanidae, such as the development of the femoral pores and surrounding scalation, the elimination of the gular fold, the development of enlarged head shields, the development of mucronate and keeled dorsal scalation, etc., will all show that Sceloporus is advanced or derived with respect to other sceloporine genera, including its osteologically closest relatives Uta, Urosaurus, and possibly Sator (which otherwise might be derived from within Sceloporus). These relationships imply that the primitive Sceloporus were xeric adapted ground lizards and radiated from the northern deserts southward and/or into more mesic habitats.
Significantly, except for Sceloporus, all of the sceloporine genera are largely or entirely xeric in their distributions. Also, except for the rock-dwelling Petrosaurus (which have been tested) and the insular endemic Sator (which has not been tested), all other sceloporine genera regularly "shimmy bury" (Axtell, 1956) in loose sand or soil for sleeping and escape (Hall, pers. obs., see also Stebbins, 1943, 1944, 1948; Pough, 1969a, 1969b), which is clearly an adaptive behavior in a xeric habitat. The "sink trap" type of nasal apparatus, characteristic of all sceloporine genera (Savage, 1958), appears to have evolved in conjunction with the "shimmy burial" trait as an adaptation for excluding sand from the nasal passages (Stebbins, 1948). Both are developed to extremes in the "sand swimming" Uma (Stebbins, 1943). Even the mesic tropic species of the formosus group have the "sink trap" type nasal apparatus (Hall, in prep.[see attached material]) and S. formosus collected from the high mountain (3600 m elevation) rain forest of the Sierra de Juarez in Oaxaca will "shimmy bury" almost as readily as do S. magister taken from desert sand dunes of the southwest United States (Hall, pers. obs.). Needless to say, the formosus have little chance to "shimmy bury" in their present habitat (because of a lack of a suitable substrate) and it is highly unlikely that such a trait would have evolved in the present habitat. Except for Urosaurus, bush and rock users, and the rock dwelling Petrosaurus, the remaining sceloporine genera are all basically ground dwellers, and presumably loose soil may frequently be the only escape cover readily available for many of them. Similarly, many of the small-sized, small-scaled Sceloporus are ground and rock dwellers.
From this analysis, it appears that the basic assumptions of Smith's phylogeny are false. I do not dispute either Smith's concept that the variabilis group of Sceloporus (especially S. couchii) is closely related to Uta and Urosaurus, or that the southern species of the spinosus group are very closely relatetd to the formosus group. But, on the other hand, it is clear from the evidence cited above
This analysis would suggest that the large-sized, large-scaled radiation in Sceloporus is a comparatively recent proliferation growing out of the older small-sized, small-scaled radiation (Hall, in prep.) and that evolution in Sceloporus has been away from xeric habitats and the invasion of mesic environments is comparatively recent. If these assumptions are correct, it then becomes more reasonable that the grouping of crevice-using species is a natural one.
Although Larsen (1972, pers. comm.) accepts the conclusion that Sceloporus derive from the primitively xeric adapted sceleporine genera, he subdivides the genus into three major divisions based on a multivariate analysis of 80 characters (40 skull ratios + 40 external characters which are also mostly ratios). His phylogenetic conclusions are summarized in Figure 13. Except that the plant-crevice-users were placed in his division II, Smith's large-sized, large-scaled branch was left intact in Larsen's division III. The small-sized, small-scaled branch was split into divisions I and II. Larsen then subdivided each of these three major divisions into species groups. Note that the rock-crevice-using torquatus group remains intact in Larsen's analysis and that the species of the grammicus group, plus megalepidurus, pictus, and asper, were all placed together in a single species group of division II. Placement of cryptus, a truly cryptic sibling of formosus, in this group is probably due to an erroneous identification of a plant-crevice-user as cryptus. Larsen's analysis fully supports the ideas that the wood-crevice-users and the rock-crevice-users each form natural groups, but, if this analysis is representative of the true phylogenetic relationships of these species groups, the crevice-using assemblage as a whole cannot be natural. Nor, does Larsen's analysis indicate a close relationship between the clarkii group and either of the crevice-using groups.
Although I have no desire to review the whole controversy revolving around the philosophy of numerical systematics, since this has been adequately discussed by Mayr (1965, 1969b) and others, it should be noted here that there are several problems inherent in the phenetic approach itself:
In sum, a more reliable phylogeny can probably be constructed through the analysis of only a few characters, the evolution of which is thoroughly understood, than can be constructed from the computerized analysis of many characters picked at random.
Besides my general objections to the use of numerical systematics as an "objective" approach to constructing a phylogeny, there are some specific problems in Larsen's application of the multivariate technique to Sceloporus:
Now, to look at Larsen's results (Fig. 13), I note that there is a strong tendency to group similar sized species, even when their genetic relationships are probably not close (e.g., see Smith's phylogeny, Fig. 12). This is particularly evident in his group II. Also, the great separation of the plant-crevice-users and the rock-crevice-users in Larsen's analysis could mainly be a function of three characters: the degree of dorsoventral flattening, the size of the scales, and absolute size.
By these criticisms I do not mean to denigrate this massive work of Larsen's, or even many of his conclusions. However, I do think it wise to emphasize that his conclusions should not be blindly accepted simply because they are the result of an "objective" procedure. They should be used with common sense.
There are now before us three different phylogenetic schemes for the clarkii group plus the crevice-using Sceloporus; the purely karyotypic one presented above. Smith's (1939) treatment, and Larsen's (1972) multivariate analysis. Each suggests different derivations for the taxa treated in the present paper. Can these differences be reconciled?
Additional phylogenetic analyses of independent suites of characters are either now under way (isozyme variation by Sheldon Guttman, Miami Univ. [note added 2003 - not completed], and microdermatoglyphics by Hobart Smith, Colorado Univ.) or are planned (immunological distance) which should eventually serve to unequivocally elucidate the phylogenetic relation- ships of the different species. But, even now, some lines of evidence exist which seem to support the karyological phylogeny and which may also account for the great morphological differences between the rock-crevice-users and the plant-crevice-users.
I have alluded to the propensity of the standard and Em karyotype species to use crevices for escape and sleeping cover. This may be an especially significant aspect of their biology, both indicative of a common ancestry and sufficient to account for the morphological differences seen between the users of different kinds of crevices. The comments which follow on escape behavior and the use of cover by the various species of Sceloporus, although not the result of a planned series of studies, are based on observations from five full summers of field work and many observations of captive lizards under suitable conditions.
When chased in the field, most Sceloporus will attempt to escape by running away into a bush, up a tree, or into open burrows in the ground. However, if no other cover is present and loose soil or sand is available, such as in a holding pen, most species will quickly "shimmy bury" (Axtell, 1956) to escape being caught. Also, when kept in sand bottomed cages most Sceloporus species, including even the mountain rain forest dwelling formosus, will usually "shimmy bury" for sleeping, even if other cover such as loose boards or cardboard is available. In the field I have night-collected magister and clarkii sleeping in the dry sand under stone bridges that provide them cover and perches during the day. The only Sceloporus which I have observed under suitable conditions and which I have never seen "shimmy bury" or found sleeping buried are species given the crevice-using designation. However, only the plant-crevice-users pictus and standard grammicus (from Oaxaca) and the rock-crevice-users torquatus and mucronatus have been specifically tested for "shimmy burial" under controlled conditions. Also, unlike the species which run or "shimmy bury" for escape, the crevice-users are always found closely associated with some kind of crevice into which they will retreat at the slightest disturbance.
The torquatus groups species, excepting serrifer which is reputed to be at least semiarboreal (Smith, 1936), and cyanogenys which are not infrequently found on Joshua trees (Yucca), are almost always restricted to areas of rock outcropping (or stone fences) where they take cover in the crevices under exfoliating chips or in the cracks formed along bedding or fracture planes. On rare occasions, individuals of most of these species can be found on other substrates, such as split logs or hollow trees, that provide suitable crevices for cover. But, in many thousands of observations, I have never seen torquatus group species use burrows in the ground for cover. Nor have I ever found them very far away from the crevices they normally use for cover.
The plant-crevice-users exploit all varieties of crevice-like spaces in plants, such as spaces within the macerated and dried remnants of prickly pear (Opuntia) pads, spaces under and between the dead and dry thick sword-like leaves of both Yucca and Agave, the cracks in split logs and trees, and under all classes of loose bark on Yucca, Opuntia, dead trees, fallen logs, and stumps. The various grammicus types, especially standard, are found in all of these habitats, and even occasionally in rock crevices; while megalepidurus and pictus, which occur syntopically with standard grammicus, seem to be much more restricted to Yucca and Agave and spend much more time on the rocky ground close to these plants than do the grammicus inhabiting them.
The conclusion from these observations is that crevice-using is a derived character. The apparently complete loss of the behavioral routine for "shimmy burial" also appears to be derived, and its loss in the crevice-users, which frequently live in areas where loose soil is present in close juxtaposition to their usual perches and cover, is probably indicative that the crevice-users have made a major qualitative change in their use of the environment. The facts that the plant- crevice-users will occasionally use rock crevices for cover and that the rock-crevice-users will in turn use plant crevices indicates that the availability of crevices of whatever form is probably more important to the lizards than is the specific substrate in which the crevice occurs. By contrast, formosus, which probably have less potential opportunity to use the trait in their present habitat than do most crevice-users, will still readily "shimmy bury" when put in a sand-bottomed cage. Many other Sceloporus use trees, rocks, or other plants as preferred substrates and for cover, but they are not restricted to them. For example, besides formosus, spinosus (a rock and Yucca user), olivaceus (a tree user), and melanorhinus (a tree user) will all "shimmy bury" almost as readily as do magister collected from sand dunes. It therefore seems reasonable that the crevice-using habit and concomitant loss of the "shimmy burial" behavior were evolved only once in Sceloporus. If so, by this criterion, the crevice-user grouping clearly would be natural.
Given that the original adaptation of the ancestral crevice-user was a use of crevices in whatever substrate, an early split into rock- and plant-crevice-using stocks seems entirely adequate to account for the evolution of the obvious morphological differences between the two stocks. Crevice-users are found only in areas where crevices provide them with cover, with their population densities in these areas generally roughly proportional to the available cover. They are never found in areas where the habitat seems to otherwise provide an abundance of food and perches but is deficient in suitable crevices. Clearly, the availability of cover is a major factor limiting their populations. It then follows that selection pressure for effective use of the preferred crevice type must be rather high. The obvious morphological differences between the two stocks are clearly adaptations to the physical characteristics of the kinds of crevices exploited.
S. megalepidurus and pictus exploit the complex crevice systems found between the leaves of Yucca and Agave. They generally escape a predator by moving away through the complex system of spaces and only occasionally do they wedge themselves in the crevices of the leaf axils or under the loose bark (on dead Yucca branches and trunks). Specialization for this class of crevice-using is mainly behavioral, but their scales are intermediate in size between those of the grammicus and torquatus groups and are comparatively smooth for any large-scaled Sceloporus, presumably so as not to impede passage between the thick leaves. In general, the megalepidurus are cryptically striped, as are some of the grammicus populations which primarily inhabit Yucca away from the range of the megalepidurus group.
Grammicus group species are generally wood-crevice-users, and achieve their greatest population densities in areas where there are many logs or dead trees with loose bark. Up to 15 adults may live on a single large log, where all take cover in the space between the bark and wood. In these habitats the grammicus are particularly good at wedging themselves into an opening crevice as the bark is being pulled away by a predator or herpetologist. If the lizards had large spiny scales, as do the rock-crevice-users, they would be trapped in the crevice once the tension on the bark was released. However, the small, almost granular scales of the grammicus allow them to work their way out of the crevice once the tension on the bark has been released. It also seems likely that the kinetic skull may be used as a wedge in helping the lizard to force its way out of a crevice. These lizards are intermediate in their degree of dorsoventral flattening, presumably because the juveniles are small enough to use the round holes bored in the solid wood by insect larvae, and selection for a cylindrical body form at this age prevents dorsoventral flattening from becoming extreme in the adults. Most grammicus have dorsal patterns of undulating bars, which are exceptionally cryptic against bark.
The torquatus group species use rock crevices which are in general quite rough-textured and completely rigid to the normal predator. These lizards escape predation by wedging themselves into a crevice facing away from the predator and then arching their back and inflating their body to set the stiff spines of their large scales against the rough rock surface. The thick, generally spiny tail is then waved back and forth to prevent the predator from grabbing a leg or the body. And, even if a leg is successfully grabbed, in my experience it is frequently easier to pull the lizard's leg off its body than it is to get the animal unstuck from its crevice. Yet, when the predator leaves, the lizard deflates and easily extricates itself. Large and small rock-crevice lizards exploit structurally identical cover, so the pronounced flattening observed in these species is clearly adaptive at all ages. Coloration in the rock-crevice-users is generally not especially cryptic, but most species live in habitats where vegetation is sparse and the animals can usually see a predator coming from a great enough distance so that they can easily reach cover. From my experience, in contrast to some grammicus populations where the animals may be relatively easily caught if they are seen, all torquatus group species will take cover at the slightest provocation and clearly do not attempt to rely on being cryptic.
Size differences between the various species groups of crevice-users are probably at least partially due to trophic competition as constrained by the physical characteristics of the classes of crevices used. In many areas of the Mexican Plateau, two torquatus group species and a grammicus group species are sympatric (syntopic where habitats overlap), and within the range of the megalepidurus group, this group may add a fourth species to the sympatry or replace a torquatus group species. The size distribution of adults in areas of sympatry is always in the sequence: megalepidurus group < grammicus group < torquatus group A < torquatus group B. The physical limitations of the crevices exploited would seem to allow only this sequence of sizes, as
Presumably the first crevice-using Sceloporus exploited a wide range of crevices or at least had this potentiality available. After speciation began in the complex and derivatives became sympatric, trophic competition or competition for cover would demand ecological differentiation, perhaps most readily in size, as suggested by Williams (1972) for Anolis. However, the availability of different size distributions of crevices in structurally rather different substrates would inevitably lead to specialization for the use of different substrate classes as well, leading in time to just those morphological differences observed between the different groups of crevice users.
Virtually nothing is known about the behavioral ecology of asper, except that all specimens seem to have been taken on live trees. Based on subjective comparisons, it seems to be morphologically intermediate between the other crevice-users and the other large-sized, large-scaled Sceloporus, perhaps being closest to shannorum (omitted from Larsen's work) and heterolepis among the crevice-users, as indicated by Larsen (1972). It may well be that asper is a comparatively unmodified derivative of the ancestral stock from which the other crevice-users evolved.
The reproductive biologies of the crevice-using species of Sceloporus are also derived and similar enough to indicate a possible common ancestry. Published observations on their reproductive biology (Mulaik, 1936; Axtell and Axtell, 1970; Axtell and Axtell, 1971; Goldberg, 1970, 1971) and my own observations on pregnancy and testicular activity when preparing karyotypes show that mating, testicular activity, and oogenesis in these lizards is maximal in the fall; that pregnancy begins in the fall or winter, and that birth takes place in the spring. Some grammicus populations and megalepidurus may have a second litter during the summer, but there is little doubt that most mating takes place in the fall. By comparison, most of the egg laying species show maximal testicular activity and mating in the spring, with their young hatching generally during the summer. Other live-bearing Sceloporus belong to the formosus and scalaris groups. S. malachiticus (formosus group) from Costa Rica show maximal testicular activity during the summer, are pregnant during the fall and early winter, and most have given birth by early March (Marion and Sexton, 1971). From my own observations, formosus from Oaxaca were also pregnant early in the fall, S. aeneus and possibly some populations of the related scalaris are also live-bearing, but they are so distantly related to the crevice-users that this reproductive system has unquestionably been evolved independently.
Live-bearing is a derived condition in lizards and is generally associated with adaptation to high elevation (Greer, 1967, 1968; Greene, 1970), and all of the live-bearing Sceloporus belong to basically high elevation assemblages; but the details of the reproductive cycles within the crevice-using species seem more similar to one another than they do to the formosus group species. Excepting asper, which Smith placed in the formosus group, but which Larsen and I agree is actually closely related to the grammicus and redefined megalepidurus groups, the formosus are clearly allied to the egg laying southern spinosus group radiation. Presumably the formosus have also independently evolved the live-bearing habit. On the other hand, the closely similar reproductive biologies of the two classes of crevice-users suggests a common origin, although these similarities taken alone certainly are not sufficient to prove a common ancestry.
Again, the geographic relationships of the species treated in the present work are fully consistent with their genetic relationships as suggested by their karyotypic resemblances, although other geographic derivations may be proposed with some justification. Figs. 15, 16, 17, & 18 illustrate the approximate ranges of these species.
As background information, the only species in the large- sized, large-scaled branch known to have the primitive 2n=34,XY male karyotype of the other sceloporine genera are three species included in orcutti by Smith (1939), Cole (1970), Hall (1969, and in prep. [Hall and Smith, 1979]). These are restricted to the Baja California Peninsula (Fig. 14). Two of the species are rock dwellers in comparatively xeric habitats, and the lizards of the third species [S. hunsakeri (Hall and Smith 1979)] are found mainly in the trees of the relictual oak-conifer woodland of the Cape Region, south of the Isthmus of La Paz. Their closest mainland relatives are the small-sized, small-scaled species nelsoni, with the primitive 2n=34, XY male karyotype, and pyrocephalus, whose karyotype differs from the primitive by a macrochromosomal inversion (Cole, 1971b; Hall, in prep.). The nelsoni and pyrocephalus are mainly ground and rock dwellers, with nelsoni being found in dry thorn scrub habitats, and pyrocephalus living in moderately open areas of the generally more mesic tropical deciduous forest. These two species appear to be separated by the lower reach of the Rio Grande de Santiago, which drains the Lago Chapala-Rio Lerma system. Note that the chromosomally primitive species occur around the periphery of the hottest parts of the Colorado-Sonoran Desert, north of the Rio Grande de Santiago. This, of course, coincides with the supposition that Sceloporus are primitively xeric adapted.
The distributions of the two clarkii group species are similar to, though more extensive than, those of nelsoni and pyrocephalus. S. clarkii the chromosomally more primitive species, occurs north of / the Rio Grande de Santiago, in generally more xeric habitats, while the chromosomally more derived melanorhinus are found to the south of the river (Fig. 15), in generally more mesic habitats. The clarkii are tree and rock users during the day, although they frequently retreat to the ground at night to sleep buried in sand or in burrows (pers. obs.). In Sonora and Arizona the species is generally found in the foothills below 1800 m in dry oak woodland situations. In Sinaloa and Nayarit the clarkii spread down onto the coastal plain, where they are more completely arboreal than they are to the north, although they can still be seen migrating to burrows in the ground at dusk. The melanorhinus occur to the south of the Rio Grande de Santiago in more mesic forest and woodland situations along rivers, etc., where they are highly arboreal.
S. asper, possibly the earliest derivative of the crevice-user radiation, are found at intermediate elevations (1000-2000 m) in the valley of the Rio Grande de Santiago and on the western parts of the Sierra Volcanica Transversal (Fig. 16). Asper, like melanorhinus and the southern clarkii, appear to be highly arboreal. Although asper are generally found at higher elevations, in cooler and more mesic areas than generally inhabited by clarkii, I would not be surprised to find these two species sympatric in some areas of the valley of the Rio Grande de Santiago, nor would I be surprised to see some sympatry with melanorhinus in the area south of the river.
The species megalepidurus and pictus are probably early derivatives of the radiation leading to grammicus, to judge by their morphology. These species are found in the xeric basins at the eastern end of the Sierra Volcanica Transversal (Fig. 16). S. shannorum and heterolepis are two other well differentiated species which are more closely related to standard grammicus than are megalepidurus (Fig. 17). (I do not accept here Webb's 1969 lumping of these species, because the morphological differences between them--the conspicuous rows of enlarged paravertebral scales in heterolepis are completely lacking in shannorum--are far more obvious than differences separating the proven cryptic species of grammicus. And, as will be seen below, there is no reason to think that these populations are currently connected by gene flow.) Somewhat as in clarkii and melanorhinus and in nelsoni and pyrocephalus, the deep, hot, and sometimes xeric barrancas of the Rio Grande de Santiago and/or its major tributaries, the Rios de Huaynamota and Bolanos, would appear to completely separate heterolepis and shannorum, with the morphologically more derived species, heterolepis, occurring to the south of the barrier. North of the barrier, shannorum seems to be restricted to intermediate elevations (1500-2000 m), while to the south of the barrier, heterolepis seem to be found only above 2000 m. Although standard grammicus are probably parapatric with shannorum at present, the grammicus generally occur at higher elevations and along the crest of the Sierra Madre Occidental and generally on the east facing slopes of the Sierra. It seems likely that the distributions of shannorum and standard grammicus would have been separated by the crest of the mountains during the coldest Pleistocene periods.
Although sequences of speciation can be deduced with some confidence in the plant-crevice-users, because the torquatus group species are so well differentiated from other Sceloporus, it is difficult to say that one species is any more primitive than another. However, it may be significant that Larsen indicated that jarrovi, mainly a species of the western edge of the plateau north of the Lerma-Rio Grande de Santiago system, is comparatively primitive by his analysis. It also appears (Fig. 18) that somewhat more speciation has occurred on the west side of the plateau, perhaps indicating the greater ages of these populations.
If it is assumed that the clarkii group and all of the crevice-users derive from a close common ancestry in the large- sized, large-scaled Sceloporus, then a simple phylogeny may be constructed which provides a straightforward account of the derivations of the modified karyotypes and of all other biological details summarized above. This phylogeny is summarized in Fig. 19 (cf. also Fig. 11).
As can be seen from figures 16, 17, 18 & 19 and the discussions above, the major stocks we are considering in the present paper have rather linear ranges along the west coast of Mexico, which follow the major vegetational zones of that area as they are determined by elevation and humidity. Five major zones of climate and vegetation structure may be distinguished:
These vegetational zones are indicated in Fig. 19 by appropriate tone bands. Early divergences in the radiation under examination seem to have been restricted to the western coastal slopes of mainland Mexico north of the present Rio Grande de Santiago, and seem to have involved ecological shifts outward from the thorn scrub community.
The present ecologies of two of the three surviving karyotypically primitive species of the large-sized, large-scaled radiation (i.e., the orcutti complex species) and the related nelsoni and pyrocephalus indicate that the prototypical large-sized, large-scaled Sceloporus were probably ground and rock using inhabitants of the thorn scrub community. In Baja California, the orcutti complex species have survived in this community with probably little modification. However, on the mainland, competition with the related nelsoni stock and a host of other predominantly terrestrial sceloporines living in the hot desert and thorn scrub pushed the clarkii-crevice-use progenitor into the dry woodland habitat, where it began to make use of tree trunks as well as rocks for perches. Presumably this shift took place approximately when the Em polymorphism spread and the modified sex chromosome and reduced micro number (which survive with no further modification in clarkii) became fixed in the line.
The divergence between the 40 chromosome clarkii group lineage and the primitive 32 chromosome crevice-using lineage seems to have involved the fixation of the four macrochromosomal fissions in the ancestral clarkii. The clarkii stock then probably fully invaded the arboreal niche as a result of this speciation, thereby restricting its ancestral 32 chromosome stock more to the ground at lower elevations and/or pushing it into more mesic forested situations at higher elevations. As more chromosomally derived and predominantly terrestrial species of the large-sized, large-scaled radiation (these will be discussed in a later work) began to invade the area at low elevations, the clarkii stock was pushed even further up into the trees, and the 32 chromosome (prototypical crevice-user) line then became completely restricted to the slopes of the volcanic Sierra Madre Occidental. This cooler mountainside habitat is now occupied by the crevice-using asper, shannorum, and bulleri. Here, the evolution of live-bearing habits would be favored by the generally lower temperatures, shorter growing season, high humidity, and a probable scarcity of suitably loose and sun-warmed soil for egg incubation (there are certainly many areas in the present habitat where soil is scarce because of the precipitous slopes). It also seems likely that crevices were the most abundant class of cover readily available in this generally rocky zone, which would undoubtedly encourage the evolution of specializations for using this cover. The standard X1X2Y sex chromosomal fusion probably spread through this prototypical crevice-using stock some time before it became geographically subdivided.
Once forced to high elevations, the ancestral crevice-user became specially adapted to them and thereby during climatic fluctuations became increasingly liable to geographic disruption, isolation, and speciation on or between different mountain masses. Subsequent contacts and sympatry during climatic optima would inevitably lead to size and/or ecological displacement within the broad range of crevice- using niches, as described in the preceding sections above. Also, once the crest of the Sierra Madre was breached by the crevice-using radiation, the whole of Mexican highlands became available for its proliferation.
This is not the only possible phylogenetic reconstruction which could account for the observed geographic relationships of the different chromosomal stocks treated in this report, but it does have the advantage of simplicity to support a plausibility arising from its complete consistency with present geography. And, more importantly, it is completely consistent with the relationships predicted from the karyotypic data alone (compare Figures 11 and 20). On the other hand, Larsen's (1972) phylogeny, for example, requires rather extensive longitudinal shifts in some of the faunas to derive present day species from their supposed ancestors. Hopefully, data from the analyses of additional suites of characters will provide a more definite picture.