[Summary and Contents]

[CONCLUSION]

NOTES

1. Wilson, et al. (1975:5065) suggest resurrecting Goldschmidt's monster to account for the rapid phyletic evolution of the chromosomally variable mammals:

   ...Placental mammals have experienced unusually rapid evolution at both the chromosomal level and the organismal level, though not at the structural gene level. Hence, gene rearrangement may have a major role in organismal evolution, as Goldschmidt [1940] suggested 35 years ago. Although the mechanism involved is not known, one possibility is that gene rearrangement provides new phenotypes by altering the patterns of gene expression during embryonic development. [Italics mine.]

   As noted in the text, I do not doubt that the correlation between chromosomal evolution and phyletic evolution is real; but Dobzhansky (1941) and Mayr (1942), among others, quite rightfully buried Goldschmidt's monster as a plausible explanation for this correlation, and the monster should stay buried--at least until there is quite solid evidence from developmental biologists that any random major chromosomal rearrangement can lead with any significant probability to a selectively advantageous change in development. Certainly there is no evidence to date that this is so. We must therefore seek the causal basis for the relationship between evolution and chromosomal differentiation in the aspects of the genetic system which regulate a species phylogeny rather than its ontogeny.

    

2. Larsen and Tanner (1975) would remove 7 species from the small-scaled radiation to form the genus Lysoptychus, which would be primitive to the remaining Sceloporus but probably derived to most of the other sceloporines. Although I retain an open mind on the validity of this grouping, I will continue to follow Smith's (1939) dichotomy in the remaining discussion here. Larsen and Tanner (1974, 1-75) also present new phylogenetic interpretations for the remaining, non-Lysoptychus Sceloporus which are based on Larsen's (1973) multivariate statistical analyses of variation in Sceloporus. The approach offers useful insights, but I believe some of the phylogenetic interpretations to be fundamentally unsound: 

   1. because of the misuse of distributional data as a variable in some of the analyses and 
   2. because no attempt was made to consider or weigh for the adaptive significances of the morphological variation studied. 

   My reasoning for these objections may be found in Hall (1973) and will be developed more fully in later papers which will deal in detail with the evolution of Sceloporus.

    

3. On two different trips I searched both the Carneros, Coahuila, and the Charcas, San Luis Potosi, localities for goldmani during good weather conditions, and I was unable to find it. At both localities, because of extreme overgrazing, there is no sign of more than a thin stubble of annual grass showing between the exposed stones. Certainly no bunch grass now exists anywhere near these localities that is accessible by road. Based on several attempts to find bunch-grass habitats anywhere in the region determined by the three known localities (in some cases quite far from any reasonably passable road), I fear that this species may now be extinct. The last confirmed specimen was collected in 1962 (Thomas and Dixon, 1976). In any event, given the ever increasing habitat destruction resulting from cattle and goat grazing, if it still does survive in some isolated montane pocket, extermination is still probably a foregone conclusion.

    

4. Cole (1970) suggests that the mutation may have been a tandem duplication.

    

5. The cytological quality of some of this material is poor and it is sometimes difficult to identify which metacentric is present in a mostly fissioned karyotype. The 4 FM2 individuals suspected to be heterozygous carriers of the metacentric 1 were all collected in or on the edge of the hybrid zone with S, and could easily be backcrosses. Except for these 4 individuals, all samples, especially those closest to FM1 populations were homozygous for the fission of pair 1.

    

6. The fact that the formosus group has only very recently invaded humid montane habitats is indicated by their retention of the behavioral trait of shimmy burial (Axtell, 1956). Most sceloporines have a very characteristic way of submerging themselves in loose sand for escape or sleeping cover which is clearly an adaptation for living in sandy desert habitats where other kinds of cover (e.g. vacant mammal burrows, holes under rocks, wood crevices, etc.) are at a premium. Based on personal observations, all sceloporines tested expect Petrosaurus and the radiation of crevice using Sceloporus will readily shimmy bury if placed in sand-bottomed cages. None of the crevice users I have tested (grammicus, megalepidurus and several torquatus group species) will do so, even in the absence of other cover, which is consistent with the idea that they have had a long history of adaptation to montane habitats. On the other hand, Sceloporus formosus that were collected from mountain rain forests above 3000 m elevation in Oaxaca, where there is certainly no loose dry soil, would sleep buried in the sand of sand bottomed holding cages, frequently in preference to using the more usual (for them) kinds of cover provided. Furthermore, when chased, they would as readily dive down into the sand for escape as would Sceloporas magister collected from sand dunes in the Rio Grande Valley of New Mexico. I regard retention of this trait, which is most likely useless in the present adaptive zone they exploit, to be fairly strong evidence that they have only recently invaded this zone.

    

7. Woodruff (1973) reviewed various geographical relationships possible between hybridizing populations and presented a carefully defined vocabulary to describe them. Chromosomally differentiated populations which hybridize peripherally with one another without overlapping sympatrically (i.e. where the hybridizing populations have parapatric distributions) will form either allopatric or parapatric hybrid zones according to Woodruff's terminology. Since the phenomena discussed in this section may include both kinds of hybridization, I will use the somewhat more general term, "contact hybridization," of Littlejohn, et al. (1971) for those situations of parapatric and narrow allopatric hybridization where there is assumed to be no gene flow or introgression away from the hybrid zone (see below). Wider zones of allopatric hybridization, such as the hybrid zone on the Jutland Penninsula of Denmark between Mus musculus subspecies musculus and domesticus (Hunt and Selander, 1973), will be referred to as zones of intergradation or introgression.

    

[REFERENCES]

[Summary and Contents]