FORMATION OF THE RECENT SPECIES
The morphological difference between the extinct B. minimus and the living B. musculus is not great, and musculus seems to be the product of the B. sawrockensis-B. minimus line of development. Morphological characters of the parental stock of the two living species, musculus and taylori, may have been intermediate between those of B. minimus and those of B. musculus. The principal part of the range of Baiomys today is in México, and probably was there through much of Pleistocene time. Extension northward of the species and retreat southward of those northern populations of pygmy mice would not only have left isolated populations in the north, but would have allowed the mice that retreated south to share a common gene pool. Therefore, populations of pygmy mice occurring to the south in central México might be expected to maintain a relatively high degree of heterozygosity in morphological and behavioral characters. The occurrence of any physical or biotic barrier that would have separated this homogeneous group would be conducive to speciation. There is evidence that a barrier occurred in the Pleistocene in central México sufficient to separate the supposed interbreeding, relatively homogeneous populations of pygmy mice. According to Sears (1955:529) and De Terra et al. (1949:51), parts of the higher regions in the Valley of México, and the transverse volcanic zone in central México were glaciated. On the mountain Ixtaccihuatl, De Terra (op. cit.:52) found evidence of four marked advances of ice, from oldest to youngest, as follows: Salto, ice advanced to 3100 meters; Xopano, ice at 3200-3300 meters; Trancas, ice to 3400 meters; Ayolotepito, ice to 4350 meters. The Salto advance is correlated by De Terra (loc. cit.) with the Iowan glacial period. The advance of ice down the mountain sides in the transverse volcanic zone was accompanied by cool moist climates or pluvial periods. Such climates probably altered habitat formerly suitable for Baiomys. There is no record of Baiomys known to me exceeding 8000 feet in elevation, although the lower edge of the ice on Ixtaccihuatl is at approximately 15,300 feet (4600 meters, Sears, loc. cit.). Presumably, the advance of ice down the mountains forced the pygmy mice to move to lower altitudes. Pluvial conditions possibly rendered the habitat even at lower altitudes uninhabitable for the mice, with the result that none continued to live in the transverse volcanic zone, but only north and south thereof. Long-continued separation of these northern and southern segments allowed species formation to occur. As climatic and habitat conditions became more favorable in central México, the two species moved back toward each other, and eventually their geographic ranges overlapped.
An analysis of external and cranial characters of pygmy mice (see [Figure 12]) reveals that both species are essentially largest to the north and smallest to the south. There are exceptions to this cline in both species. For example, B. taylori analogous is a large subspecies; it lives allopatrically in the southern part of the range of the species. B. musculus pallidus is not the largest subspecies; it lives allopatrically in the northern part of the range of the species. In west-central México, where the two species are sympatric, B. taylori is smaller than elsewhere and B. musculus is larger than elsewhere. B. t. analogous lives in the mountains of the transverse volcanic zone in central México. Its large size may be a result of the cooler climate in the mountains. B. t. allex, the smallest subspecies, lives sympatrically with B. musculus musculus at lower elevations in west-central México. The small size of allex could be a result of the warmer climate of the lower elevations. B. m. pallidus, at lower elevations in southern Oaxaca, is smaller than other subspecies of musculus to the south at higher elevations. B. m. musculus lives at low elevations along the coast of west-central México. Unlike B. m. pallidus, B. m. musculus is large at lower elevations. It occurs sympatrically with B. t. allex. It is my idea that during the period of separation, when the two species were evolving, larger subspecies evolved to the north or at higher altitudes where climates were cooler; smaller subspecies evolved to the south or at lower elevations; the two cognate species, musculus and taylori, made contact at lower elevations where individuals of taylori may have been smallest, but individuals of musculus were not the largest of the species. The differences, therefore, between the two species in their initial contact probably were slight. Hybrids, if they occurred, were probably inviable, sterile, or ill-suited for occupancy of the habitat of either of the parental stocks. The occurrence of hybrids, therefore, would result in what geneticists call "gamete wastage," and any further divergence in the parental stock, either in external characters (size and shape of body and head), or behavior, useful in recognition of species, would be favored by natural selection (see Dobzhansky, 1951:225; and Koopman, 1950:147). The two species seem to have diverged more in external characters where they occur together than in areas where they live separately (see [Figure 12]). The two species could be confused if a sample of adults of taylori from 7 mi. S La Belle, Jefferson County, Texas, were compared to a sample of adults of musculus from Tehuantepec, Oaxaca (see [Figure 12]). No confusion in species identity would arise, however, if a sample of adults was taken from the area where the two species live together (see [Figure 12]). Brown and Wilson (1956:49) pointed out that where two closely related species occur together, characters (morphological, ecological, physiological, or behavioral) of each species are easily distinguished. However, where the two species are allopatric, the two closely related species so resemble one another that the species are not easily distinguished. This phenomenon has been called "character displacement" by Brown and Wilson (loc. cit.).
In the area where the two species of pygmy mice occur together, there seems to be a disparity in numbers between them. Hooper (1952a:91) has recorded the collection of both B. musculus and B. taylori in a single trap line. A series of pygmy mice collected from San Gabriel, Jalisco, contained one taylori and 33 musculus; another sample from La Resolana, Jalisco, had a ratio of 25 taylori to 6 musculus. The disparity in numbers where the two species occur together has been further substantiated by collections of the University of Kansas. Possibly this disparity in numbers is a result of interspecific competition. Hooper (op. cit.:90) pointed out that where the range of B. musculus (typical of arid tropical lowlands) meets that of B. taylori (typical of arid temperate highlands), the two geographic ranges interdigitate with parts of the range of musculus extending into the highlands and parts of the range of taylori extending into the lowlands. In the lowlands, musculus may be better adapted to environmental conditions and, therefore, more successful in competition with taylori for available habitat. The reverse situation may exist in the highlands. Also, the fact that musculus is more of a diurnal animal than is taylori may account for the difference in numbers of individuals of the two species taken in trap lines. Many collectors set their traps in late afternoon or evening and retrieve them in early morning. Such a schedule might not yield many musculus. If interspecific competition does occur in the area where the two species occur, any change in habits or microhabitat by either species that would reduce this competition would be favored by natural selection (see Mayr, 1949:518; Lack, 1944:262-263; and Brown, 1958:154-155). Brown (op. cit.:154), as I understand him, pointed out (taking account of Gause's principle) that when two species having similar ecological valences move into the same niche in the same locality, one of three things must eventually happen: (a) the two species occupy different geographic ranges; (b) they compete and one is eventually eliminated; (c) the two species, because of differentiation or specialization, exploit different aspects of the niche. In Baiomys, (c) seems to apply. Natural selection probably would favor a continuation of diurnal activity in musculus and nocturnal activity in taylori, thereby preventing frequent meeting of the two species.
AREAS OF PRESENT DIFFERENTIATION
In both species of Baiomys, the most distinct subspecies, B. t. allex and B. m. musculus, occur in the area where the two species are sympatric. Seven subspecies, or 44 per cent, occur either in or adjacent to the transverse volcanic zone. This area is the major area of active differentiation. Incipient subspecies are also evident in these areas. A secondary area of differentiation is indicated within the range of B. musculus in Guatemala, El Salvador and Honduras. Three subspecies occur in this area (grisescens, handleyi and nigrescens) and incipient subspeciation is in evidence there.
Hooper (1949:25) regards Baiomys as a member of the rodent fauna of the arid, western Sonoran region, whereas Hershkovitz (1958:609) suggests that Baiomys is a nearctic-neotropical varicant (a kind that occurs in contiguous zoogeographic regions without our knowing in which region the taxon originated). The findings from my study do not contradict either of the above suggestions. Because of the close resemblance of Baiomys to certain hesperomine mice of South America, it is postulated that Baiomys, in more primitive form than now, occurred farther south in past times than it does now. Fossils show that primitive stocks of the genus in late Pliocene or early Pleistocene times occurred also north of the present range of the genus. The belt in west-central México between nearctic and neotropical regions is the current center of distribution of the genus and probably has been for a considerable time.