FIGURE 1. An example of a chromosome spread showing Giemsa banding. The spread
illustrated is from a Columbian ground squirrel (Spermophilus columbianus ruficaudus).
Geoscience
Research Institute
Origins 13(1):9-35 (1986).
Related page — |
IN A FEW WORDS |
Do different species of mammals have similar chromosomes? What is the meaning of chromosomal similarities? Are they evidence of common ancestry? Do they provide any support for common design? This article will discuss some of the evidence and attempt to propose answers to these questions.
INTRODUCTION
The genetic instructions for an organism are located in the
chromosomes of the cells of the organism and are transmitted to the offspring by
inheritance. A logical prediction of evolutionary theory is that closely related species
should have similar chromosomes. Techniques of chromosome banding have now been available
for a long enough period of time that some trends have been discovered, and the results
can be examined profitably.
Comparisons of karyotypes (sets of chromosomes) can be based
upon differing levels of detail (see White 1978:47). The first comparisons were made on
the basis of the number of chromosomes. In some cases the number of one-armed
(acrocentric) and two-armed (metacentric) chromosomes were included in the comparison, and
the sex chromosomes were identified. Extensive lists of chromosome counts can be found in
Matthey (1973a,b) for placental mammals and in Sharman (1973) and Hayman (1977) for
marsupials. However, attempts to infer relationships based upon unbanded karyotypes have
not been satisfactory (Atchley 1972). Frequently, individual chromosomes could not be
identified, making comparisons of uncertain validity. Differences in arm number due to
gain or loss of heterochromatin (tightly condensed chromatin, generally
considered to have little genetic activity) were not correctly interpreted using
conventional staining (Duffy 1972).
The development of banding techniques overcame these difficulties and
made comparisons more meaningful. Structural changes in chromosomes (chromosomal
rearrangements) can now be identified precisely. However, much remains to be learned about
the meaning of banding and the structure of chromatin (the chromosomal material), and
further developments can be expected to add to the value of comparative karyology. Several
methods of chromosomal banding are available, but the most widely used method is G-banding
(Giemsa-banding). This technique produces a characteristic pattern of contrasting dark and
light transverse bands on the chromosomes (see Figure 1). The banding pattern is different
for nearly all species studied, although sometimes the differences are slight. A large
number of mammal species have been G-banded, but the number of species remaining to be
studied is much larger.
FIGURE 1. An example of a chromosome spread showing Giemsa banding. The spread illustrated is from a Columbian ground squirrel (Spermophilus columbianus ruficaudus).
PURPOSES OF COMPARISONS
Homeology
Comparisons of banding patterns often reveal nearly identical
patterns in closely related species. The corresponding bands are believed to be
homologous, but to allow for minor genetic difference, the term "homeology" is
often used (e.g., Dutrillaux, Couturier and Fosse 1980).
Chromosomal similarities have been noted between groups of species in
different genera or higher taxonomic categories. A significant degree of homeology has
been found among all three families of seals (Arnason 1977). Homeologies at the suborder
level have been noted within the primates (e.g., Dutrillaux, Couturier and Fosse 1980).
Comparison of cat and human banding patterns (Nash and O'Brien 1982) did not show
significant banding homeologies, although their gene mapping studies suggest similar gene
arrangements (O'Brien and Nash 1982). Claims of banding homeologies between primates and
carnivores by Dutrillaux and Couturier (1983) are of uncertain validity, because the
proposed homeologies are based on portions of chromosome arms with a small number of bands
involved. Reliable homeologies should include entire arms or be supported by other
evidence (see Ponsa et al. 1981).
The actual genetic homology of similar banding patterns is supported by
comparative gene mapping. Genes for equivalent enzymes are indeed often present on
chromosomes with similar banding patterns in different species (Lalley and McKusick 1985).
Genes which are found close together on a chromosome are said to be linked. Groups of
genes which are linked in humans are generally also found linked in other species. In
fact, gene groupings appear to be similar in many species even when chromosome banding
homeology has not been detectable (Nash and O'Brien 1982, Kiel et al. 1985). Several
equivalent linkage groups have been found on chromosomes with similar banding patterns in
humans and other primates (Lalley and McKusick, loc. cit.). Several linkage groups are
also common to human and cat chromosomes; in fact, the similarities in linkage patterns
between cats and humans are almost as consistent as between chimpanzees and humans
(O'Brien and Nash 1982), although based on fewer gene loci. Several mouse linkage groups
are similar to human linkage groups, if one allows for the correspondence of one two-armed
human chromosome with two one-armed mouse chromosomes. Whether the genes controlling such
characteristics of organisms as development and morphology are also linked in similar ways
in similar species is not known.
Classification
Comparative studies of chromosomal banding patterns have been useful
in classification. Sibling species are species which appear alike morphologically, but
have been discovered to be reproductively isolated. Giemsa-banding is not necessary to
detect sibling species, but it can assist in identifying the differences between the
species more precisely. Chromosomal sibling species have been discovered within cotton
rats (Sigmodon; see Elder 1980), grasshopper mice (Onychomys; Hinesley
1979), and shrews (Sorex; Olert and Schmid 1978).
The use of G-banding has sometimes been helpful in clarifying the
taxonomic position of species which do not have clear affinities based on other
characters. Chromosomal differences have been used in determining the taxonomic placement
of the African rat Mastomys (Lee and Martin 1980), the golden mouse (Ochrotomys;
Engstrom and Bickham 1982), and Neotomodon alstoni (Yates, Baker and Barnett
1979). The giant panda, Ailuropoda, has been variously classified with the bears,
raccoons, or in a family by itself. Although previous researchers were unable to find good
banding matches (Wurster-Hill and Bush 1980), a more recent study (O'Brien et al. 1985)
identified several banding matches linking the giant panda with the bears. The authors
conclude by suggesting the giant panda be classified in a separate subfamily within the
bear family Ursidae.
Sometimes chromosomal comparisons give unexpected results. For example,
the karyotypes of some South American genera of rodents were more like the karyotypes of
wood rats (genus Neotoma) a primarily North American genus, than they were like
the karyotypes of cotton rats (Sigmodon), a primarily South American group
(Baker, Koop and Haiduk 1983).
The validity of using chromosomal banding comparisons to assist in
determining degree of relatedness was supported in a study by Mascarello, Stock and Pathak
(1974). The banding pattern of a species of woodrat (Neotoma) was compared with
that of six other rodent species, progressively more distantly related taxonomically. The
degree of matching was near total for another woodrat species from the same subgenus and
showed a general decrease in comparison with species of increasing taxonomic distance.
About one-third of the chromosomes matched those of species from other tribes or
subfamilies. Comparison with a species of kangaroo rat (Dipodomys, different
superfamily) revealed no detectable banding homeologies with the woodrat. These results
show general agreement with traditional methods of classification.
Constructing phylogenies
Giemsa banding has made possible the identification of specific
chromosomes involved in rearrangements (Seabright 1972), permitting one to determine
whether similar species possess the same rearrangement. Shared chromosomal rearrangements
in similar species are interpreted as evidence of a common ancestor which had the
rearrangement (see Rofe 1976). Several species of antelopes which share a Y/autosome
translocation provide one such example (Benirschke et al. 1980).
The ability to identify similar banding patterns in different species
and to identify chromosomal rearrangements has led to interest in reconstructing the
historical sequence of rearrangements which have accompanied speciation in a group of
species (see Spotorno 1977). The construction of such "family trees" is based on
several assumptions. One assumption is that the ancestral karyotype can be determined with
reasonable accuracy. This requires that the species to be compared be chosen carefully
(Dutrillaux and Couturier 1983) and that homeologies be accurately identified. Another
assumption is that the best tree is one which requires the fewest reversals and
convergences (principle of parsimony; see Farris 1978).
The method is not without difficulties. One problem is that reversals
and convergences do occur (e.g., see Baker, Barnett and Greenbraum 1979; Baker, Koop and
Haiduk 1983; Searle 1984), probably because certain points on a chromosome are more
susceptible to breakage than other points (Bush 1981, Nevers and Saedler 1977). Another
problem is the possibility of mismatches, especially when only portions of chromosomal
arms are involved (see Ponsa et al. 1981). Despite these difficulties, cladograms
("family trees") based on chromosomal characters are useful in testing for
congruence with cladograms based on other data.
Cladograms based on chromosomal characters have been constructed for
groups at several taxonomic levels, for example the genus Peromyscus (Robbins and
Baker 1981), the bovid tribe Tragelephini (Benirschke et al. 1980), and several genera of
murid and cricetid rodents (Koop et al. 1984). An especially comprehensive study at the
superfamily level has been done for phyllostomoid bats (Patton and Baker 1978). Obviously,
such cladograms cannot be more accurate than the identification of banding
homeologies.
RESULTS OF COMPARATIVE G-BANDING STUDIES
This section discusses numerous examples of studies in which banding patterns of various species have been compared. It forms the basis for the discussion in the next section (A creationist viewpoint). Readers not interested in details may skip to the next section, referring to this section only if more details are desired. No attempt is made here to present an exhaustive list of references on G-banding results in mammals. Instead, I will discuss briefly a sample of the literature available, emphasizing studies of special interest or taxonomic breadth. Preference is given to papers which compare the banding patterns among several species or higher categories.
Monotremes
The taxonomic relationships of the egg-laying mammals are somewhat uncertain, as they show some skeletal similarities with reptiles and others with mammals (see Nowak and Paradiso 1983:1). There are three living genera, divided into two families. The anatomical uniqueness of the species is paralleled by the unusual nature of the karyotypes. The platypus has 52 chromosomes in each sex (Bick and Sharman 1975). Like most mammals, males have an XY sex chromosome pair, and females have two X chromosomes. Tachyglossus, the more widespread genus of echidna, is the only monotreme for which G-banding has been published (Murtagh 1977). There are 63 chromosomes in the male, which has one Y and two X chromosomes (X1X2Y). The female has two pairs of X chromosomes (X1X1X2X2) with 64 chromosomes in all. Zaglossus, the other genus of echidna, apparently has the same system. Unlike most other mammals, all of the monotreme species studied have unpaired elements at meiosis, which participate in a chain multiple with the sex chromosomes. The three genera of monotremes are karyotypically more similar to each other than to any other mammal.
Marsupials
The marsupials are of special interest because of their unusual
characteristics and biogeographic distribution. Chromosome numbers for over 100 species of
marsupials are listed by Hayman (1977), and trends analyzed. The chromosome numbers range
from 10 to 32, with 14 being the most frequent number, and 22 the next most frequent. The
greatest diversity of chromosome number is among the Macropodidae (kangaroos etc.).
Comparing Australian and American marsupials. The relationship
of Australian marsupials to those from South America is a question of continuing interest.
An important interfamilial comparison of G-banding by Rofe and Hayman (1985) may shed some
light on the question. The study included one American species and 14 Australian species,
representing four or five superfamilies (depending on the classification scheme). All
species had 14 chromosomes, and their banding patterns showed remarkable agreement. These
results were interpreted by the author as supporting the common ancestry of both
Australian and American marsupials, with the ancestral karyotype being most like the one
shared by a wombat (Vombatus ursinus), a dormouse possum (Cercartetus
concinnus), and a bandicoot (Isoodon obesulus). Differences between species
can be accounted for on the basis of pericentric inversions (an inversion
including the centromere) and small variations in heterochromatin. The presumed increase
in 2n number from the proposed ancestral number of 14 to as many as 32 is attributed to
chromosomal fission.
The South American marsupial Dromiciops has sometimes been
placed in a family separate from other living species. Its chromosomes have not been
G-banded, but they appear to be similar to those of the presumed ancestral marsupial 2n=14
karyotype (see Sharman 1982). Karyotypic similarities have also been reported among
several species of American opossums (Yonenaga-Yassuda et al. 1982; Casartelli, Rogatto
and Ferrari 1986), and among several species of Australian dasyurid marsupials (Young et
al. 1982, Baverstock et al. 1983b, Rofe and Hayman 1985).
Kangaroos. The chromosomes of kangaroos and their allies
appear to be distinct from those of other marsupials. Rofe (1976) compared the G-bands of
ten species of kangaroos and their allies, with 2n ranging from 10 to 22. Numerous
chromosome arm homeologies could be identified, suggesting Robertsonian fusion
(fusion of two chromosomes by their centromeres) to be the predominant type of chromosome
rearrangement. The karyotype of the red-bellied pademelon (Thylogale billardierii),
with 22 chromosomes, was interpreted as being closest to the ancestral condition for the
group. Karyotypes for seven species of kangaroos (Macropus) were derivable by
various fusions. The banding patterns of the swamp wallaby (Wallabia bicolor: 10
chromosomes in males, 11 in females) and the rock wallaby (Petrogale penicillata:
22 chromosomes, but a different karyotype) displayed greater divergence from the presumed
ancestral state. Pericentric inversions and centric shifts (change in position of the
centromere) were proposed to explain the differences.
Insectivores
One of the first examples of chromosomal polymorphism to be discovered was the European shrew superspecies, Sorex araneus. At least 12 chromosomal forms have been described (see Seale 1984). Differences can be explained on the basis of Robertsonian fusions involving different acrocentrics of an ancestral karyotype. Chromosome numbers range from 20 to 32. Some hybridization occurs, but partial reproductive isolation exists between some of the races. An X-autosome fusion, giving rise to a sex chromosome system of XX in females and XY1Y2 in males, is found in some, but not all of the races (Olert and Schmid 1978).
Bats
Bats comprise the second-most-diverse order of mammals. Two
suborders are present, the Old World fruit bats and the rest of the bats. Eight species of
African fruit bats, representing eight genera, were compared on the basis of their G-bands
(Haiduk et al. 1981). In spite of the fact that chromosome numbers ranged only from 34 to
36, substantial differences in banding patterns were detected, requiring at least 34
rearrangements to explain the differences among the eight species. The mechanism for
nearly half the rearrangements could not be conclusively identified. This study
illustrates that significant karyotypic differences may exist between karyotypes which
appear similar superficially.
Most G-banding studies of bats are concerned with the insectivorous and
nectar-feeding bats. Patton and Baker (1978) concluded that the ancestral karyotype for
the largely tropical American superfamily Phyllostomoidea is most like that of the
big-eared bat (Macrotus waterhousii). In two different genera, a comparison of
banding patterns of two similar species suggested that a total rearrangement of the genome
had occurred in one species but not in the other. Another interesting discovery in this
study was that the fisherman bat (Noctilio), placed in its own family on
morphological grounds, has very similar G-banding to that of the mustache bat (Pteronotus
pamellii), family Mormoopidae. The existence of similar karyotypes in morphologically
distinct species and of different karyotypes in morphologically similar species can be
interpreted as evidence against the theory that chromosomal rearrangements promote
speciation by disruption of genes which regulate development (e.g., Wilson, Sarich and
Maxson 1974).
Other good studies of chromosomal banding in bats include those of
Haiduk and Baker (1982) on the long-tongued bats (Glossophaginae) and on evening bats by
Bickham (1979a,b) and Zima (1982). A summary of the kinds of chromosomal rearrangements
proposed in various studies of New World bats was published by Baker and Bickham (1980).
Primates
Because of the great interest in their relationship to humans,
primates have been the object of special attention in comparative karyology. Numerous
banding homeologies have been claimed for some 60 species of primates, including man
(Dutrillaux et al. 1978). They conclude that "it is likely that all the euchromatin
[genetically active chromatin] ... is identical in all the species". This statement, if
true, would appear to reduce the significance of chromosomal banding comparisons.
As is often the case, different types of chromosomal rearrangements are
typical of different taxonomic groups (karyotypic orthoselection; White 1973). In
most lemurs Robertsonian rearrangements are the most common type of rearrangement, except
for one genus in which tandem fusions are common (Rumpler and Dutrillaux 1976, 1978, 1979;
Rumpler et al. 1983b, 1985). If their karyotypes are derived from the presumed ancestral
karyotype for the group, Robertsonian rearrangements predominate in the species of Galago,
while pericentric inversions are more important in Perodicticus, a loris
(Dutrillaux et al. 1982, Rumpler et al. 1983a). Chromosome fissions are reportedly very
frequent in the Old World monkeys, but have not been found in the other families. In the
apes pericentric inversions are the most common type of rearrangement.
One of the most variable genera karyotypically is the New World owl
monkey, Aotus. Nine different karyotypes have been reported, differing by
fissions, fusions, and inversions (Ma 1981, Galbreath 1983). The number of sex chromosomes
differs among the races. An ancestral karyotype for platyrrhine (New World) monkeys was
proposed by Dutrillaux and Couturier (1981). A bibliography of cytogenetic studies in New
World primates is available (Mudry de Pargament, Brieux de Salum and Colillas 1984).
Different workers have sometimes obtained different results from study
of the same material. This can be illustrated in studies comparing the banding patterns of
the grivet (Cercopithecus aethiops) and the rhesus monkey (Macaca mulatta).
One pair of investigators (Stock and Hsu 1973) reported complete matching of the euchromatin
(genetically active chromatin), with differences explainable as the result of
heterochromatin additions or fusions. Another group of investigators (Estop, Garver and
Pearson 1978) were unable to match some of the chromosomes in the two species. The claim
of nearly complete homeology of banding among the Old World monkeys (Dutrillaux 1979,
Dutrillaux et al. 1978) has been questioned by Ponsa et al. (1981), who suggest that the
extent of banding homeologies among the primates may have been overstated. They emphasize
the need to base homeologies on characteristic banding patterns, not merely short
segments, and criticize the construction of karyotypes of hypothetical ancestors as
"paper cytology".
Apes and humans. The chromosomes of the great apes have
received a great deal of study, and detailed banding patterns have been published and
compared with human banding patterns (Yunis and Prakesh 1982). The similarities between
chimpanzee and human chromosomes are very striking. Only ten of the 23 pairs of human
chromosomes show banding differences when compared with chimpanzee chromosomes. The
banding patterns of nine chromosomes are identical in humans and gorillas. The three
species differ in their banding patterns by various inversions and a Robertsonian fusion.
The fusion involves chimpanzee chromosomes 12 and 13 as equivalent to human chromosome 2
(see Sun, Sun and Ho 1978a,b). No differences in the gene maps of humans and chimps have
yet been noted (Lalley and McKusick 1985). In contrast to the usual phylogeny proposed for
the group, it is the human karyotype that is considered to be closest to the ancestral
type (Yunis and Prakesh 1982). The karyotypes of chimps and gorillas are more similar to
the human karyotype than to that of the orangutan.
Carnivores
Banding similarities among the cat, raccoon and mongoose families
were reported by Wurster-Hill and Gray (1975). More recently, attempts have been made to
propose an ancestral karyotype for the order Carnivora (Dutrillaux and Couturier 1983,
Couturier et al. 1986). This hypothetical karyotype is quite similar to that of the palm
civet (Paradoxurus hermaphroditus, family Viverridae). Seal karyotypes show
banding similarities with those of the carnivore families, but bears and dogs have
karyotypes that are quite different.
The hypothetical ancestral carnivore karyotype (see above paragraph)
was compared with the hypothetical ancestral karyotype previously proposed for the New
World monkeys (Dutrillaux and Couturier 1981), prosimian primates (Rumpler et al. 1983b)
and the squirrels (Petit et al. 1984, cited by Couturier et al. 1986). Although banding
homeologies are claimed for significant portions of the karyotype, the method used has
been criticized (Ponsa et al. 1981). Dutrillaux and Couturier invoke gene mapping
similarities to support their view of actual homology of the chromosomes.
Karyotypically, one of the most homogeneous families known is the cat
family (Wurster-Hill and Gray 1973). Mongooses show significant but varying degrees of
similarity with cats (Wurster-Hill and Gray 1975). Translocations involving a sex
chromosome are known in at least two genera of mongooses (Pathak and Stock 1976, Fredga
1972).
Seals and their allies
There are three families of pinnipeds: true (earless) seals, sea lions (eared seals), and the walrus. These families all share considerable banding homeology, with only four different karyotypes known (Arnason 1977). Differences among the karyotypes were not described thoroughly, but at least one fusion is involved. A striking resemblance to certain carnivore karyotypes was reported, especially to the coati mundi karyotype, but it is not clear whether this similarity was based on banding patterns.
Whales
Whales are generally divided into two major groups, toothed whales (Odontoceti) and baleen whales (Mysticeti). Karyotypes of members of both groups are very similar (Arnason 1974, Arnason et al. 1977), except for the sperm whales, which have distinctive karyotypes. Several species have interstitial heterochromatin and similar C-bands. Some homeologies were reported, but differences have not been described.
Odd-toed ungulates
Horses are the only members of this order for which I have seen comparative G-banding studies. All seven living species of the horse family have been studied (Ryder, Epel and Benirschke 1978). Each species has a different 2n number, ranging from 32 to 66. Only the X chromosome and a single autosome show the same banding pattern in each species. The other chromosomes all show differences, most commonly involving Robertsonian fusions and pericentric inversions. The mechanism for many of the rearrangements is unknown. The two species of horses have similar chromosomal banding patterns, as do the two species of asses. Two of the three species of zebras have similar patterns, but the pattern in Hartman's zebra is so different that little homeology can be determined in comparisons with the other species.
Even-toed ungulates
Interfamilial G-band homeologies have been identified (Buckland and
Evans 1978) among the cow family (Bovidae), deer family (Cervidae) and the giraffe family
(Giraffidae). A hypothesis of chromosomal evolution involving fission has been outlined by
Todd (1975) for the order, but our knowledge of this group is still very incomplete.
Camels. Camels have a disjunct distribution, with four species
in South America and two species in the Old World. A study comparing banding patterns in
two South American species and the Bactrian camel found the G-banding patterns to be
indistinguishable (Bunch, Foote and Maciulis 1985). The distributions of heterochromatin
were also indistinguishable, a rather unusual result. The lack of chromosomal divergence
despite the geographical isolation is unexpected, and suggests either a very stable
karyotype or a relatively short period of isolation, or both.
Cattle family. The karyotypes of the various species of sheep
and goats are very similar, with differences attributed to fusions (Bunch, Foote and
Spillett 1976). Cattle chromosomes show large homeologies with those of sheep and goats
(Schnedl and Czaker 1974). Buckland and Evans (1978), using the goat karyotype as a
standard, found nearly complete agreement in banding patterns among several species of
bovids, representing three subfamilies. The goat and the horse-like antelope karyotypes
were more similar to each other than either was to the cattle karyotype.
A rearrangement which is shared by several similar species is
considered to be a good indicator of common ancestry (Rofe 1976). A Y/autosome
translocation is found in several species of African cattle-like antelopes (Benirschke et
al. 1980), including the eland and the bongo. Differences among the species appear largely
due to Robertsonian fusions, with a few tandem fusions and some other unidentified
rearrangements. An X/autosome tandem fusion is found in several species of gazelles
(Effron et al. 1976, Benirschke et al. 1984). These examples illustrate variability in
species which has probably come about relatively recently.
Deer family. One of the most unusual examples of chromosome
modification yet discovered is found in the muntjacs, a group of small Asian deer. Two
species, the Indian muntjac, Muntiacus muntjak vaginalis, and M.
rooseveltorum, share the distinction of having the lowest 2n number known among
mammals, six in the female and 7 in the male (Wurster-Hill and Seidel 1985). The Chinese
muntjac, M. reevesi, looks very similar but has 2n=46 in both sexes. A comparison
of the chromosomes of the Indian and Chinese species (Liming, Yingying and Xingsheng 1980)
suggests that essentially the same genetic material is present in each and that the lower
number of chromosomes is probably derived by tandem fusion from the 2n=46 karyotype. A
fourth species, M. feae, has 2n=13 (Soma et al. 1983). Karyotypes of these
species illustrate the usefulness of G-banding in comparing karyotypes and show that
caution is in order in drawing phylogenetic conclusions based solely on chromosomal data.
Lagomorphs
Species from both the pika family (Ochotonidae) and the rabbits and hares (family Leporidae) have been studied. The two families appear to share very little or no detectable banding homeologies (Stock 1976), but extensive chromosomal similarities are present among the leporids. Hares (Lepus) have similar karyotypes, while cottontails (Sylvilagus) show considerable variation (Robinson, Elder and Chapman 1983a,b, 1984). The ancestral karyotype for the group appears to be like that of the hares. Other genera of rabbits appear to be related karyotypically to the hares (Robinson and Skinner 1983, Robinson 1980, Stock 1976). Many of the differences can be ascribed to Robertsonian rearrangements.
Rodents
This is the most diverse order of mammals, and a great amount of
cytogenetic study has been done with rodents. However, much more remains to be done. Most
comparative studies of rodent G-banding have been done with rats and mice, and many
families have not yet been studied. A recent bibliography of rodent karyological studies
is available (Jotterand-Bellomo 1984).
The G-banding patterns of thirteen species of ground squirrels (Spermophilus)
have been published (Nadler et al. 1973, 1975, 1984), and extensive homeologies
determined. An interesting geographical pattern has been discovered in this group. The
arctic ground squirrel, S. parryi, is found on both sides of the Bering Strait,
both populations having identical karyotypes. The arctic ground squirrel populations
separate two other species: S. columbianus in North America and S. undulatus
in Siberia. The banding patterns of these two species are identical. The highest 2n
numbers in the genus are found in the Asian S. xanthoprymnus (2n=42) and the
North American S. vigilis (2n=46). These two species differ primarily by two
fusions. In contrast to the karyotypic variability of the squirrel genus Spermophilus,
most chipmunks (genus Tamias), have very similar karyotypes (Nadler et al. 1977).
By far the largest family of mammals is the mouse family, and a large
number of chromosomal studies have been conducted among its members. Only a few studies
can be described here. All species of white-footed mice (Peromyscus) studied so
far have 48 chromosomes (Robbins and Baker 1981). The most primitive karyotype was
proposed to be that of P. boylii. A modified Peromyscus karyotype was
proposed to be ancestral for the family by Koop et al. (1984). They noted that karyotypic
differences may be more extensive among species in a genus than between genera.
One of the most interesting cases of chromosomal speciation is found in
the house mouse (Mus musculus complex). Chromosome numbers range from 22 to 40,
with differences due to Robertsonian rearrangements (Gropp and Winking 1981). Such a
situation is called a "Robertsonian fan". Several other species from the same
subgenus share identical banding patterns with Mus musculus, but at least some
species in other subgenera have quite different banding patterns (Hsu, Markvong and
Marshall 1978). Another Robertsonian fan has been described in the European mole-vole, Ellobius
talpinus (Lyapunova et al. 1984). Here the 2n number varies from 31 to 54 within a
geographic distance of only 150 km. All chromosome numbers from 31 to 54 have been
found, indicating extensive introgression.
Several genera of native Australian rodents, representing three tribes,
have been studied. Each of the three tribes contains a species with a common banding
pattern (Baverstock et al. 1983a). These Australian rats have virtually no banding
homeologies with species of Rattus, indicating only a distant relationship. One
genus (Zyzomys) has apparently had its genome completely rearranged.
Banding patterns of ten genera of murid rodents, mostly of African
origin, were compared by Viegas-Pequignot et al. (1983), and an ancestral karyotype
proposed for the murid rodents. Several examples were noted in which a particular type of
rearrangement appears to have accumulated in a particular lineage (karyotypic
orthoselection). This ancestral karyotype for murid rodents was compared with that of a
South American cricetid, Akodon arviculoides to test for similarities between the
two subfamilies (Viegas-Pequignot et al. 1985). About 40% homeology was claimed. It would
be interesting to compare the "ancestral" karyotype proposed by these authors
with that given by Koop et al. (1984).
South American hystricomorph rodents. Several families of
mostly South American rodents are included in the hystricomorphous rodents. Not many
studies of G-band comparisons have been published, and fewer yet in English. A review of
unbanded karyotypes of hystricomorphs was published by George and Weir (1974). Three
species of Caviidae, representing three genera, were compared by Maia (1984). Differences
reported were primarily due to heterochromatin content. Chromosomal speciation appears to
be taking place among populations of a superspecies of spiny rats (Proechimys,
Family Echimyidae) in Venezuela (Reig et al. 1980). Chromosome numbers range from 42 to
62. Differences are due to Robertsonian fusions, except for the extreme chromosome
numbers, where pericentric inversions are also involved.
Miscellaneous rodent families. Five species of gundis (family
Ctenodactylidae), representing four genera, have been shown to have similar chromosomal
banding patterns (George 1979a). Differences can be explained by a pericentric inversion,
and perhaps several very small translocations. The same author (George 1979b) found very
close similarity in the banding patterns of two species of African mole-rats (family
Bathyergidae). This contrasts sharply with the variability seen in some other families of
burrowing rodents and casts doubt on the hypothesis that chromosomal evolution is
especially promoted by the kind of social structure found in burrowing rodents (Wilson et
al. 1975; see also Gileva 1983).
A CREATIONIST VIEWPOINT
A challenge for creationism
Although much remains to be learned about the meaning of chromosomal
structure, enough data on chromosomal comparisons have been gathered to raise some
important questions for creationists. That changes have occurred in organisms since
creation is not in question, but the extent of those changes is uncertain.
Species which are similar morphologically generally have similar
karyotypes, although there are significant exceptions (e.g., see Liming, Yingying and
Xingsheng 1980). This is quite reasonable if species with similar morphology (e.g., in the
same genus) are thought of as being related through common ancestry. The occasional
exception merely shows that chromosomes can be extensively rearranged with no significant
morphological effect.
More problematic is the finding that species which are quite different
morphologically may have similar karyotypes. The chromosomal similarities among many of
the Australian marsupials (Rofe and Hayman 1985), between goats and giraffes (Buckland and
Evans 1978), between seals and terrestrial carnivores (Arnason 1977) and between humans
and the great apes (Yunis and Prakesh 1982) raise some significant questions for
creationists. Perhaps the two most important questions are:
These questions will be amplified below, and then various hypotheses regarding the
relationship of chromosomal evidence to these questions will be discussed.
Problem 1. The extent and mechanism of morphological change.
There is circumstantial evidence that mammal species may change significantly in their
morphology. This evidence comes from the study of island populations (e.g., Lawlor 1982,
Simpson 1956), from the results of selective breeding of domestic animals (e.g., Wayne
1986), from the ability of some animals to hybridize (Van Gelder 1977), and from
distributional patterns of living mammals (Darwin 1859). However, there seem to be limits
on the amount of morphological change possible (Lester and Bohlin 1984).
If groups such as the Australian marsupial families are considered to
share a common ancestry in spite of their diverse morphology, one is challenged to propose
some mechanism by which such change could be brought about. The standard neodarwinian
gradualistic explanation for morphological change is that small changes arise by mutation
and accumulate over time by natural selection to produce large changes (e.g., see
Charlesworth et al. 1982). However, the lack of fossil intermediates, or even conceivable
intermediate stages, has led many scientists to search for other explanations. Several
alternative mechanisms for macroevolutionary changes have been proposed (e.g., Gould 1977;
Oster and Alberch 1982; Wilson, Maxson and Sarich 1974; Wright 1982), but none has been
satisfactory. The possible role of chromosomal rearrangements in speciation was discussed
in a previous article (Gibson 1984).
For creationists, the origin of diversity in mammals is an important
question. If enough morphological change has occurred since the Genesis flood to explain
the origin of diversity among marsupials, it seems reasonable to think that the same
amount of change could also have happened among placental mammals, although placentals as
a group do not have such similar chromosomal banding patterns as marsupials. However, in
the absence of a plausible genetic mechanism for creating new adaptations, creationists
are somewhat skeptical that such changes have occurred, even though there is no scriptural
prohibition against large changes in species.
If the marsupials are considered to be unrelated, then one has the
problem of explaining why they share so many unique characteristics, including chromosomal
similarities and such structural traits as their reproductive anatomy, the presence of
epipubic bones, and the inflection of the angular process of the lower jaw. Their
geographic distribution is also difficult to explain.
Problem 2. The relationship of man and the apes. Questions
concerning the origin and nature of man have deep philosophical significance.
Evolutionists have long held that humans and apes share a common ancestry, a belief based
largely on morphological similarities. Fossil discoveries have not clarified the picture,
but seemingly have made it more confused, perhaps due to the subjective nature of
interpreting the fossils (Washburn 1973). However, striking similarities have been
discovered between apes and humans in their proteins (Bruce and Ayala 1979), their
chromosomes (Yunis and Prakesh 1982), and in their DNA (Sibley and Ahlquist 1984).
To say that humans and apes are not related by common descent is to
emphasize their difference in anatomy and behavior, and to downgrade the importance of
their similarities in anatomy, biochemistry and chromosomes. Although the human karyotype
is considered to be closest to the ancestral condition for humans and apes (Yunis and
Prakesh 1982), I am not aware of any serious examination of the possibility that humans
might be ancestral to apes.
The meaning of chromosomal similarity
As an explanation for similarities in chromosomal structure, four
distinct possibilities come to mind, each presented as a separate hypothesis below.
Hypothesis 1. Chromosomal similarities are the result of common
design. This would mean that organisms which are similar morphologically were created
with similar karyotypes, just as they were created with similar anatomical and biochemical
features. If the karyotypes have not undergone much change since creation, we should be
able to see the similarities. Whether a karyotype should be shared by all mammals or only
by those with some degree of morphological similarities is uncertain.
If a karyotype is shared only by species with similar morphology, one
might infer that the structure of the chromosomes is somehow related to the morphology of
the organism. It is true that, in general, groups of species with similar G-banding
patterns are also similar morphologically. However, it is known that major changes in the
karyotype, as shown by G-banding, do not cause morphological change (e.g., see Baker,
Bickham and Arnold 1985). It is also known that different types of chromosomal change may
be found in groups of species which could plausibly have a common ancestry (e.g., see Koop
et al. 1984). These facts cast doubt on any fixed relationship between chromosome
morphology and anatomical morphology, although they do not disprove a possible original
relationship between them.
There have been some suggestions that karyotypic structure has adaptive
significance (Baker et al. 1983, Kiel et al. 1985), but this has not been demonstrated
conclusively. It is of interest to note that there is frequently a correlation between
anatomical distinctiveness and karyotypic distinctiveness between groups at high taxonomic
levels.
Hypothesis 2. Chromosomal similarities are exclusively the result
of common ancestry. According to this hypothesis, if two species have similar
chromosomes (including banding patterns), they are related. This would require that each
original species group was created with its own unique karyotype. If this hypothesis is
correct, one must accept a common ancestry for apes and humans, for at least the majority
of the marsupials, and for cattle, goats, antelope and giraffes. Acceptance of this
hypothesis is the basis for phylogenies based on chromosomal similarities.
There are problems with this hypothesis. One is that despite some
circumstantial evidence for major morphological change in mammals, no mechanism is known
which would account for the kind of changes here suggested. Among the Australian
marsupials, for example, there are considerable morphological differences between the
wombat, the bandicoot and the "native cat". A second problem is that in
comparing banding patterns which are similar but may not be identical, preconceived ideas
of ancestry can bias one's conclusions (e.g., see above under Primates). If one
assumes that two similar species do in fact have a common ancestor, then one is committed
to finding a way of matching the banding patterns. In view of the subjectivity involved in
matching chromosomal banding patterns, one might wonder about the significance of a 25% or
50% match of banding patterns, especially it no entire arms can be matched.
Hypothesis 3. Chromosomal similarities are due to random changes
which happen to produce the same banding pattern in different species. This
hypothesis implies that each originally created species had a unique karyotype. It also
implies that chromosomal similarities have no real significance. If a series of patterns
is made by randomly arranging dark and light bands, it is inevitable that some patterns
will be repeated by chance. Thus the similarities of chromosome banding in humans and apes
and within some other groups could be held to be merely a result of chance.
This hypothesis does not seem reasonable for two reasons. Similar
chromosome banding patterns are not found randomly distributed throughout all taxonomic
groups, but rather are found in groups which share morphological similarities. This argues
strongly against any random cause of the banding patterns. In fact, it is known that
breakage points in chromosomes are not random (Jacky, Beek and Sutherland 1983), and so
changes in a karyotype will not be random. The phenomenon of "karyotypic
orthoselection" also shows that chromosomal changes are non-random. If chromosomes
have non-random breakage points, similar (parallel) rearrangements could occur
independently in similar karyotypes (see below), but it is unlikely that convergent events
would occur in different karyotypes to produce similar results.
Hypothesis 4. Chromosomal similarities are the result of non-random
changes due to viruses or transposable elements. This hypothesis requires that either
1) karyotypes which were once different have been caused to become similar, or 2)
karyotypes which were once similar have been changed in a similar way, due to the action
of transposable elements or some similar mechanism.
Transposable elements (TEs) are known to increase the rate of
chromosomal rearrangements in a non-random way (Nevers and Saedler 1977). But if the
karyotypes were substantially different, there is no reason to expect them to change to be
the same, since the insertion sites of TEs appear to be at least partially
sequence-dependent (see Inouye, Yuki and Saigo 1984; Shapiro 1979).
Similar (parallel) changes do sometimes occur in similar species, as
has been shown in several studies (e.g., Robbins and Baker 1981; Baker, Koop and Haiduk
1983; Baker, Bickham and Arnold 1985; Searle 1984). A common ancestry is plausible in each
of these cases, and it is likely that the rather similar species have undergone numerous
chromosomal changes during speciation, some of which happened to be the same.
It seems more likely that transposable elements could cause karyotypes
which were originally similar to become different. This could occur if different species
were infected by different TEs or retroviruses having different effects on the genome (see
Rose and Doolittle 1983). It seems possible that different TEs might affect a genome in
different ways. As a hypothetical example, it seems possible that the ancestors of
oryzomine and peromyscine rodents (groups of rats and mice) could originally have had
similar karyotypes when infected by different TEs. The TE(s) infecting the peromyscine
lineage might have caused a series of heterochromatin additions and pericentric
inversions, while the TE infecting the oryzomine lineage might have caused a series of
fusions.
CONCLUSIONS
It is possible that chromosomal similarities have different
explanations in different groups of animals. It this is true, then one must be cautious in
using chromosomal comparisons to determine relationships. Nevertheless, chromosomal data
can serve as a useful check on data from other sources.
Hypothesis 3, that chromosomal similarities are due to random
chromosomal rearrangements which happen to produce similar banding patterns, is not
reasonable, for reasons discussed above. Hypothesis 2, that chromosomal similarities are
exclusively the result of common ancestry, does not seem consistent with creation theory
and does not seem a necessary conclusion from the scientific data. The fact that very
large genomic rearrangement does not seem to affect morphology, and yet animals with
different body plans ("Bauplan") appear to have very different kinds of
karyotypes suggests to this writer that some different groups had different starting
points and do not share a common ancestry.
Hypotheses 1 and 4 seem consistent with both creation theory and the
evidence available. It seems likely that species which were morphologically similar were
created with similar chromosomes, reflecting their genetic similarity. It is evident that
large changes have occurred in chromosomes since creation. These changes have often
resulted in karyotypic divergence and have contributed to the multiplication of species.
Chromosomal rearrangements seem to occur so frequently that one would
expect to find very little banding homeology between species which supposedly diverged
long ages ago, such as the marsupials. The existence of numerous banding homeologies can
be explained as the result of a common design which has been preserved only because a
relatively short time has been available for changes to occur.
How much anatomical change has occurred since creation is still an
unanswered question. Chromosomal comparisons suggest that new genera may have arisen since
creation, for example among the antelopes which share a Y/autosome translocation
(Benirschke et al. 1980). Whether larger changes have occurred cannot be determined from
chromosomal studies. At the present time there is no known mechanism by which changes in
organisms can take place which are large enough to account for the differences among, for
example, the Australian marsupials or the various families of artiodactyls (cattle,
giraffes, deer). The absence of fossil evidence linking different groups by a common
ancestry, together with the lack of biological evidence of a mechanism for such change,
seem consistent with the hypothesis that they have separate ancestries.
LITERATURE CITED
COVER PICTURES
Front Cover: The chimpanzee (Pan troglodytes). The banding patterns of chimpanzee chromosomes show extensive matching with those of human chromosomes. For a discussion of the significance of chromosomal matching, see the article by James Gibson, pp. 9-35.
Back Cover: The markhor (Capra falconeri), a species of goat native to the mountains of Afghanistan, Pakistan and nearby regions. G-banding patterns of goat chromosomes match well with those of many other species of artiodactyls (mammals with split hoofs), leading to the suggestion that many artiodactyls descended from an ancestor with a goat-like karyotype. Cover photographs taken at the Los Angeles Zoo by Katherine Ching.
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Geoscience Research Institute. All rights reserved.
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