There are those on the board more qualified than I to speak to this, but I
always follow the advice of that old song, "Fools rush in..."
I don't know the technical details of how this happens but some animals seem
to evolve by chromosomal fusion and chromosomal breakage. Przewalski's horse
is the horse that was drawn on cave walls in Europe. It is now extinct in
Europe but was found alive in central Asia. It has 66 chromosomes. The
modern horse has 64. It is believed that either Przewalski's horse arose from
the modern horse by chromosomal breakage or that the modern horse arose from
Przewalski's by chromosomal fusion. These two horses can breed and produce
fertile offspring. The two broken chromosomes of the Przewalski align with
the unbroken one of the modern horse. By this means, gene flow can take place
between these two creatures. There is other evidence of this type of
evolution in equines. The donkey has 62, Burchell's zebras (or "plains zebra")
have 44 chromosomes, but some have 45 chromosomes due to a chromosome that
broke in two. If two 45-chromosome zebras mated, it is possible that
you could get a 46-chromosome Burchell's zebra! Persian onagers can have 55
or 56 chromosomes; kulans can have 54 or 55; and kiangs can have 55 or 56.
References below
This process has been observed in modern times. On the island of Mauritius,
there were no rats 400 years ago until western ships started going to the
island. This is known from history. Some rats escaped from the rat infested
ships. However, the chromosomes of the Mauritius rats are unique and found
nowhere else on earth. Yosida et al tell of the chromosomes of normal rats.
"Depending on chromosome numbers, the black rats, Rattus rattus, are
classified into three geographical types, such as Asia (42 chromosomes),
Ceylon (40 chromosomes) and Oceania (38 chromosomes)."~T.H. Yosida, et al,
"Mauritius Type Black Rats with Peculiar Karyotypes Derived from Robertsonian
Fission of Small Metacentrics," Chromosoma, 75: 51-62 (1979), p. 51
but those on Mauritius are described as follows,
"Four among the 17 black rats from Mauritius were found to have 43 and
44 chromosomes. Among them three rats had similar karyotype to the 13 black
rats described above by having 10 small metacentrics and 10 small
acrocentrics."~T.H. Yosida, et al, "Mauritius Type Black Rats with Peculiar
Karyotypes Derived from Robertsonian Fission of Small Metacentrics,"
Chromosoma, 75: 51-62 (1979), p. 55
They conclude:
"There are many researchers who have studied the chromosomes of the black rats
from several locations of the world, but none has observed in them the
karyotype characterized by the Robertsonian fission as seen in the Mauritius
type. Chromosomes of the black rats in Madagascar which is located near
Mauritius Island were of the typical Oceanian type . Chromosomes Recently we
had an opportunity to study on chromosomes of the black rats from the
Seychelles Island in the Indian Ocean, but they were of a regular Oceanian
karyotype. Based on the above consideration, it seems to be possible that the
Mauritius-type fission has occurred in Mauritius itself at some time since the
16th century. Fission of took place in the small metacentric pairs no. 14 and
no. 18. Probably during the last 400 years rats carrying small acrocentrics
would have propagated in Mauritius and replaced the Oceanian type ancestor
rats. One rat studied here still showed a heteromorphic pair no. 18 showing
that the original metacentric no. 18 still remains in this island."~T.H.
Yosida, et al, "Mauritius Type Black Rats with Peculiar Karyotypes Derived
from Robertsonian Fission of Small Metacentrics," Chromosoma, 75: 51-62
(1979), p. 59
Pigs, dogs etc seem to have evolved in this fashion. The interesting thing is
that we have 46 chromosomes and the African apes have 48. It would appear
that what happens with rats, horses, pigs and canids also happened with man.
This is especially true since we share pseudogenes(broken genes) at the same
DNA locations with the African apes.
(see ~R. V. Short, A. C. Chandley R. C. Jones and W. R. Allen, "Meiosis in
interspecific equine hybrids II. THe Przewalski horse/domestic horse hybrid"
Cytogenet. Cell, Genet., 13: 465-478 (1974), p. 476
and
~A. Trommershausen-Bowling and L. Millon, "Centric fission in the karyotype of
a mother-daughter pair of donkeys (Equus asinus), Cytogent. Cell Genet. 47:
152-154 (1988), p. 153
and
~O.A. Ryder, "Chromosomal Polymorphism in Equus hemionus, Cytogenet. Cell
Genet.21:177-183 (178), p. 178-179)
glenn
Foundation,Fall and Flood
http://members.gnn.com/GRMorton/dmd.htm