At 09:25 PM 7/18/98 -0400, Francis Maloney wrote:
>A horse and a donkey can interbreed but produce sterile
>offspring. Assuming a common ancestor, have they diverged to the point
>where they are no longer of the same "kind"?
> On the other hand, shifts in relative gene frequencies certainly are
>not evidence of evoution.
A minor correction here. About 1 in 10,000 donkeys are fertile!
(."~Lorraine Travis, The Mule, J. A. Allen, 1990, p 68-69)
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 was not found alive until the late 1800's. 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
(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)
The horse, equus equus, has 64 chromosomes, Przewalski's
Horse has 66 chromosomes. However, the horse and Przewalski's horse can mate
and produce fertile offspring. The reason is that one of the horse
chromosomes broke in to giving rise to Przewalski's horse. During mating,
the two broken halves in Przewalski's horse can align with the unbroken equus
chromosome. One might be tempted to say big deal except that all horse-like
animals appear to have arisen this way. The plains zebra has 44 chromosomes,
but some have 45. Assuming that two 45 chromosome zebras mated you could get
a 46 chromosome zebra. Burchell's zebra is such a beast. Donkeys have 62
chromosomes (one has been found with 63); Persian onagers have 55 or 56
chromosomes, kulans have 54 or 55 and kiangs have 55 or 56. This seems to be
an excellent example of gradual chromosomal and morphological change.
glenn
Adam, Apes and Anthropology
Foundation, Fall and Flood
& lots of creation/evolution information
http://www.isource.net/~grmorton/dmd.htm