>>As indicated above, a chihuahua will breed with XXX, and XXX will
breed with a St. Bernard. >>
True a chihuahua will breed with xxx and xxx with the St. Bernard. My point
was that if they intermediates were eliminated, then the link would be
severed. You would have two non-interbreeding populations. But this
reproductive isolation can be accomplished in a more subtle fashion in some
animals.
The horses make an even more interesting situation. The modern horse has 64
chromosomes. Przewalski's horse has 66. Przewalski's horse is the horse
depicted on cave walls in France. What has happened is that one of the horse
chromosomes has broken giving Przewalski an extra pair of chromosomes.
Amazingly, Przewalski's horse and the modern horse can produce fertile
offspring. The zygote gets 32 chromosomes from the horse and 33 from
Przwalski's horse. Two of the Przewalski's chromosomes align with the
unbroken equus equus chromosome and a fertile offspring is created
. But there is an indication that this process has been going on in other
species for a longer time. The donkey which can not produce fertile
offspring with the horse, because of inversions on the chromosomes and the
fact that they have 62 chromosomes and can not have them correctly align..
Other horse-like animals show different chromosome numbers also. The Persian
onagers can have 55 or 56 chromosomes, kiangs can have 55 or 56. Kulans have
54 or 55. So we can see cellular morphology changes which lead to
reproductive isolation and morphological change between these species. Where
is the evidence of a limit? How can I predict exactly what the extent of
morphological change is in these equines? If this is the process which gave
rise to the multitudinous species of fossil horse, all of whom display
relatively gradual morphological change in relation to the age of the fossil,
(the further back in time the more unlike the modern horse the fossils are)
why does this not constitute evidence of massive morphological change?
David Tyler wrote:
>>YECs and PCs do (or should!) "believe" in speciation. Evidence for
speciation does not constitute proof for large-scale evolution, as
the same evidence fits happily within the YEC and PC thinking of
variation within a kind.<<
Unfortunately, not all do believe in speciation. They run into quotations
like.
"An average of at least one species has become extinct every day since
records have been kept, but no new species have evolved during that time."
Henry M. Morris, "The Logic of Biblical Creation," Impact, 205, July, 1990,
p. ii. Notice the date here - only 5 years ago.
One can see an article by David Paul Licata in Creation Research Society
Quarterly, June, 1979, which says the same thing.
David Tyler wrote:
>>This is why picked up the issue of limits to variation: the
neoDarwinians must justify this assumption if their evolutionary
theory is to be considered science.<<
Why should they have to be the only ones to prove their point? Why are
anti-evolutionists exempt from having to justify their position? What you
are asking is the impossible. Since logically it is very difficult to prove a
negative (it can't be done), it seems that it would be more fruitful for the
anti-evolutionist to prove the positive i.e. that there is a limit to
variation. No one can prove that there is NO limit because that violates the
laws of logic. But you as a believer in limits, should be able to prove the
extent of variation.
references
Ryder, O.A. 1978. Chromosomal polymorphism in _Equus hemionus_.
Cytogenet.Cell Genet. 21:177-183.
Ryder, O.A. and L.G. Chemnick. 1990. Chromosomal and molecular evolution in
Asiatic wild asses. Genetica 83:67-72.
Short, R.V., A.C. Chandley, R.C. Jones, and W.R. Allen. 1974. Meiosis in
interspecific equine hybrids, II: the Przewalski horse/domestic horse hybrid.
Cytogenet. Cell Genet. 13:465-478.
Trommershausen-Bowling, A., and L. Millon. 1988. Centric fission in the
karyotype of a mother-daughter pair of donkeys (_Equus asinus_).
Cytogenet. Cell Genet. 47:152-154.
Whitehouse, D.B., E.P. Evans, W. Putt, and A.M. George. 1984. Karyotypes of
the East African common zebra, _Equus burchelli_: centric fission in a
pedigree. Cytogenet. Cell Genet. 38:171-175.