Since I picked on Steve, I might as well also pick on Glenn.
Speaking of those two, I really appreciate their contributions,
but how about getting greater participation from others. This is
one of the few forums where thoughtful folks can raise academic
questions about the evolutionary theory within the framework of
their evangelical faith. In any case, Glenn raised polyploidy
as a solution for increasing DNA in organisms. He said (on Fri
09 Jun 1995 12:07 CT in a post entitled, self-order,organizing &
program): "The universe can easily tolerate small reductions of
entropy in a stepwise fashion but not large ones all at once. In
plants a similar type of process (a doubling of the amount of
DNA) has been observed to create new species and is believed to
have been the basis for the formation of wheat, corn and many
other useful plants. It is called polyploidy. In one
generation, a new species of plants is generated. There is no
gradual evolution of parent to daughter species. And the new
species are incapable of crossing with the parent."
I would like to address some questions about polyploidy and
the role it plays in evolutionary theory and would especially
like to hear from some biology types whose genetics courses were
taken a more recently than mine. It sounds from this post that
polyploidy is an easy way to get new species and my understanding
of biology (which I try and teach at the colegatee level) is that
it doesn't provide the mechanism at least directly.
Since everyone on this reflector is not a biologists, let me
share my BASIC understanding of polyploidy and then move to ask
some questions. I am not as strong in genetics as I am in other
areas, so let me know if I have it wrong somewhere. Polyploidy
involves doubling the number of chromosomes. In normal
reproduction of eukaryotic cells, the chromosomes must double and
then separate. In the case of polyploidy, the cell division
after doubling does not take place and one is left with a cell of
double the number of chromosomes. But this will be two identical
sets, without any new information. If you had 23 pairs you will
now have 46 pairs. Polyploidy can be induced experimentally with
an antimitotic drug like colchicine (Its effect is to disrupt the
microtubules that are needed to pull the chromatids apart into
separate cells). In fact, I think it has been used on some
commercial fruits like ?apples. They are similar apples to the
non polyploid root with a larger fruit. In this case, I am not
sure they would be considered new species although with the
different chromosome number you could not cross them back to the
non polyploid parent stock. Glenn is I believe right that Corn
and other crops are usually considered polyploids of smaller
native species. But I don't think polyploidy is used to account
for most traits besides larger size in the crops. The other
traits would be produced by selective breeding of genes already
in the gene pool or any that would be produced by mutation. It
is also my understanding that some organisms are more apt to
produce polyploidy. It is more common in plants than animals,
where it is fairly rare, although I believe some amphibians are
polyploid. Even among plants some groups are more apt to have
polyploids. I believe some of the highest polyploid numbers
occur in the ferns.
After I talked this over with one of my Ag brethren, who knows
more genetics than me, he agreed that most commercial polyploids
would not be considered different species. In fact, one of the
common results is sterile offspring and many of our seedless
varieties are polyploids. He mentioned bananas as examples (they
are 3N) and seedless watermelons.
With that as a background, let me ask a few questions.
1. In evolutionary theory is polyploidy seriously considered as a
mechanism for increasing base chromosome numbers. It struck me
that it is about the only mechanism I know of that would increase
the number of chromosomes (other than the case of sexual
reproduction of diploid gametes - which is not that common). I
don't think you can as easily add to the DNA in a chromosome as
you can in a prokaryotic (bacteria).
NOTE:
I would still like some answers to this question, but after
talking to the same knowledgeable Ag brother, he mentioned the
Robertsonian translocations, which if I understand it allows
acrocentric (centromere or kinetochore at one end) chromosomes to
fuse a meiosis. Apparently you can also get it to go the other
way and do in some cattle (#1 and 21 combine). He used the
example of goats and sheep. That would be a more powerful
argument for you Glenn. The goats (assumed progenitors) have 60
acrocentric chromosomes and sheep have 48 acrocentric and 6 meta
centric. Although that will give you only 54 total. if you
divide the 6 meta you have your 60 acrocentric. Apparently there
has even been some crossbreeding and you can get nice match of
the chromosomes. You still do not get any new genetic information
but it at least gives you a mechanism of changing number without
doubling. If you are making a distinction between micro and
megaevolution, goats are still pretty similiar to sheep and you could
call this micro.
2. If you use this mechanism and I do want to know if it is
seriously cited in the literature, you still have the same genes
you started with, just more of them. Unless you can change the
rates of mutation or modification of identical genes of different
chromosomes (and I don't know how you would do that) I am not
sure it gives you any advantage. In other works, you want to
protect the integrity of the genes on one set and let the other
sets change.
3. Is there any indication that chromosome numbers reflects
supposed evolutionary phylogeny on a large scale. I am not sure
I have heard of this and am pretty sure it would have been
brought to my attention if it were the case. In fact some of the
more "primitive groups" like the ferns have high chromosome
numbers and I am not sure that the angiosperms are generally
higher than "lower vascular plants".
4. Of course another problem is how to hypothesize the transition
from erkaryotic DNA with the DNA integrally bound to histomes and
in definite chromosomes from the prokaryotic DNA which is not
membrane bound and is a circular ring. I don't think
endosymbiosis will work for that. The silence on that point in
my cell texts makes me think like Mike Behe would say that there
is NO, ZILCH, NO experimental evidence that would provide a model
for how you could get that transition. Am I right or is there a
model I have missed.
-- :James F. Mahaffy e-mail: mahaffy@dordt.eduBiology Department phone: 712 722-6279Dordt College FAX 712 722-1198Sioux Center, Iowa 51250