Different numbers of chromosomes?

I have a question for the biologists in the group. How exactly can a new species end up with a different number of chromosomes than the species from which it descended? For example, hares have 24 pairs of chromosomes, while domestic rabbits have 22 and cottontails have 21. Back when I was a creationist, I would use this discrepancy as an argument against macroevolution. I'm just curious about the mechanism of how this works, thanks.

I'm not a biologist, but...
one excellent example of this is that all the great apes except humans have 24 pairs of chromosomes, while we have 23 pair. Our chromosome 2, though, looks exactly like two great ape chromosomes fused end-to-end - it even has the remnants of a spare centromere and two telomeres in its middle. Yunis, et al, in Science, 208, 1145-1148 (1980) have very nice pictures of this, but they are also on the web somewhere - Internet Infidels discussion board for one place, with "Scigirl" (from Bozeman!) explaining them.
It seems pretty reasonable that a rare fusion event like this might not be a big obstacle to reproduction, either: unfused chromosomes from one gamete should pair right up with a fused version. It might get tricky when the zygote tried to divide, but hey - it only had to happen once.

Dear Coragyps,For a elegant falsification go here:http://www.europa-universalis.com/forum/showthread.php?th...It is a bit much, but the final page says it all.Best wishes,
peter

Whenever this issue raers its ugly head I always
say look up Down's syndrome.I'm not sure exactly why or how such a thing happens,
but Down's syndrome individuals have a different number of
chromosomes to their parents, and most are fertile.That means it's possible for chromosome numbers to change
within a population.

Hi Ryan,There are a number of possibilities - chromosome fusion being one of them. Polyploidy (chromosome doubling) can also happen, although it's waaay more common in plants than in animals. As long as the mutation/doubling doesn't present a reproductive barrier, then the organisms are usually still considered the same species. The point is "species" is something of a squishy term - most are composed of discrete populations (called demes on the genetic level, or clines on the population level), that blend into each other where these populations overlap.If you'd like to look up further references, and have access to a decent library, here are three articles that you might find interesting from the journal Heredity:Castiglia, R and Capann, E wrote a series of three articles for the journal Heredity describing this observation:
Contact zones between chromosomal races of Mus musculus domesticus. 1. Temporal analysis of a hybrid zone between the CD chromosomal race (2n=22) and populations with the standard karyotype.
Heredity. 1999 Sep; 83:319-26Contact zone between chromosomal races of Mus musculus domesticus. 2. Fertility and segregation in laboratory-reared and wild mice heterozygous for multiple robertsonian rearrangements.
Heredity. 2000 Aug; 85:147-56Contact zones between chromosomal races of Mus musculus domesticus. 3. Molecular and chromosomal evidence of restricted gene flow between the CD race (2n = 22) and the ACR race (2n = 24).
Heredity. 2002 Sep; 89:219-24.If you're lazy (like me ) I posted links to several more articles discussing the genetics of incipient speciation in this post, although not all are dealing specifically with chromosome (karyotype) changes.If you have any further questions, let us know. Happy reading!

I posted what I reproduce below in response to the post by "Ilion" that borger links to. Ilion, like Borgewr, is an admitted 'darwin attacker' and non-scientist. As is Ilion's custom, he ignoirewd the falsification of his claims:

quote:

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I quote from a resident creationist:

quote:

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"Don’t let the fact that in quote above Lindsay says If a human-chimp hybrid were to exist mislead you. We don’t know the species of our hybrid (2n=47) in question or its parent stock (2n=48), and we don’t need to know, because this problem isn’t species specific.

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As I stated above at the point when I asserted that our hybrid (2n=47) is actually sterile, after all, I realize that I could be wrong: I could be mis-remembering a lesson from long ago or new evidence may have shown the fact I learned to not always be true. Since I haven’t found anything that specifically backs up this fact (or my assertion of it) , I must rely on what I hope is correct understanding of the Britannica article I quoted. Specifically, Homologous paternal and maternal chromosomes pair up along the midline of the cell. Each pair of chromosomes--called a tetrad, or a bivalent--consists of four chromatids. At this point, the homologous chromosomes exchange genetic material by the process of crossing over In our hybrid (2n=47) the chromosome 2 pair are not homologous, implying that I am correct in my remembered fact."

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In response, I offer this, emphasis mine:

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Cytogenet Cell Genet 1978;21(4):177-83

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Chromosomal polymorphism in Equus hemionus.

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Ryder OA.

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The first cytogenetic studies of the Turkmenian kulan, Equus hemionus kulan, are reported, and a polymorphism in diploid chromosome number is described. Chromosome fusion is apparently involved in the alterations of the karyotype of E. hemionus kulan (2n = 55, 54) when compared to the karyotype of the onager, E. hemionus onager (2n = 56). Additionally, the rearrangement involved has been identified in animals unrelated through captive breeding; inheritance of the fusion chromosomes has also been observed.

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In other words, individuals of species with polymorphic karyotypes can produce viable offspring. The 'hole' supposedly poked is getting patched...

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I also offer this:

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quote:

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A minor correction here. About 1 in 10,000 donkeys are fertile!
(."~Lorraine Travis, The Mule, J. A. Allen, 1990, p 68-69)

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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.

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References

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(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

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and
~O.A. Ryder, "Chromosomal Polymorphism in Equus hemionus, Cytogenet. Cell
Genet.21:177-183 (178), p. 178-179)

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The above was written by (gasp!, eyeroll) Glen Morton. That is, of course, irrelevant as he provided references.

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It is obvious then that the claims of Ilion regarding the breakage/fusion of chromosome 2 is an evolution stopper are baseless. The exact mechanisms of how this happens are apparently unknown for certain at this time, but from these horse and horse-like exmaples, it is clear that differing karyotypes are not necessarily lethal nor do they necessarily produce infertile offspring.

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The poked hole is now fully sealed. And it did not take 14 pages to do it.

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That was not the point, Page, and you know it. The point was that the mechanism between fusion and transloctaion are distincly different.And you evo-guys put them on the same pile.Why do I still discuss with you, while you do not even understand molecular biology mechanisms? You are far out of date. I recommend you to buy yourself a recent biology book! And read it!Best wishes,
Peter

You gotta be shittin me?

That's it?

Peter B,Why not? You bailed on the intermediate fossil thread, & still think you have credulity re. Fossils, so I can reasonably ask, "that's it"?Mark------------------
Occam's razor is not for shaving with.

bailed out? I still wait for your reply.Best wishes,
Peter

You are in absolutely no position to criticize the educational background of anybody Peter..you are the most poorly informed biology Ph.D. I have ever encountered...but then you do medical research so that explains a lot

Hi Mammuthus,The 'Page Lane' is a dead end alley. You didn't know that?Best wishes,
Peter

PB: The 'Page Lane' is a dead end alley. You didn't know that?PB: "Why do I still discuss with you, while you do not even understand molecular biology mechanisms? You are far out of date. I recommend you to buy yourself a recent biology book! And read it!"M: So you mean you are retracting this post of yours to Mark24? Glad to hear it.My post was accurate, you are the most poorly informed Ph.D. I have ever encountered...a personal observation not an insult....my jibe at medical geneticists was a joke.As to dead ends....you are the expert at that, MPG, GUToB, creatons, morphogenetic fields, evolution is only about the origin of genes (microbe to man..etc) regardless of what real evolutionary biologists say.

Mammuthus: As to dead ends....you are the expert at that, MPG, GUToB, creatons, morphogenetic fields, evolution is only about the origin of genes (microbe to man..etc) regardless of what real evolutionary biologists say.PB: GUToB it's not only the end, it's also the start.Best wishes,
Peter