Perusing the comments at STACLU can be an amusing way to pass the time. You think the main writers there are clueless? You should see their fans. I particularly liked this little tidbit about evolution from kerwin_brown:
To change from horse to a donkey or vise versa requires at least one and possibly two monstrous changes because of the chromosome difference. This is very unlikely to occur in nature and yet there is evidence that it happens on a regular basis. It sounds like miracles are normal in nature.
If it's very unlikely in nature, yet it happens regularly, can it really be said to be unlikely? What kerwin calls a "monstrous change" is not at all unusual; the chromosome counts of various plants and animals can differ wildly because of duplication and fusion. The horse has 64 chromosomes; the donkey has 62. But even within horses, you have Przewalski horses from Mongolia, the most ancient extent species of horse, and it has 66 chromosomes). How could such distinctive chromosome counts happen in nature? Generally, through either duplication or fusion.
Duplication happens during recombination under certain circumstances and it has happened many times throughout natural history. We can identify duplicated chromosomes the obvious way - by sequencing all of the chromosomes in a genome and comparing them to one another. Likewise, we can do the same for chromosomal fusions. Indeed, this provides us with powerful evidence of common descent with the great apes. Why? Let me explain.
All other species of ape have one more pair of chromosomes than humans do - 24 pairs rather than 23 pairs. Geneticists had long assumed that this was the result of the fusing of two chromosomes at some point after humans and apes split off from a common ancestor, and now with the ability to sequence the entire genome and compare chromosome to one another, that has been confirmed. Chromosome 2 in humans is a fusion of two chromosomes shared by all other apes, and we can tell this because right in the middle of chromosome 2 in humans there is a fused telomere (telomeres are the ends of a chromosome), indicating two chromosomes fused together at their ends to form one larger chromosome. The banding patterns are a clear match and were predicted before hand.
Thus, what we have here is powerful evidence for evolution, an accurate prediction made in advance based upon the common ancestry of humans and the other apes. Given the chromosome counts, if we shared a common ancestor then one of two things must have happened: either a chromosomal duplication took place in the line leading to modern apes, or a chromosomal fusion took place in the line leading to modern humans. The two can be easily distinguished with genetic sequencing, and voila, we have our answer. It's unfortunate that so many are so ignorant of such facts.
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Anybody with only a rudimentary knowledge of evolution would know that it is all about cumulative very minor changes. Genetic mutations that lead to monstrous changes are pretty much doomed to the Darwinian shitpile.
I love it when these cretins attempt to argue science rather than demons and archangels as it only reenforces their ignorance.
You gotta like that logic. I can't imagine how it could happen in Nature, therefore it can't. But since it does, it can only mean that a miracle has occurred.
Hallaluah!
We should also note that having different chomosome numbers may prevent giving birth to fertile offspring (as is the case between horses and donkeys), but that this is not necessarily true in every instance. The wild Przewalski horse (n=33) will quite happily mate with the domestic horse (n=32) to give birth to fertile horselets.
Chromosome transloctions are surprisingly common even with humans. It is estimated that about 1 in 625 individuals have a mix up of chromosomes of some kind. All the genetic information is still present; just not on the usual chromosomes. Such individuals are usually completely healthy, but have troubles reproducing. See Chromosome aberrations at biologyreference.com.
It is possible to have a chromosome abberation passed on and even evntually fixed in a population, and this is presumably what happened in our ancestry. For some reason, mice are particularly likely to gain such chromosomal changes. See House mice on the island of Madeira
Ed,
I think you are spending way too much time on STACLU. They are becoming your version of "Uncommon Descent" and this thread is becoming a version of "Official Uncommonly Dense Discussion Thread."
I'd like to hear your take on this post over at Effective Measure: Wayne's World, the UN and guns
http://scienceblogs.com/effectmeasure/2006/06/waynes_world_the_un_and_g…
While I tend to agree with Spike regarding the amount of STACLU material, I like this particular post for the crunchy science.
Spike-
That cracks me up. I've seen a few references to the same thing on the Worldnutdaily. This notion that the UN is going to sweep in and take away our guns is a popular delusion among the hard right, but it's simply absurd. The UN has no standing army, folks, and the US sits on the security council with a permanent veto. Even if they wanted to, the US wouldn't let it happen. But this conference has nothing to do with individual gun ownership, it has to do with small arms trafficking, which is a HUGE problem for UN peacekeeping troops around the world and an even bigger problem for at-risk populations. In situations like Rwanda and Somalia, with warlords and butchers sparking civil wars and genocide, arms traffickers frequently sell weapons to both sides to keep it going. That's how they get rich. It's a hell of a problem that helps destroy millions of people and something surely needs to be done about it. If we can make it illegal to sell weapons to Iran, surely we can make it illegal to sell weapons to Sudanese and Somali warlords.
How many chromosomes have the colts from a Przewalski sire and non-Przewalski mare? And how does the oocyte manage that? (I'll confess up front to only nominal knowledge of biology.)
Russell wrote:
That I do not know, but from what I've read they can produce fertile offspring with a regular horse. That surprised me.
Russell asks:
2n=65. Or n=32.5
Good question. The Przewalski sire (Equus przewalskii) has 66 chromosomes arranged as 33 pairs and his mare (E. caballus) has n=64, or 32 pairs. What has happened here is similar what happened in the ape/human split. The Przewalski has 2 chromosomes which when put together are nearly identical to one of the horse chromosomes. The remaining chromosomes are also similar. Now the Przewalski/Horse split only happened 200,000 years ago or so, compared with the ape/human split of about 6,000,000 years. Not really very long. Consequently the 2 Przewalski genes together still line up quite nicely with the single matching horse gene.
As mentioned, this foal will have 65 chromosomes in total, 32 from mom and 33 from dad. When he grows up his own gametes will be produced via meiosis. In that process, during Metaphase I, the chromosomes match up in pairs and align at the metaphasic plate. But the two "broken" chromosomes from the Przewalski side align with the single corresponding gene from the horse side. So rather than 2 chomosomes as is normal, there are 3. The rest are just pairs as normal. So you'd see 31 pairs of chromosomes, and one triplet chromosome. These triplet chromosomes have actually been observed in the testes of young Przewalski/Welsh pony colts. The young colt ends up with half his haploid sperm domestic horse and half Przewalski horse, both being able to impregnate a mare.
It works only because the two "broken" bits are together nearly identical to the single chromosome of the horse. In animals that are most distantly related like the donkey, the fusion is good enough to produce viable youngsters, but not good enough to allow them to produce further young.
Literature support -
J. Reprod. Fertil. Suppl. 1975. Oct;(23):356-70
Cytogenetic studies of three equine hybrids.
Chandley AC, Short RV, Allen WR.
A detailed investigation of testicular meiosis in a mule, a hinny and a Przewalski horse/domestic horse hybrid were made. Abnormalities of pairing were observed in the mule and hinny in most germ cells at the pachytene stage of meiotic prophase, and spermatogenesis was almost totally arrested. A few mature spermatozoa were recovered from the ejaculate and epididymal flushings of the hinny. The Przewalski horse/domestic horse hybrid was fertile and showed normal spermatogenesis. Chromosome banding studies showed a close homology between the karyotypes of the Prezwalski horse (Equus przewalskii, 2n = 66) and the domestic horse (E. caballus, 2n = 64), and it is evident that a single Robertsonian translocation has occurred transforming four acrocentric chromosomes of E. przewalskii into two metacentric chromosomes in E. caballus. The investigations showed that a trivalent is formed at meiosis in the hybrid (2n = 65), segregation from which gives two classes of genetically balanced spermatozoa. Both of these are capable of producing normal offspring if they fertilize the eggs of a domestic mare.
Dave, thank you for the answer! That means the Przewalskii horse provides a living mammalian example of something that is a necessary part of evolution: a single species with populations that vary in chromosome number. It's pretty interesting that there are individuals whose somatic chromosome count is 65, while their gametes are either 32 or 33 by chance, and that the two varieties of haploid are mutually compatible, so that descendants of two 65 chromosome parents can have 64, 65, or 66 chromosomes. My mediocre biological knowledge increases by the blog!
However, fertile mules are likely more common than people believe. It does seem that these mares (why mares? see belows) generally produce eggs with a complete set of horse chromosomes. For instance, there was a fertile mare mule in the 20s at Texas A&M (Old Beck) who was bred repeatedly - she gave birth to horses when bred to a stallion, and to mules when bred to a jack. Others have been recorded, and most seem to follow Old Beck's pattern, though one in China apparently produced a foal with a mixed set of chromosomes.
Why only the mares? Probably because male mules are so routinely gelded that any fertile ones have been missed.
Caveat - it's been thirty years since my animal husbandry classes at university! More data may well be available now.
This does indicate, however, that a slight mismatch of chromosome number may not be an insuperable barrier to reproduction.
I feel kind of bad arguing with Kerwin, because it's kind of like punching a retarded baby. I've pretty much given it up. He's a Borkian Constitution Party quack who might just be a parody - and a very good one at that. No amount of evidence or reasoning will sway him from his ultra-ultra-ultra-right-wing ideas. He's so far to the right that he fell off the spectrum years ago and there's no coming back. Please, Hammer; don't hurt 'im.
Russell -
Something similar in the plains (Burchell's) zebra. Most have 2n=44, but some have 2n=45 chromosomes. And don't get me started on plants!
From what I have read, it is known that there are humans with more than the normal pairs of chromosomes, particularly among the X and Y chromosomes. XXY, XXXY and XXYY patterns are known. Apparently, most of them have female attributes, and apparently they have no trouble reproducing.
I know a few posters on other boards with similar attributes, but you'd think a parodist would gett tired after a year or two. These folks just keep going, and going, and going . . . They make that damned bunny look lazy.
Even more for Russell: The tufted deer (Elaphodus cephalophus) also has polymorphic karyotypes with three different diploid numbers: 46, 47, and 48.
More interesting is its close relatives: the Chinese muntjac deer has 2n=46, the Indian muntjac has just 2n=6fem,7male. They produce viable but sterile hybrids -- rather, sterile-to-date, since only a handful of crosses are known, but some with good sperm. The 5 other muntjac "species" have intermediate 2n numbers.
Three cheers for large mammals diverging before our eyes. (All the multi-karotypic mice and rats get a raspberry.)