A while ago read Dr. Kenneth Miller discussing the fusion of human chromosome 2. To paraphrase what he said: When the fusion first occurred an individual would be born with only one chromosome less, then that began to spread through the population, until, eventually, a pair of hominids that each had the fused chromosome bred together and the offspring had 2 missing chromosomes. But I was just reading an article discussing speciation of mice on Madeira, it says:
“In order to reproduce successfully, both parents must have the same number of chromosomes. So when a population develops a chromosome fusion, suddenly that group cannot mate with the other members of its species.”
If this is right then when human chromosome 2 first fused and an individual was born with 1 chromosome less, he/she couldn’t reproduce, so the variant couldn’t spread through the population, and if it couldn’t spread through the population a pair of hominids each having the fused chromosome would never exist and so wouldn’t ever breed together, and so there would never be any offspring born with 2 chromosomes missing. I’m assuming I must be missing something here?
I originally came across the above information while considering the creationist claim that there’s some limit to evolution so that it’s not possible to ever have a new species evolve (by “species” I specifically mean a population that can no longer breed with the “original” population). I know about the ring species concept, but as Jerry Coyne has pointed out, there are no examples of ring species in nature, and the commonly cites examples really don’t fit the definition if ring species. So if anyone knows of an example of a new species (according to the above definition of species) that show that there is no limit as creationists claim (and hence what they call a “kind” can change) that would be helpful, thanks.
As I understand it, a missing chromosome in one parent doesn’t automatically mean they can’t reproduce (ei: the human Y chromosome is a partly missing X).
I found this quote in an old Biologos post. I knew it was unlikely to be from the scientific literature, because it’s not true. (In biology, there are few stark laws like the one in that quote, which is the first hint that there’s a problem.) Aneuploidy is indeed a barrier to successful reproduction in general, but “suddenly that group cannot mate” is just much too strong a claim. I would suggest to @Elliot_Mudd that he identify his sources to help discussion, and that he give thought to how hard-and-fast law-thought can make biology impossible to understand.
I’ve seen Coyne’s older article (blog post) on this topic, and it’s great. The reason that “true ring species” examples are hard to find is that there are indications that the “rings” are interrupted by barriers to gene flow. (Such as geographical barriers.) The irony, IMO, is that the ring species concept is hard to see in nature because very well-known mechanisms of speciation (that’s the whole topic, isn’t it?) are right there in the middle of it.
Thanks for the reply. Yes, I forgot to mention the source of the quote. I didn’t think it sounded right, but guessed I must have misunderstood them as I’m not an expert in the field. Thanks again.
I would recommend not using them as a source of information in the future. They are clearly not sufficiently competent.
When a fusion happens, carriers will produce mostly unbalanced chromosomes. By carriers I mean individuals with a fused chromosome and a set of normal chromosomes, and by unbalanced I mean gametes either with the fusion and one of the normals, or just one of the normals. They will also produce balanced gametes, with either both normal chromosomes or just the fusion chromosome. When mating, balanced gametes will be viable, and the balanced gametes with the fusion will pass that fusion to the next generation. Since the carrier mostly makes unbalanced gametes their fertility will be lower, but they will absolutely still be capable of mating successfully.
What was your source? It’s best to give a complete citation. At any rate, what it says is not true, at least for the general case. In a chromosomal fusion, nothing is lost. The two original chromosomes can pair with the homologous parts of the fused chromosome with only a small loss of fecundity.
There is actually one ring species that’s still a live proposition: the greenish warbler Phylloscopus trochiloides. Irwin DE, Bensch S, Irwin JH, Price TD. Speciation by distance in a ring species. Science. 2005;307(5708):414–416.
Thanks for explaining that. If I may just check if I’ve got that:
So when the fusion occurred there was a set of 23 chromosome and a set of 24 chromosomes. That individual would produce gametes with:
A single set of normal chromosomes (1 x 24)
A set of 23 chromosome and a set of 24 chromosomes
A single set of chromosomes including the fusion (1 x 23)
Two sets of normal chromosome (2 x 24)
So some offspring will have a set of 23 chromosome and a set of 24 chromosomes. So I’m presuming that at some later point two individuals both with the fusion mated and thus the offspring had 46 chromosomes, as we do today. Eventually two individuals both with 46 chromosomes mated and due to genetic drift the entire population eventually 46 chromosomes?
So you mean the carriers of the unbalanced gametes are less fertile, but also viable and as such the fusion will still be passed on?
Needless to say, the cited papers do not support the quote. Even other bits of the same article don’t support it. For example, “In only a few hundred years, the original populations of Mice had separated into six genetically unique species. The first mouse populations had 40 chromosomes altogether. But the new ones were quite different. Each new variety had its own unique combination of chromosomes, which ranged in number from 22 to 30.” This is not a single fusion: it’s a combination of many large chromosomal mutations, most of them likely postdating speciation.
Nope. All gametes are haploid, so 2 and 4 are out. The problem is potential aneuploidy. In addition to 1 and 3, you might end up with the fusion plus one of the unfused chromosomes, or possibly one of them minus the fusioin. It all depends on whether the two unfused chromosomes are pulled into the same gamete (good) or into different gametes (bad).
Here’s a great nugget from that PZ Myers about Luskin’s ludicrous genetics:
Does anyone else have this mental image of Discovery Institute “scholars” poring over science papers with almost no comprehension, but happily plucking out random sentences here and there that they can misuse? I suspect they have a compendium of such fragments that their fellows use, without the need of ever having to actually read any science.
Yes, I do have that mental image of Discover Institute “scholars.” I’ve always assumed that it was in the official job description for all staff members.
This old webpage also explains what happens when chromsome fuse (or more accurately, it’s called a Robertsonian translocation).
Below we have one individual who has inherited two-fused chromsomes from one parent, and two seperate chromosomes from the other. These chromosomes are still able to line up and undergo homologous recombination like usual. Furthermore, the gametes that result from meiosis can still be viable if one gamete gets the fused chromosome and the other gets the 2 that remains seperate.
Although, there is a downside that this situation will increase the prevalence of gametes that have an extra/missing chromosome.
This means that some gametes will be non-viable. Although this could reduce fertility, but viability is still plausible. People could be walking around with a newly fused chromosome, carrying 45 or 44 chromosomes (instead of the common 46) and not even notice it for their entire lives. We actually know of some of these people. For example, a family in Finland have been carrying a Robertsonian translocation of chromosomes 13 and 14 for nine generations. One individual has inherited this chromosome from both parents, thus having 44 chromosomes in total.
(This no doubt dates me but my concept is meant to play off of what I recall was a New Yorker cartoon series of long ago with the title: “Another Day in the Salt Mines” or something like that. It was always about workplace humor.)
If you are looking for examples of things bringing forth other than according to thier kind, I have your back.
I did a series of 2 debates with Kent Hovind himself about this issue. I believe I make a fairly compelling case that CVTV (canine venereal tumor disease) and DFTD (devil facial tumor disease) as well as the HeLa cells are all fairly strong examples of living things bringing forth other than according to thier kind.
I had a bunch of other examples, but we never really got to them because Hovind couldn’t really deal with the ones I gave him.
If you want to know what else I had cooking, let me know.
Not in the way creationists mean when they expect a dog to give birth to pig. Obviously that’s utterly insane. I just mean an example of a population diverging into two separate species that continue to evolve to the point that they cannot bred with one another at all. I was particularly looking for an example that involves species that are multicellular. Thanks for the video, I’ll take a look. I can guess Kent’s response even before I watch it: “but it’s still a type of cancer sir, it didn’t change into something else, ok.”
The beauty of it is that this is an example of a dog giving birth to a venereal disease. Cells in the dog stop being part of the dog and start living on thier own as a single celled parasite in other dogs, and spread like a pathogen.
Yes, he does try to say “Well that’s just a type of cancer” but unfortunately for him, that was after he had already agreed that if the cells in my arm stopped cooperating with my body and started living as single celled organisms that this would count as an animal bringing forth other than it’s kind. He then asks why we are talking about this, and it is at that point that I tell him that fundamentally that is what cancer is. And I give him a paper to show that from an evolutionary perspective cancer is the result of a mutation of genes that code for co operation, and that when such mitations occur the cells start acting like single celled organisms and reproducing without regard for the body.
In the second debate he gets pinned down to the point where he refuses to answer the question of whether he thinks dogs and venereal diseases are the same kind of life.
It really is a great watch.
Also as I said, I have more. Here is a little gem I never got to use (in my mind he is beaten at this point):
If you are looking for the example of a scene out of “Alien” where the mother gives birth to something completely different from herself, but it can’t just be where she is an incubator or ecosystem, her cells have to manufacture and bring forth something that is completely different from her using thier reproductive machinery, then I would have one question for you: how do viruses reproduce?
