…and this is a 
Basics: How can chromosome numbers change?
Category: Evolution • Genetics • Science
Posted on: April 21, 2008 10:43 AM, by PZ Myers
There in the foaming welter of email constantly flooding my in-box was an actual, real, good, sincere question from someone who didn't understand how chromosome numbers could change over time — and he also asked with enough detail that I could actually see where his thinking was going awry. This is great! How could I not take time to answer?
So here's the question:
How did life evolve from one (I suspect) chromosome to... 64 in horses, or whatever organism you want to pick. How is it possible for a sexually reproducing population of organisms to change chromosome numbers over time?
Firstly: there would have to be some benefit to the replication probability of the organisms which carry the chromosomes. I don't see how this would work. How is having more chromosomes of any extra benefit to an organism's replicative success? Yes, perhaps if those chromosomes were full of useful information... but the chances of that happening are non existent and fly in the face of 'small adaptations over time'.
Secondly, the extra chromosomes need to come from somewhere. I'm not sure about this, but I believe chromosome number are not determined by genes, are they? There isn't a set of genes which determines the number of chromosomes an organism has. So the number is fixed, determined by the sexually reproducing parents. Which leads me to believe that if the number does change, and by chance the organism is still alive and capable of sexual reproduction, that the number will start swinging back and forward, by 1 or 2, every generation, and never stabilising. The chances of this happening are also very very slim.
Let's clear up a few irrelevant misconceptions first. Life probably started with no chromosomes — early replicators would have been grab bags of metabolites, proteins, and RNA that would have simply sloppily split in two, with no real sorting. DNA and chromosomes evolved as accounting and archiving tools: they were a way to guarantee that each daughter cell in a division reliably received a copy of every gene. Also, most living things now just have one 'chromosome', a loop of DNA, and perhaps a small cloud of DNA fragments. So to keep this simple we're going to ignore all that, and consider only us diploid eukaryotes, where the question of chromosome numbers becomes a real issue.
Normally, I'd be scribbling madly on a whiteboard, so we'll have to make do with some scribbles on the computer screen. Here, for instance, is a typical cartoon chromosome. It's a string of DNA, and scattered along it we have sequences for genes, that I've labeled "A", "B", "C", "D", and "E". I've also drawn a circular blob in the middle: that's important. It's not a gene, it's a structure called the centromere, which gets all wrapped up in proteins to form a kinetochore. It's a sort of anchor point; when the cell needs to move chromosomes around, as it does during cell division, it hitches motor proteins to the kinetochore and using drag lines called spindle fibers, tows it to a new destination.
I mentioned that this was a diploid organism — that just means that every chromosome comes in pairs. This cell would have a similar chromosome to the one that has the ABCDE genes on it; here I've draw it as containing the same genes, but in slightly different forms: abcde. This matters because during meiosis, when gametes (sperm and egg) are formed, the two chromosomes line up with one another and the cell machinery tows one chromosome to one daughter cell, and the other to the other daughter cell. It's accounting; it makes sure each daughter gets a copy of all of the genes, one A or one a, one B or one b, etc., for instance.
For now, put the fact that there are two copies of each chromosome at the back of your mind and don't worry about it. Let's think about a single chromosome and ask what can happen to it.
Here's something fairly common. An error in copying the DNA can lead to the loss of a piece of DNA. This happens with a low frequency, but it does happen — if we sequenced your DNA, we might well find a few bits missing here and there. We can get situations like this, where a whole gene gets lost.
Don't panic! Remember that we have two copies of every chromosome, so while this one is missing the "D" gene, there's that other chromosome floating around with a "d" gene. This is not necessarily bad for the individual, it just means he doesn't have a spare any more.
Another kind of error that can happen with a low frequency is a duplication, where the machinery of the cell accidentally repeats itself when copying, and you get an extra copy of a piece of a chromosome, like so:
This person has two copies of D on this chromosome now (and remember that other chromosome, with it's d gene — he actually has 3 copies in total now). This is not usually harmful: it gives the individual a little extra redundancy, and that's about it. It can change the total amount of the D gene product in the cell, and if it's a gene for which precise dosage is important, it can have visible effects…but in most cases, this is a neutral change.
You may have noticed that nothing has changed the chromosome numbers yet. Here's a situation that can lead to the formation of a new chromosome: what if there is a duplication of the centromere, rather than a gene?
Remember, I told you that the centromere/kinetochore is where the cell attaches lines and motors to haul the chromosome to the appropriate daughter cell. In this case, two lines are attached; what if one tries to pull one centromere to the left, and the other tries to pull the other centromere to the right? Tug of war!
The end result is that the chromosome is broken into two chromosomes. I think this is a key concept that the questioner is missing: chromosome numbers really aren't significant at all! You don't need to add significant new information to create a new chromosome, and as I'll show you in a moment, a reduction in chromosome numbers does not represent a loss of genetic information. Chromosome are disorganized filing cabinets, nothing more; we can shuffle genes around between them willy-nilly, and the cell mostly doesn't care. A fission event like the one described above basically does nothing but take one pile of genes and split them into two piles.
But there are some important effects. This may not be an entirely neutral situation. Let's bring back that abcde chromosome, and pair it up with our two new chromosomes, AB and CDE.
The accounting is accurate. This cell has two copies of the A gene, an "A" and an "a", just like normal, and the two new chromosomes can still pair up efficiently with the old chromosome in meiosis, just like before. This is a healthy, functioning, normal cell, except for one thing: if it goes through meiosis to make a sperm or egg, it's going to make a larger number of errors. There are three centromeres there, to be split into two daughter cells! Never mind what the Intelligent Design creationists tell you — the cell is really, really stupid, and it will more or less decide by eeny-meeny-miny-moe how to divvy up those chromosomes. If by chance the split is that one daughter gets AB + CDE, and the other gets abcde, both daughters have the full complement of genes and all is well. However, the split could also be that one daughter gets AB and nothing else, while the other gets CDE + abcde … and that's no good. One is missing a whole bunch of genes, and the other has an overdose of a bunch.
The net result is that although this individual is fine and healthy, a significant number of his or her gametes may carry serious chromosomal errors, which means they may have reduced fertility. They aren't sterile, though; some of their gametes will have the full complement of genes, and can similarly produce new healthy individuals who will probably have fertility problems. (Note: the significance of those fertility problems will vary from species to species. Organisms that rely on producing massive numbers of progeny so that a few survive to adulthood would be hit hard by a change that cuts fecundity; species that rely on producing a few progeny that we raise carefully to adulthood, like us, not so much. So you have to have sex 20 times to successfully produce a child instead of 5 times; that won't usually be a handicap.)
So our two chromosome individual will have a reduced fertility as long as he or she is breeding with the normal one chromosome organisms, but those split chromosomes can continue to spread through the population. They are not certain to spread — they're more likely to eventually go extinct — but by chance alone there can be continued propagation of the two chromosome variant. Which leads to another misconception in the question: something doesn't have to provide a benefit to spread through a population! Chance alone can do it. We don't have to argue for a benefit of chromosome fission at all in order for it to happen.
So we can have a population with a low frequency of scattered chromosomal variants, some carrying the rare two chromosome variant and others the more common one chromosome form. What if two individuals carrying the two chromosome variant breed? They can produce offspring that look like this:
How many centromeres are there? Four, not three. This is a situation the cell machinery can handle reliably, and this individual will consistently produce good gametes that accurately carry AB + CDE, nothing more, nothing less, and will have no reduction in fertility. Now we have a potentially interesting situation: individuals with the one chromosome situation have full fertility when breeding with other individuals with one chromosome; individuals with two chromosomes have full fertility when breeding with other individuals with two chromosomes; it's when individuals with two different chromosome arrangements try to breed that fecundity is reduced. This is a situation where speciation is a possibility.
One last thing: what about reducing chromosome numbers? That's easy, too. Here's an organism with an AB chromosome, and a different chromosome with the genes MN on it. They can simply fuse in the region of the centromere.
This happens with a low frequency, too, and has been observed many times (hint: look up Robertsonian fusions on the web.) I think the key issue to understand here is that chromosome number changes are typically going to represent nothing but reorganizations of the genes — the same genes are just being moved around to different filing cabinets. This has some consequences, of course — you increase the chances of losing some important file folders in the process, and making it more difficult to sort out important information — but it's not as drastic as some seem to think, and chromosome numbers can change dramatically with no obvious effect on the phenotype of the organism. These really are "small adaptations over time", or more accurately, "small changes over time", since there is no necessary presumption that these are adaptive at all.
I've discussed fusion events and how they relate to evolution before, and there's an interesting difference in context there, too. My prior article was a response to Casey Luskin, an ignorant creationist who used his misunderstanding of genetics to foolishly assert the existence of a major problem, and that's where we have a conflict: ignorance is not a problem, but stupidly using your ignorance to push invalid ideas is. This question in my mailbox is also ignorant — the fellow really doesn't understand the basics of genetics — but it's self-recognized ignorance that, in a good way, prompts him to ask a sincere question.
If you want to dig a little deeper, there are many ways genetic information can be rearranged on chromosomes, and this has opened the doors to some interesting evolutionary research. I've described how we can map the reshuffling of chunks of genetic information over time, a process called synteny mapping, which allows us to reconstruct ancestral chromosomes. A fish might have 42 chromosomes, and we might have 46, but we can still trace how the ancestral arrangement was scrambled in many different ways to generate the modern arrangements.
Life Science
Comments
Wow. Thank you for answering a zillion questions in one fell swoop!
Posted by: Milo Johnson | April 21, 2008 10:55 AM
Thanks PZ! I asked my biology teacher the same question in high school but she couldn't tell me. Not that I was trying to disprove evolution or anything, I was just curious! Ever since then I've always had a mild curiosity about this very question but never got around to digging up the answer. Now (15 odd years later) I finally know!
Posted by: Wiggy | April 21, 2008 11:00 AM
Superb!
Posted by: GBruno | April 21, 2008 11:02 AM
Now THAT was a great answer, PZ! And it disproves the notion that you're an old poopy-head who just wants to make fun of the sincere god-bots. You treat others as they treat you; a respectful and intelligent question gets a response in kind, whereas a threatening, abusive letter filled with cant and deliberate gets either chucked, or held up for contempt.
Just to repeat: great answer -- thorough, thought-provoking, and clear. Why the hell couldn't the National Geographic specials have stuff like you just did, instead of the pretty, but vapid and (mostly) dumbed-down stuff they produce?
Posted by: Hairhead | April 21, 2008 11:03 AM
I've just started reading Relics of Eden, by Daniel Fairbanks. It describes these kind of things very well for a layman like me, and I have been writing down parts of it that I know will be very good ammunition for my debates with IDers and creationists.
Posted by: boomer | April 21, 2008 11:04 AM
Thank you, I've wonder the same thing for a long time. I'm sure I'll recover to normal or increased levels of "self-recognized ignorance" after I've thought about this awhile.
Posted by: dorght | April 21, 2008 11:05 AM
Thanks for addressing this topic. This has been something I have been wondering about since I learnt some basic genetics at school.
Posted by: Andy | April 21, 2008 11:05 AM
hurrah *much applause*
Posted by: alex | April 21, 2008 11:06 AM
This is one of the questions that I had always wondered about but never taken the time to look up. Thanks for answering it.
Posted by: Eric | April 21, 2008 11:08 AM
And remember, it actually takes good working replication machinery even to keep chromosome numbers the same over generations.
In cancer cells, aneuploidy, the "wrong number" of chromosomes, is very common. Yet cancer cells are notorious for surviving, and even adapting through darwinian selection. Even many normal cells in our bodies can be aneuploid.
The fact is that it is all too easy to change chromosome numbers, and what is amazing is how meiosis usually manages to keep the right number. Selection has to weed out many changes in chromosome number, while occasionally a change in numbers is either beneficial, or perhaps simply neutral yet able to give rise to a new species. The fusion that gave rise to our chromosome 2 may be an example of the latter.
Glen D
http://tinyurl.com/2kxyc7
Posted by: Glen Davidson | April 21, 2008 11:10 AM
Excellent.
Posted by: Kitty | April 21, 2008 11:11 AM
Oh SNAP!
Kudos to whomever asked this question, your answer PZ helped me understand how the process works. Thanks again.
Posted by: firemancarl | April 21, 2008 11:13 AM
Wow. That was marvelously lucid. Thank you:)
Posted by: Spencer | April 21, 2008 11:14 AM
Some examples: Humans have 1 fewer chromosome than the rest of the great apes - our chromosome #2 is recognisably the fusion of 2 of the ape chromosomes (now called 2A and 2B).
Horses have 64 chromosomes, donkeys have 62 - they can interbreed, and the result is almost always sterile, no idea whether the success rate is lower than normal.
As I understand it, plants do this far more often than animals - including by simply doubling up - i.e. the child plant has 2x the chromosomes (and evolution can then create differentiation, assuming that the plants are fertile (domesticated wheat) or just by slow mutation even if they're not, but helpful humans propagate cuttings (banana).
Posted by: Lurker #753 | April 21, 2008 11:22 AM
(.)(.)
Posted by: wÒÓ† | April 21, 2008 11:23 AM
As a long time lurker, time to stick my head above the parapet and say 'brilliant'. Thanks for a great explanation.
Posted by: PaulH | April 21, 2008 11:24 AM
Awesome! I hadn't actually wondered about that--I'd assumed arrangements were made. (I guess that makes me a dogmatic believer in evolution, like the creationists are always accusing us of being? Except that I've always known the answers to these questions existed, if I cared to look for them.) Nonetheless, this is a very cool thing to know.
One thing I'm wondering about, though: what's the likelihood that several organisms within a species would have a break in the same place? Is it happening when the cousins get to breeding mostly? Or is that "same place" an oversimplification, where there's various nonsense to either side of the break? Or is there something about certain segments of DNA that make them more likely to sprout an extra centriole? Or is it just a large numbers fallacy--it's going to happen, because there are a huge number of trials?
Posted by: lytefoot | April 21, 2008 11:29 AM
That was refreshing, thanks :)
Posted by: Beowulff | April 21, 2008 11:29 AM
That's what I like about this blog! It is at the same time very entertaining and very educational. Creationists have actually done me a favor: I would never have learned so much about biology if it hadn't been for their making claims out of ignorance. Thanks once again, PZ!
Posted by: cynthax | April 21, 2008 11:30 AM
Very descriptive, thanks PZ.
Posted by: Chris | April 21, 2008 11:34 AM
Ah! Written so even a physicist can understand it.
Thanks, unnamed emailer!
Posted by: Patrick Conley | April 21, 2008 11:35 AM
I seem to recall reading about a species of fern that has a ridiculously large number of chromosomes, like on the order of 20,000 or so... but I can't seem to find the name of it now. Does anybody know anything about this, or was this a lie I read on wikipedia?
Posted by: boomer | April 21, 2008 11:35 AM
A THEORY IN CRISIS!!!!!!
Posted by: Liam | April 21, 2008 11:37 AM
I believe there are examples of balanced translocations being dominant in populations of Drosophila on some islands. If the translocation break points don't wreck genes, the duplicated translocation can be perfectly viable, an least in the balanced form. I don't have the time to go into the details (they are in many genetics books), but this is a pretty straight forward way that two populations can lose the ability to interbreed.
And I used to think that Hu Chromossome 2 was an ape Robertsonian, but then they had to go find a telomere or centromere in there somewhere...
that was such a nice nice hypothesis until it ran into a little data....
-d
Posted by: don kane | April 21, 2008 11:37 AM
I once wrote a pair
Of Haikus, related, but
Willing to fuse--please
Don't ask me how one
Limerick now replaces
The Haikus in twos
*****
I once wrote a pair of Haikus
Related, but willing to fuse
Please don't ask me how
One limerick now
Replaces the Haikus in twos
(OT-- http://digitalcuttlefish.blogspot.com/2008/04/i-just-love-xkcd.html )
Posted by: Cuttlefish, OM | April 21, 2008 11:38 AM
I wish you were my prof, PZ!
Posted by: NP | April 21, 2008 11:38 AM
Yay! Learning!
Posted by: Will Davies | April 21, 2008 11:40 AM
One of the arguments more learned creationists have raised is that the great apes have 48 chromosomes, but humans have only 46. How could humans have possibly evolved from the same phylogenetic ancestor as we have? One answer is in a great paper:
Hillier LW, and lots of others.
Generation and annotation of the DNA sequences of human chromosomes 2 and 4.
Nature. 2005 Apr 7;434(7034):724-31.
Apparently, in the course of our evolution, two ancestral primate chromosomes fused to make chromosome 2 in humans, an illustration of the point PZ Myers makes toward the end of his nifty piece. Added beauty is a stretch of telomeric TTAGGG repeats --inside-- the chromosome. So, not only does this find support the idea that we evolved from a common ape ancestor, it also shows just how it must have happened.
Posted by: dubiquiabs | April 21, 2008 11:40 AM
Chromosomal numbers can vary within the same species. This number is very plastic. Mice colonized Madeira 1000 years ago and the 6 populations all have varying karyotypes. This is fast in evolutionary terms.
Same thing is observed in Tunisian mice. The latter are thought to be undergoing speciation, driven in part by karyotypic differences and lower fitness in hybrids.
Posted by: raven | April 21, 2008 11:45 AM
Your smackdowns of creationists are fun, but stuff like this is priceless for us laypeople. Have you considered putting these in a book?
Posted by: jck | April 21, 2008 11:45 AM
Ahhh, always a good day when gains new understanding before lunch. Even if it is from the gooier, more icky sciences. =)
Seriously though, very enlightening, many kudos PZ.
Posted by: Bouncing Bosons | April 21, 2008 11:46 AM
Lovely dip into the refreshing pool of knowledge. Besides helping the general public, stuff like this helps people like me see easier ways to explain things than I can come up with myself. Thanks!
Posted by: Carlie | April 21, 2008 11:48 AM
Rarely do I ever get that deep into biology, precisely because its specifics are not easily explained. Thanks for the detail - I can now say I understand at least one involved concept in biology.
(Though I finally got my brain around the explanation, it took just enough time to confirm that my choice to enter philosophy and anthropology was the correct one...)
Posted by: brokenSoldier | April 21, 2008 11:49 AM
Great post, PZ!
And Cuttlefish, I am always in awe of your mad skilz, but that is astoundingly inventive! Perhaps there are other biological phenomena you could represent in concrete poetry...
Posted by: Tulse | April 21, 2008 11:49 AM
Happy, happy, joy, joy.
Every now and then somebody puts real information in front of me, and I unwittingly learn something. Rarely so elegantly presented, however. Thank you. I'll show this to my wife when she gets off work.
Posted by: freehand | April 21, 2008 11:52 AM
There's an interesting twist on the breakage fusion business pointed out in a nice paper (full text available) -- that since female meiosis is asymmetric (two cell divisions produce only one egg and some polar bodies, so some chromosomes get thrown away) there is competition between chromosomes for getting into the egg. If you have one breakage, or one fusion, you will have an odd number of total centromeres (like PZ drew above). It turns out there is a slight preference for the 'extra' centromere to either go to the egg, or to the first polar body - different ways for different organisms. This is why if you look at mammalian karyotypes, you will see that they are either all-metacentric (centromeres near the middle) like humans, or all-acrocentric (telomeres near one end) like many mice, but never a mixture of both, as would be expected if inheritance were random. That's chromosome evolution put into high gear by a 'selfish' phenomenon.
Posted by: PeteC | April 21, 2008 11:54 AM
Excellent. I often wondered about this too. And I've heard the same assertions from idiots thinking that chromosomes make speciation impossible.
One answer that I give is to repeat the line that they often use: "interspecies hybrids are usually sterile". As in the horse donkey example, as in ligers and tigons, as in many others. But there was an important word in there. They are usually sterile. Not always.
Posted by: R N B | April 21, 2008 11:54 AM
The more you post like this, the more I consider going to Minnesota to finish my bio degree.
Hint: Move to Arkansas. We need the education.
Posted by: Jason B | April 21, 2008 11:55 AM
One thing I found of interest when I looked some of this stuff up for a discussion on a Christian webforum I inhabit is the frequency of fusions in humans, right now.
Something like 1 in a 1000 human individuals in the general population have a Robertson translocation, in which different chromosomes are fused into a single larger chromosome. Details are available in Changes to Chromosome Structure (pdf file); fact sheet 7 from the Australian Centre for Genetic Education.
There is probably no real effect for such individuals, until they try to have children, when they are likely to have some difficulty with fertility.
I passed this information on without the depth of education that a biologist would bring to the subject. Posts like this one here are a great help.
Posted by: Duae Quartunciae | April 21, 2008 11:56 AM
Wow PZ, that was great!
Posted by: Mr_P | April 21, 2008 11:58 AM
There are other mechanisms for changing numbers that I didn't discuss: polyploidy and alloploidy, for instance. This post only describes a few of the ways we can get variations. Don Kane also mentioned translocations, which are sneaky ways for a chromosome to gradually scramble itself around, piece by piece.
Posted by: PZ Myers | April 21, 2008 11:58 AM
This is great. I envy your students. None of my biology professors ever explained this question so clearly, and although I've had a general notion of it, I've always still been a bit confused. (Bad me for not doing the research to clear up any confusion, I know.) Now I understand much better. Thanks, PZ!
Posted by: Etha Williams | April 21, 2008 11:59 AM
I've wondered about this recently, thanks, this is a really good answer
Posted by: D | April 21, 2008 12:01 PM
Great great great post - thanks again PZ for a fine post!! An additional point is that depending on the literature source you read, somewhere between 1 in 600 to 1 in 1000 people carry a balanced translocation (the numbers vary depending on the type of study performed). That is, one chunk of a chromosome has switched places with another - no gain or loss of genetic material, but instead the re-shuffling that PZ discussed above. Such people are perfectly normal, and may never be aware that they carry a balanced translocation. Or, they may have a history of infertility or spontaneous abortions, or have a child or children with congenital anomalies. This is why it is standard practice to have chromosome analysis performed as part of a reproductive-issues work up, or if a child is born with anomalies. Chromosome rearrangements in humans (well, all animals with chromosomes) are neither common nor rare.
BTW - Cuttlefish is a poetry ROCK STAR. Love the fusion!
Posted by: ctenotrish | April 21, 2008 12:01 PM
If it hasn't been done already, this needs to be sent to Wilkins as a "Basics of Science" post.
Posted by: KeithB | April 21, 2008 12:04 PM
Great answer. The question asked about selective advantage and your answer (accurately) says that there needn't be a necessary selective advantage to propagate through the population. However, there is a potential selective advantage (and disadvantage) to having material scattered across several chromosomes (for a diploid eukaryote). With only one chromosome, all of your material is in one cabinet, inherited wholesale from parent to child, child to grandchild. You'll necessarily have an exact copy of one of your ancestors chromosomes (leaving aside meiotic recombination), bringing both the good and the bad copies of genes along. Splitting up your genetic material into chromosomes that can be inherited independently means you will get a mixture of the grandparental genes. You won't be an exact genetic copy. This allows increased mixing and matching of genetic material, potentially improving the chances of a beneficial combination of genes.
Of course more chromosomes also means more record keeping. More centromeres and telomeres to handle during duplication and meiosis/mitosis. I think the jury is still out on whether there might be some optimum number of chromosomes (rhinos have 84!).
Interestingly, most avian species have numerous microchromosomes. Up to 25% of the DNA (and 50% of the coding sequence) is not in conventional chromosomes but rather in a pool of smaller chromosomes. In these microchromosomes, recombination and mutation rates are significantly higher than in standard chromosomes, which may have significant impacts on their evolution.
There is interesting research into the origin of these microchromosomes and chromosome number in general. Nakatani Y et al (2007 - Genome Research 17:1254) suggest that microchromosomes may represent an ancestral vertebrate state where fusions have produced the 20-30 large chromosomes found in most vertebrate clades.
Posted by: drerio | April 21, 2008 12:04 PM
True. I personally know 2 people who have altered karyotypes, balanced. They were picked up on the basis of a family history of fertility problems and miscarriages. These weren't severe though, one of the ancestral families ended up with 6 kids.
Posted by: raven | April 21, 2008 12:08 PM
Wow, a post from PZ Myers about science, on Scienceblogs of all places!
Posted by: Anonymous | April 21, 2008 12:10 PM
Great post, PZ!
But one critical point: perhaps you could have included a little more about what the *evidence* is for what you've just said. I can just hear the creationists saying "But how do YOU know?" I think that one general problem with the way science is often taught is that it is taught in abstraction from the evidence for/context of discovery of the claims being presented to the student. Although you usually do a pretty darn good job of not making that mistake.
Here's something to ponder: how many big bang doubters have you met that could name one single piece of alleged evidence for the big bang? I sure haven't met any.
Not surprisingly, those who are aware of the evidence believe, those who aren't, don't.
Posted by: Dustin | April 21, 2008 12:12 PM
I wish you were my prof, PZ!
The archive here makes him everybody's prof.
Dig in - it's the best part of the place.
Not least because the prof can really employ multimedia.
I recommend "How to make a vulva". I won't link to it directly because I have no idea whether the comments are worksafe in the land of patriarchal womb-control ;-)
Posted by: Nan McIntyre | April 21, 2008 12:19 PM
Thanks PZ, this was a wonderful read.
I'm a long time lurker, occasional commenter.
This kind of stuff is so much cooler than just saying "Goddidit."
Posted by: Tim Foreman | April 21, 2008 12:22 PM
Thankyou for this, P-Zed. It's always a treat to learn something from an expert without having to become an expert first.
Honestly, if more people were able to communicate technical concepts like you communicate this one, I doubt we would have nearly as much problems getting people to accept sound science and reject nonsense.
Posted by: Tim Mills | April 21, 2008 12:23 PM
It's such a pleasure seeing a pro on top of their game. Well done, sir.
Posted by: Paul Lundgren | April 21, 2008 12:23 PM
That was a beautifully concise explanation! Well done.
I'm given to sentimentality on occasion, and PZ's mini-lecture brought me back to the class on genetics and heredity I took way back in 2001. The instructor was Dr. Mike Harrington here at the U of A, and he shared PZ's talent for clarity as well as a sense of humour. (Plus, he wrote some of the best exams I've ever had the pleasure of taking--yes, you read that correctly: a well-written exam can be a pleasure for a prepared student.)
Man, I loved that class. Now I feel like quitting my job and going back to school.
Posted by: Brownian, OM | April 21, 2008 12:27 PM
Wow... that's all I can say. That was an amazing explanation. Good job!
Posted by: benny | April 21, 2008 12:31 PM
I wish I were smart enough to understand all this stuff, but I read a few paragraphs and my brain does a short circuit. Bzzzt. But even not understanding what you are saying, I realize you have an infinitely better argument for evo than the creos.
Posted by: Ignorant Atheist | April 21, 2008 12:31 PM
Weird, my post deleted all my spacing between sentences.
Posted by: Ignorant Atheist | April 21, 2008 12:34 PM
Cuttlefish, this is honestly your best yet!
I bow in awe.
Posted by: Kenneth Oberlander | April 21, 2008 12:34 PM
Great post, great thread.
Regarding the reduction of chromosome numbers in Humans vis a vis the great apes, some work by Simon et al. several decades ago uncovered some circumstantial yet intriguing evidence that the process was specifically influenced by the mother. I don't have the text handy, but one section I recall rather clearly said something to the effect "Mama, don't take my Chromosome away."
Posted by: jeff | April 21, 2008 12:35 PM
Sorry, the link to the listing of articles is stripped from my bad html.
This is it:
http://pharyngula.org/articles.html
Posted by: Nan McIntyre | April 21, 2008 12:36 PM
There is plenty of evidence for this. Take some of the human genetic illnesses. Some of these involve extra chromosomes, and yet they are people who function, albeit in these cases with varying degrees of handicap.
It's clear that some mutations like this aren't noticed because the effects aren't extreme. If they lead to an advantage, you would get the growth of organisms with extra chromosomes.
Nick
Posted by: Nick | April 21, 2008 12:40 PM
@Raven:
karyotypes:
AAAAAAAAAAAAAAhhhhhhhhhhhhhh!
lol.
Posted by: Ignorant Atheist | April 21, 2008 12:44 PM
Boomer, I did a quick google search and found indications that the species is Ophioglossum reticulatum, which apparently has 630 pairs (1260, a bit less than your recollection of 20000, but still ridiculous consider that animals seem mostly to be in the double digits). Note, this is after about 5 seconds of looking by a nonbiologist, so it's possible that I've missed something. The website I found is http://www.vivo.colostate.edu/hbooks/genetics/medgen/basics/minmax_chromos.html
Posted by: Charles | April 21, 2008 12:56 PM
Thanks, PZ. I enjoy reading such explanations. That's one reason why I visit this site regularly.
Posted by: shrimplate | April 21, 2008 12:59 PM
Nice post! This will make a fantastic addition to the things I send to brash, ignorant creationists. I can't wait until the creationists read it. I'm sure their response will looks something like this ...
PZ Myers has admitted that, on the subject of evolution: "Chance alone can do it."
Posted by: Greg Gyetko | April 21, 2008 12:59 PM
Lucid lesson! I really wasn't interested in this subject, but, I started reading and I was hooked on learning more.
Posted by: bigjohn756 | April 21, 2008 1:12 PM
Nothing original in my comment, I just want to chime-in and say how fantastic and accessible that explanation was. I just got a biology lesson from a professor for free (complete with whiteboard illustrations)! If people really do learn something new every day, this will probably be the most interesting thing I'll learn this week (now that the John Adams miniseries is concluded). Thanks PZ!
Posted by: Dave | April 21, 2008 1:13 PM
OK, I'm officially in love with Cuttlefish! Haiku to limerick? Let me catch my breath....
Posted by: ildi | April 21, 2008 1:14 PM
(OP Kudos)
What an excellent post. Thanks PZ.
(snark)
Where are Nisbet and Mooney? This is a frame they should really see.
(Comment Kudos)
Cuttlefish, that was brilliant, and inspiring seeing as my timestamp math says you did it in less than an hour.
Posted by: Vic | April 21, 2008 1:24 PM
This is very illuminating indeed ... However: What about species that have holocentric chromosomes (i.e., no centromeres?). Many insects, for example, are notoriously variable in chromosome numbers, and often with no correlation to their phylogenetic history. And they have holocentric (or holokinetic) chromosomes.
Posted by: Vazrick | April 21, 2008 1:29 PM
There is one part of this explanation that I don't understand.
When there is a single chromosome with two centromeres, the chromosome is split when one part (AB) is pulled in the opposite direction of the other (CDE). However this would produce one cell with the chromosomes (AB) and (abcde) and another with (CDE) and (abcde). I imagine these cells wouldn't do that well, but even worse in the case of gametes we would have 4 cells with (AB), (abcde), (abcde) and (CDE).
So how do we go from a cell splitting a single chromosome into two different cells, to the state with a cell containing (AB) (CDE) and (abcde).
Thanks PZ
Posted by: 938MeV | April 21, 2008 1:35 PM
Very interesting, neutral mutations propagating, then a benefit for reproduction of two chromosome offspring which could lead to speciation. Great stuff, keep on posting science stuff I learn so much from reading your posts.
Posted by: dave | April 21, 2008 1:36 PM
Good biology lesson! Here's a followup question: How does the cell pair homologous chromosomes during meiosis? Are centromeres unique (tagged, somehow) to the chromosome pair?
Posted by: Russell | April 21, 2008 1:39 PM
The whole idea of chromosome number goes much deeper. Basically prokaryotes and the vast majority of eukaryotes are haploid. If they reproduce sexually, the zygote is the only diploid stage. Only animals and the sporophyte generation of plants are routinely diploid (dikaryon in basidiomycetes). What's the advantage of being diploid? It allows you to be heterozygous for one thing, and meiosis allows for recombination of parental genotypes. So ultimately this leads to your point, more chromosomes allows for more gene combinations, more variation upon which selection can act. With the exception of the rattlesnake ferns, which have some very high chromosome numbers (paleopolyploids?), there must be some optimal range (1 to 4 dozen) of chromosome number to accomplish this efficiently.
Posted by: DrA | April 21, 2008 1:40 PM
What I find fascinating is the implication that divergence of species would occur most readily when an individual with a chromosome split or fusion becomes a member of a small isolated (in-breeding) group. The likelihood of such a mutation gaining any traction in a large population would be quite small. So island populations in particular would be prone to divergence.
Posted by: bobz | April 21, 2008 1:41 PM
I've often wondered why we don't have just one chromosome, and your explanation even offers an inkling why that would be unlikely.
Thanks for your patience.
And now I'm waiting for the inevitable question: "But have you OBSERVED this?".
I want to see some of your post-Expelled hate-mails, too. Hopefully, from someone who had never heard of you before this weekend. The more basic errors these emails contain, the better.
Posted by: MikeM | April 21, 2008 1:43 PM
Case Luskin recently returned (in typically obtuse manner) to the subject of the fused ape chromosomes 2A&2B.
Intelligent Design 101: Casey Luskin on Human Chromosomal Fusion
He argues that the similarities are caused by common design, but of course he doesn't mention the degenerate centromere and vestigal telomeres (i.e. evidence of dysteleology), and he also doesn't manage to mention the potential biological importance of looking at the genes surrounding the fusion point (PGML/FOXD/CBWD) for changes that may have led to the origins of modern human abilities.
But he has this silly scenario which he explores at the end of the podcast. To paraphrase - imagine, a population of future humans derived from modern H. sapiens. These future humans ("the double fuser people" or DFP) have inherited a new chromosome fusion which is fixed in the population (i.e. reached 100% frequency, they now have 44 chromosomes - 22 pairs).
Modern H. sapiens are extinct, all their knowledge is unavailable and DFP humans are starting again from scratch. The DFP humans compare themselves with surviving chimps. These future DFP humans would think they had a common ancestor with chimps, but with 2 chromosomes fusions in between.
However, Luskin argues that this inference would be logically invalid, as the second hypothetical fusion event took place in modern H. sapiens, and has:
Is Luskin being intentionally dense? In his scenario either of the fusion events could have taken place at almost any arbitrary generation in the history of the population - between the last common ancestor with the outgroup (chimps in this case) and long enough before the karyotype is tested to fix the variant across the population.
In his own scenario, if 'the double fuser people' humans got hold of a modern human karyotype and thought "perhaps we share a common ancestor with this organism, except for a single chromosome fusion event", they would be completely correct.
Posted by: Pantrog | April 21, 2008 1:43 PM
I second the comment at #30 about a book - why not, at the very least, put a collection of your 'science' posts together, similar in vein to Science Blogging Anthology - or at least have your own chapter in the next edition? Are you listening, Coturnix? :-)
Posted by: ColinB | April 21, 2008 1:46 PM
That was really, really good.
Posted by: DarkSyde | April 21, 2008 1:46 PM
Great. I already throw more details at the freshman than they want, and you give me a few more to rattle around and spill out.
On the question of big chromosome numbers, here's an info page - don't know how authoritative it is, but I'm betting moreso than Wikipedia:
http://arbl.cvmbs.colostate.edu/hbooks/genetics/medgen/basics/minmax_chromos.html
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