January 16th, 2011, 06:46 AM
I am revising my biology after a good few years.
I was thinking that as a female has two X chromosomes, and a male has an X and a Y chromosome. This must mean (given approx the same number of females and males over evolutionary timescales) that there have only been 25% Y's as compared to 75 X's . Does this mean that as there have been fewer instances of X chromosomes there has been less opportunity for natural selection to work its wonders?
Am I making a blunder somewhere?
Zero
Y chromosomes actually tend to have higher rates of genetic change over time because sperm are produced in larger number than eggs, and all copies of Y chromosomes in individuals must have been carried as a sperm.
As much as 95% of the Y-chromosome can't undergo recombination with the X-chromosome - the so-called "non-recombining region" - probably due to some drastic inversion event in the remote past. The Y has been slowly withering away over the last few hundred million years because of this, and may in fact disappear one day in the distant future.
In humans evidence of mutation rates suggest that they have been 5-6 times faster on the Y chromosome, giving rise to the term "male driven evolution". If you're interested in this subject a google search can reveal a good amount of articles on this subject, including its application (reproductive behavior of extinct ancestors*).
*In animals like some species of seals and sea lions where a dominant male controls a huge harem, and mates with all of them, while most of the other males fail to reproduce at all would create such a bottleneck in the male genome to counteract the faster mutation rate of the male genetic material, thus causing evidence for the faster mutation rates not to appear in the genetic ancestry of these animals.
Relative differences can thus be used to provide an idea of reproductive behavior.
Zwirko, that could you provide a source and more information (sounds interesting) on the Y chromosome "withering away"?
Really? Like males born without the Y chromosome? Citation needed.Originally Posted by Zwirko
It does sound quite outlandish I admit, but it is a theoretical possibility. Note though, that I'm not saying the Y-chromosome is doomed to extinction, just that it could/maybe/possibly vanish one day. I've linked to a very short news and commentary piece published in Nature's Heredity journal. It gives a nice summary of the background and some of the evidence for and against the doomed Y chromosome hypothesis: The longevity of Y chromosomes: The Human Y chromosome is not dead (yet)
Basically, the evidence is not conclusive. The principle behind the erosion of the Y is pretty solid. There is also evidence that the Y may have stabilised and will be around for a while. Then again....
The idea of losing a sex-determining chromosome is not without precedent though. If it were to happen, there would just be a new sex-determining chromosome or something. It wouldn't be the end of males. There's a heap of pop-sci type articles and more serious papers on the subject for those who'd like to dig deeper.
I was thinking, about my (wrong) hypothesis, that one type of chromosome is more evolved than the other X over Y, Y over X, for this to happen its not the instances (numbers) of the chromosomes, but also the competition
I am still not sure if there is a natural selelection war in XY chromosomes, either one against each other, or against the same type?
Maybe sperm with more waggly tails? Eggs with larger surface area?
Surely they would evolve to be more efficient.
I remember seeing part of a documentary on some large animal (elephant I think) where i was stated that their sperm had relatively incredible endurance as they have to travel a great distance, or something around those lines.
As for competition between the X and Y chromosome, as females and males are always exactly the same successful, the ratio will always stay at about three to one. There are however differences, as the costs or raising males and raising females is not always the same. I don't remember the specifics mentioned by the Ancestor's Tale however.
I disagree with the idea, as it seems to draw upon the same sort of fallacy creationists draw upon: that mutations over time cause losses in genetic material, and thus species can only lose over time.
Mutations with negative consequences are going to be selected against, thus losses of important genetic material are not likely to be passed down. I assume this is the reason that the studies have found no recent decreases in Y chromosome size.
All of the information also present on the X chromosome would simply have been redundant, and thus losing it would not have caused any decrease in survival value (unless the X chromosome also lacked it, but this is rare enough that it is unlikely to have had more than a negligible impact). In this way the lack of recombination did allow for a huge decrease in size, and I would guess that by now the Y chromosome has largely been reduced to only the male sex specific genes.
I would wager that future significant losses are very unlikely.
I understand that the Y chromosome is going through constant renovating its genes. it is not disappearing it is just keeping up with demands of females and how many of them are in their population.
I ma not talking of competition between X & Y, but between X & X and Y & Y. I am not sure if all the essential mechanisms are in place for one X chromosome type to be selected over hte next X chromosome type.
On further reflection I suppose this happens only at the phenotype level. I.e. a successful (and good looking) male sows more seeds and his sperm type gets reproduced.
C.Elrod, I don't see any creationist-style fallacies there at all. I agree that it may well never happen, but who can tell if it will or not? The evidence points both ways, yet we have to ask whether a 6 million year period of apparent stability is enough to dismiss a 320 million year trend? Or is a simple linear extrapolation of that trend just being a little bit naive? As I hinted at previously this idea is not without precedent - it has been observed to have happened in several species of mammal. Loss of a sex-determining chromosome is not an evolutionary no-no.
As for the wager, I don't think that either one of us will be around to collect in 300 million years time.
I just looked it up.
You are right, I was wrong.
Evidently some species of rodent have lost the Y chromosome, and have evolved a new system of sex determination.
Cool! I had no idea this was the case.
I should really research more before posting; I thought what I said made sense, but I was plain and simply wrong.
Ha ha, that was my way of saying "I would guess".
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