I was told by a friend that the male ‘Y’ chromosome is gradually shrinking, and that in a few millenia, men will be nonexistent. Is this true? Is it possible? Most importantly, who would do all the driving in a world without men?
It’s a theory, but one with little evidence and a lot of flaws.
The basis of the idea is the “selfish gene” theory. Essentially any gene “seeks” to promote its own survival reproduction, even at the expense of the host. That means that X chromosomes should be seeking to eliminate the Y chromosome, since doing so would double their own reproductive success. So over time the X chromosome is going to be selected against and will eventually become extinct.
The theoretical flaws in this theory are legion, the more glaringly obvious one being that any X chromosome that any X chromosome that can survive once the sex ratio dips below 50% will have a far greater survival advantage. IOW the more successful the X chromosome is at eliminating Ys, the more advantageous it is to be a resistant Y.
The practical flaws are almost as numerous, the most obvious on being that many animals, including the common house mouse, reproduce 50 times faster than humans, and there is no sign of X chromosome degradation in these species. So if it is ever going to be a problem for humans it will occur half a million years after it has already stricken mice and rats. Not a present danger.
There is one species of mouse where the X chromosome has become fused with a neighbouring chromosome, but that’s very different to the chromosome being eliminated, and that species produces at least as many males as any other mammals species.
The version of the theory I’ve heard is that it HAS happened - again, and again, and again. That eventually a variation on the X chromosome DOES arise that prevents Y chromosomes from ending up in offspring, resulting in all females. And then, any mutant version of the Y that can resist has a huge advantage and takes over the species, replacing the old version. And then many generations later the X develops another method of sabotaging the Y, and the cycle begins again.
In which case the Y chromosome hasn’t become extinct or shrunk, it has simply evolved.
Not to mention that there seem to be no actual evidence for this. It’s pure speculation.
Is this the same friend who told you humans are devolving?
At least this one is based on a misunderstanding of (extremely speculative) “science”. It’s not pure fruitcackery like her ape-man suggestion.
Huh, I didn’t think that the shrinking of the Y chromosome is controversial at all – after all, it does only contain some 80 ‘working’ genes, compared to a rough 1500 on the X (per wiki), which, if the two have indeed originated as a pair of identical chromosomes, certainly implies shrinkage. What’s controversial (and most likely, bullshit) is that this shrinkage will lead to the extinction of males, or even of the Y chromosome (the latter, as far as I understand it, does not necessarily imply the former, since the sex-defining gene could be ‘moved’ to another, making it the new Y-chromosome).
The reason for the shrinkage is essentially that X- and Y-chromosomes don’t recombine with each other, meaning that the Y can’t eliminate harmful mutations; and since there’s only one Y-chromosome involved in the whole reproduction process, there’s also no possibility for chromosomal crossover, i.e. paired chromosomes of both parents recombining with each other. Thus, the Y has little to no possibility to get ‘refreshed’ (though I hear there is a possibility for the Y to recombine with itself, using palindromic gene sequences), and gradually atrophies.
That’s the difference between “has shrunk” and “is shrinking”.
All placentals share a common ancestral Y chromosome. That means there is no evidence that the Y chromosome has shrunk since the Cretaceous at the very most recent. So yes, it has shrunk at some stage. It hasn’t shrunk for the last umpteen million years and it isn’t shrinking today.
Canaries are much smaller than there ancestors too. Does that mean that we can conclude that canaries are shrinking and will one day vanish? Nope, it means that they shrank at some point in the past and that their size is now stable.
Sure they can.
Y chromosomes regularly undergo crossing over events with X chromosomes. Only a relatively small number of regions don’t undergo regular crossing over. Moreover the Y chromosome eliminates harmful mutations in those regions by crossing over with itself. The arms of the “Y” contain palindromes of the sequences in the other arm and the two versions regularly swap.
Precisely why the Y chromosome has shrunk remains a subject of debate. The most sensible theory IMO is that mammals have been forced to evolve a mechanism to combat the effect of maternal oestrogens in the embryonic blood supply. That meant that they had to produce a mechanism by which maleness completely swamped femaleness. The most efficient way to do that was to produce a genome that made an embryo female by default but develop male if it got even a whiff of testosterone regardless of the dose of oestrogens it was exposed to. However if such a genome underwent regular crossing over with its partner it would regularly prevent development of female traits in XX individuals, to it own detriment. The solution was to produce a chromosome with minimal information, restricting itself largely to genes associated exclusively with males.
This hypothesis gets a lot of support from the fact that birds and monotremes, where the embryo shares no blood supply with the mother, do not have a Y chromosome, rather they are male by default and develop female with even a minor exposure to testosterone. Marsupials, where there is some exposure to maternal hormones, also have a Y chromosome and are also female by default.
Well, I’m certainly not the expert here, but from what little I know, it does appear that there are legitimately differing opinions on the matter.
On the other hand, the Transcaucasian Mole Vole has completely lost its Y-chromosome, circumventing the argument that an uneven gender ratio leads to positive selection pressure for more stable Y-chromosomes by finding another way to determine maleness, so there’s apparently nothing that says it can’t shrink away. (That it most likely won’t, as long as its needed, though, receives no argument from me.) And at least according to the study quoted in this article, there’s evidence that the Y is still degrading (though it won’t disappear):
Well, the figure I’ve heard quoted is 95% of non-recombining regions – not that that undermines your general point, as it’s certainly true that the Y-chromosome does receive some reconstruction, which may well be enough to keep it in its present state for all eternity; I flat don’t know.
This is one of those “I don’t understand what some people are thinking.” things. So our Y-chromosome has been around for many 10s of millions of years. And it’s going to go away in a few millenia.
And let me know when Elvis gets here.
This is one of those stories that garner headlines in popular science articles, with the more boring truth buried 37 paragraphs down.
As in As Y Chromosome Shrinks, End of Men Pondered.
Last line:
Another problem with the claim that a shrinking Y chromosome means “the end of men” is a practical one. Even if it’s happening, either natural selection will work out a way around it; humans will just use genetic engineering to put it right back; or humanity will perish due to a lack of men. It would only lead to an end to men in the same sense that the Moon crashing into the Earth would.
There are currently no mammals that undergo parthenogenesis, for one important reason: imprinting. Some of our genes are only expressed on one parental chromosome and silenced on the other. The classic example, of course, is Prader-Willi/Angelman syndromes. So that’s another major obstacle in the way of this.
The evolution of sex is really a cool little field. There are entire evolutionary battlegrounds on the chromosomal level that we’re just beginning to figure out. During female meiosis, for example, there’s an evolutionary drive for each chromosome to make it into the oocyte rather than one of the polar bodies. People are looking for evidence that mechanisms exist to fight this battle, and there’s some intriguing stuff that’s been found.