Rumors of Y Death Are Greatly Exaggerated; Male Chromosome Evolving Like Crazy

By Andrew Moseman | January 14, 2010 12:15 pm

Young_male_chimpDon’t count out the Y chromosome just yet. Far from being in a state of decay, as some studies have suggested, a new study in Nature says the male chromosome in humans is actually evolving at a furious pace.

Study leader David Page of MIT sequenced the human Y chromosome back in 2003, and in the new study his team compares it to the male chromosome of chimpanzees. The scientists expected the two sequences to look very similar. However, while human and chimp DNA generally differ by less than 2 per cent, more than 30 per cent of the Y chromosome differed between the two species [The Times].

The prevailing theories, according to the study’s introduction, have held “that Y chromosomes evolve by gene loss, the pace of which slows over time, eventually leading to a paucity of genes, and stasis.” Originally, the Y had the same set of genes as the X, biologists believe. But the two can’t trade DNA; so as the Y shed most of its X-related genes, researchers thought it lacked a way to get fresh genes and was therefore on the road to stagnation. Earlier research showed that over 300 million years of evolution, the X chromosome retained many hundreds of genes, while the Y chromosome was reduced to only about 70 or 80 genes.

However vast differences between human and chimp Y chromosomes imply not that the male chromosome is grinding to a halt in humans, but rather the opposite—it’s evolving as a furious pace compared to the rest of the genome. In fact, that study says, while the overall genome of humans and chimps are separated by a tiny relatively amount because we diverged just 6 million years ago, the Y chromosomes differ by about as much as the overall differences between humans and chickens, which last had a common ancestor more than 300 million years ago.

Why the big Y chromosome differences? Humans, the researchers say, seem to be adding and deleting genes; chimps, on the other hand, are mainly just losing them. They point to few possible explanations, including differing mating behavior. Many male apes mate with one female in the species, leading to evolutionary success only for those whose sperm out-impregnates others, a process called “sperm competition” in evolutionary biology.  The human male chromosome, in contrast, shows no evidence of such competition, according to the study [USA Today].

The implications of the Y chromosome’s newfound vitality remain to be seen. This does not mean that men are evolving faster than women, given that the two belong to the same species, but it could be that the Y’s rate of change drives or influences the evolution of the rest of the human genome in ways that now need to be assessed. It would be “hard to imagine that these dramatic changes in the Y don’t have broader consequences,” Dr. Page said [The New York Times].

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DISCOVER: Whither the Y?

Image: Wikimedia Commons / Frans de Waal, Emory University

  • Ryan Somma

    Why is everyone using the word “evolving” to describe what is obviously the amassing of random mutations on a chromosome that is mostly a genetic dead zone? “Evolving” implies natural selection, there is no selection here. There are mutations on the Y Chromosome that aren’t being weeded out by natural selection, because the mutated genes serve no purpose.

  • CharlesDexterWard

    Although the word “evolve” is often co-opted to mean a biological process, driven by natural selection, that inexorably leads to ever more advanced and complex organisms, “to evolve” literally only means “to change over time.” Even the accumulation of random mutations=change over time, regardless of the impact on the phenotype. One can even have evolution in which a creature becomes simpler, not more complex, as when an organism evolves into a parasite, shedding much of the biological apparatus formerly used to maintain an independent existence, and which the parasite no longer needs.

  • Sundance

    @Ryan Somma: It seems premature to declare that the mutated genes serve no purpose without a better understanding of what’s going on, and why the human Y chromosome is changing at such a furious rate. If the changes are taking place at a faster rate than in chimps as a result of different mating behaviours for instance, then there may well be some selective pressure encouraging an increased rate of mutation.

  • Zachary Karmiol

    While natural selection appears to be the dominant driving force in evolution, it is by no means the only game in town. Look at the works of Stephen Jay Gould and Richard Lewontin, particularly

  • Matt T

    @Ryan (#1): Not to jump all over your argument, but the statement “…because the mutated genes serve no purpose” is a seriously presumptuous statement. Read, for example:

    It would seem that one gene’s trash is another gene’s treasure.

  • Abraham Kryger, MD

    The Y chromosome contains the SIR gene which apparently controls longevity. Perhaps the “evolution” of the Y chromosome is allowing men to live longer. At one time women outlived men by almost a full decade and it seems we are catching up.The oldest people in the world are no longer just women, a few men have hit the 120 yr mark as well. Also it seems that men ( boys ) are affected by diet more drastically than girls(women) and the fastest growing rate of obesity is seen among young males. Could it be the epigenetic modification of DNA due to diet occurs more rapidly in the Y chromosome? These are just questions and observations from a non-geneticist.

  • amphiox

    @Ryan: Since evolution = change of gene frequencies over time, by one well acknowledged definition, this certainly qualifies. Natural selection is not the only mechanism of evolution. And it’s clear that the scientists studying this feel that these changes ARE under selective pressure (ie they are consequences regarding reproductive fitness)

    At any rate, there are ways of distinguishing genetic changes subject to selective pressure from genetic changes accumulated wholly at random in a neutral pattern, based on analysis of the code itself, so the issue should in theory be resolvable.

  • Dave E.

    Well, atleast it’s good to know that my Y chromosome isn’t giving up one me!

  • Greg

    Odd how new scientific theories seem to reinforce biblical stories of creation. If male, Y chromosome “evolution” drives human evolution could this not be the cause of an ancestor of ours, who in striving to understand this complexity, concluding that Eve (female) arose from Adams (male) rib.

  • What?


    “Odd how new scientific theories seem to reinforce biblical stories of creation.”
    Huh? Only in your confused brain. Give an example, please.

    “If male, Y chromosome “evolution” drives human evolution…”
    Who says it does? And chromosomes don’t “drive” evolution, genes are selected and/or deselected.

    As for the rest of your post ~ I tried hard, but I couldn’t make sense of it at all. What “complexity”? Why would anyone conclude that males came first? Can you imagine a species composed of only males, and females evolving from them? And isn’t the whole “Adam first” nonsense a perfect example of science undermining, not “reinforcing” biblical stories?

  • Y me!

    Is it natural selection that made womens feet smaller than mens, so they can stand closer to the sink to do dishes?

  • Bob F

    I do think this evidence points to men driving the evolution of the species.

    Eggs are created at birth in women while sperm are constantly being produced. The environment can turn genes on and off in the human body. If those changes can be reflected in the sperm being produced by men because new sperm are constantly being produced but those changes cannot be reflected in eggs since they are all created at birth then it stands to reason that men would drive the evolution of the species.

    Because the Y chromosome is not paired with a chromosome that was set at birth in the egg, its changes are more rapid than the rest of the genome. Evolution of the X chromosome should be slowest of all the chromosomes if this theory is correct.

  • Bob F

    I had a brain fart, the X chromosome should evolve at the same rate as the rest of the chromosomes other than the Y chromosome since it could experience genetic changes in males due to the environment.

    If this theory is correct, diseases like austism, asthma, and other diseases on the increase could be due primarily to male exposure to harmful environmental contaminants rather than equally due to male and female exposure.


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