In this case, I presented a poster at the scientific meeting I attended, the 2013 annual meeting of the Society for Molecular Biology and Evolution, about work that was recently accepted for publication (expected to be published in January). I'm working on an accessible research blog post about this work. In the meantime, here is the description from the poster this summer, and the pdf version of the poster (you can also download the preprint version here):
Description
The human Y chromosome exhibits surprisingly low levels of genetic diversity. This could result from neutral processes if the effective population size of males is reduced relative to females due to a higher variance in the number of offspring from males than from females. Alternatively, selection acting on new mutations, and affecting linked neutral sites, could reduce variability on the Y chromosome. Here, using genome-wide analyses of X, Y, autosomal and mitochondrial DNA, in combination with extensive population genetic simulations, we show that low observed Y chromosome variability is not consistent with a purely neutral model. Instead, we show that models of purifying selection are consistent with observed Y diversity. Further, the number of sites estimated to be under purifying selection greatly exceeds the number of Y-linked coding sites, suggesting the importance of the highly repetitive ampliconic regions. Because the functional significance of the ampliconic regions is poorly understood, our findings should motivate future research in this area.
http://dx.doi.org/10.6084/m9.figshare.806296
71 Comments
Joe Felsenstein · 24 December 2013
Is the time scale which you are observing diversity long enough that what you could be seeing is purifying selection in the past? It is thought that loss of functional loci on the Y occurred, through processes like Muller's Ratchet. When you see more sites that have purifying selection than there are Y-linked coding sites, could that be an observation of past selection? So selection at locations that no longer have any function.
https://me.yahoo.com/a/UIFqpY46nexUlCvhZ8zfKDh3zX4VO81SHItDeWm6L4agSA6W#dd2ba · 24 December 2013
This should come as no suprise to anyone because all that natural selection is known to do is reduce existing genetic variation. It is Nature's executioner and eliminator, and not an agency responsible for the proliferation of diversity.
Just Bob · 24 December 2013
M. Wilson Sayres · 24 December 2013
M. Wilson Sayres · 24 December 2013
Just Bob · 24 December 2013
Thanks, M.
Do you think it might be possible for a species to become genetically 'programmed' to look for diversity in mates? And probably along with that would be a genetic tendency to actually BE more diverse -- to mutate more frequently, since an obvious (but not drastically harmful) mutation would be THE way to get laid. IOW, might a species evolve a tendency to frequent mutation, along with an evolved mating preference for the unusual?
M. Wilson Sayres · 24 December 2013
I don't think selection for the most diverse phenotype would lead to a higher genome-wide mutation rate in vertebrates, what with long generation times, relatively small effective population sizes, and a whole host of other selective pressures. But in species with short generation times and large population sizes, like viruses, perhaps it could. Some, like HIV, mutate very quickly, and it seems that having a lot of standing variation, as well as the ability to mutate quickly, allows HIV to propagate and adapt very well.
In animals, some loci are prone to accumulating more variation, but on the autosomes (non-sex chromosomes, and non-mtDNA) I don't think this is because of a higher mutation rate (that is, I don't think polymerase is more error prone in some areas versus others, but it could be). For example, the MHC complex, involved in the immune response, is a very diverse locus, and there is some evidence that animals preferentially choose mates with MHC loci that are different from their own.
Some plants also have regions of their genome with increased (relative to the rest of their genome) variability that are involved in reproduction, specifically reducing/preventing self-fertilizations.
That said, even with selection for variation at one locus, a lot of factors play into mate-choice, and there will be a lot of purifying or positive selection acting in the rest of the genome.
Shelldigger · 25 December 2013
pngarrison · 26 December 2013
The pop gen experts can correct me, but I think selection can increase diversity if it is balancing selection. That's supposed to be what drives the high diversity of HLA alleles. Of course that doesn't apply to effectively haploid regions like the Y.
John Harshman · 26 December 2013
harold · 26 December 2013
Just Bob · 26 December 2013
How's this for an example of selection increasing diversity: Females of a species, say deer, are 'programmed' to favor a certain trait in males, say big antlers. There will naturally be a bell-curve of antler sizes, with a few lucky well-endowed bucks at the extreme big end, say the 9th stanine. Since those guys will likely sire more offspring than the general run of bucks in stanines 4, 5, and 6, then (assuming there is a heritable component to antler size) there will be more unusually large antlers in the next generation. Thus the deer population has become more diverse. Instead of big antlers being very rare, there are now more of them mixed into the population.
Henry J · 26 December 2013
Maybe one needs to make a distinction between genetic diversity and phenotype diversity?
As I understand it, a genome (or a subset of it) will acquire diversity through mutation and drift, even without selection. With selection, it would still acquire diversity, I'm guessing somewhere around the same amount of it, but the differences would have some affect on anatomy or biochemistry.
But for phenotype diversity, that would seem to need variation and selection acting in a repeating loop of some sort, emphasizing or optimizing some trait that deals with something in the environment. If the something is another species that is also evolving, this could be a positive feedback loop. With lots of species evolving in different ways, and sometimes speciating, that would increase overall diversity.
Does that make sense?
Henry
https://me.yahoo.com/a/UIFqpY46nexUlCvhZ8zfKDh3zX4VO81SHItDeWm6L4agSA6W#dd2ba · 26 December 2013
Just Bob · 26 December 2013
Pierce R. Butler · 26 December 2013
... models of purifying selection are consistent with observed Y diversity.
IANABiologist, but my first thought on reading this was that relatively few males have fathered the majority of sons for enough generations to create a genetic bottleneck.
Are most of us descended from harems?
https://me.yahoo.com/a/UIFqpY46nexUlCvhZ8zfKDh3zX4VO81SHItDeWm6L4agSA6W#dd2ba · 26 December 2013
Flint · 26 December 2013
MememicBottleneck · 26 December 2013
Paul Burnett · 27 December 2013
Just Bob · 27 December 2013
Look, A Masked, some female deer selectively choose males with the biggest antlers. A mutation arises that allows bucks to grow antlers 10% larger than was ever possible before. Originally, there is only one buck out of millions with that trait: a homogeneous population. All the rest have a normal distribution of antler sizes. But Big Horn gets sexually SELECTED by a harem of does, and within a few generations Big Horns make up 20% of the population: a DIVERSE population. The diversity was created by SELECTION.
Will the Big Horn trait drive out all others and lead again to a homogeneous population? Not necessarily. Mega-antlers may have a metabolic cost that in hard times works against bucks with impressive headgear: a selection pressure AGAINST big antlers. The population could even oscillate. In good years mega-antlers proliferate because does preferentially SELECT them, increasing their component of the population, and thus DIVERSITY. When the drought hits, the well-racked guys suffer and the small-horned bucks preferentially survive, driving DIVERSITY in a different direction.
DIVERSITY means there are various traits in the population. In a world with varying environmental pressures, selection often increases diversity by favoring a trait that once was uncommon. Unless that specific selection factor is overwhelmingly strong and persists for many generations, the favored trait won't completely drive out the less-favored. As flint said, it's not a zero-sum game.
Robert Byers · 27 December 2013
Its not Surprising to the Y chromosome about its nature. Its the humans who study it who haven't figured it out!
If man has only been around 6000 years then there would, in many elements of genetics, be a lack of diversity. We haven't changed since Adam that much although some. At least since the fall.
This would be predicted by creationist(YEC) models.
John Harshman · 27 December 2013
Scott F · 28 December 2013
harold · 28 December 2013
fittest meme · 28 December 2013
. . . so glad you brought up the string on the Galapagos Fiches.
The Grants concluded that it was "Retrogressive Hybridization" that maintained the diversity of genotype and resulting phenotype within the various finch populations on the archipelago. Natural selection did indeed limit the diversity by favoring certain traits within isolated populations living within certain environments. Fortunately for the finches, this retrograde hybridization, (the ability for different populations to interbreed) maintained enough diversity so that if environmental condition did change, the highly specialized population (some would call them a species) wouldn't be wiped out.
Contrary to the commonly held belief of many in the scientific establishment, mutations were not the source of diversity displayed in these finches . . . instead it was inherent genetic diversity in he founding members of the Galapagos finch population.
By the way . . . one of my favorite observations of Rosemary Grant in this video is her revelation that all of the populations of Galapagos inches can indeed interbreed and that to become completely genetically incompatible it is supposed they would need to stay reproductively isolated or over 30,000,000 years! Quite different from what I was taught in most of my biology classes growing up.
I hope you in the field are doing a better job of teaching all of the evidence.
Scott F · 28 December 2013
harold · 28 December 2013
fittest meme · 28 December 2013
fittest meme · 28 December 2013
harold · 28 December 2013
harold · 28 December 2013
Just Bob · 28 December 2013
John Harshman · 28 December 2013
John Harshman · 28 December 2013
harold · 28 December 2013
harold · 29 December 2013
fittest meme · 29 December 2013
fittest meme · 29 December 2013
SWT · 29 December 2013
Dave Luckett · 29 December 2013
Just Bob · 29 December 2013
SWT · 29 December 2013
PA Poland · 29 December 2013
Zetopan · 29 December 2013
"Congressman Paul Ryan has frequently spoken of his obviously sincere admiration for Ayn Rand, yet also claims to be a practicing Catholic. (I think we can imagine what the current pope would have to say about Ayn Rand.)"
It's not very difficult to imagine:
http://www.salon.com/2013/12/27/paul_ryan_lectures_the_pope/
http://www.rawstory.com/rs/2012/04/26/under-catholic-pressure-paul-ryan-backs-away-from-rand-objectivism/
http://www.youtube.com/watch?v=wvvIEXQlQhc
http://www.salon.com/2012/08/14/ayn_rand_vs_the_pope/
http://www.deathandtaxesmag.com/187242/paul-ryans-former-catholic-priest-is-worried-about-his-ayn-rand-inspired-philosophy/
https://docs.google.com/document/d/1JRLM7Jh9PnrxptafWYENXdAmxnXd4gQJMYTu3H4TFHA/edit?pli=1
... etc.
fittest meme · 30 December 2013
SWT · 30 December 2013
fittest meme · 30 December 2013
DS · 30 December 2013
fittest meme · 30 December 2013
apokryltaros · 30 December 2013
fittest meme · 30 December 2013
fittest meme · 30 December 2013
pre-existing not per existing
degradation not desegregation
Mike Elzinga · 30 December 2013
apokryltaros · 30 December 2013
apokryltaros · 30 December 2013
SWT · 30 December 2013
fittest meme · 30 December 2013
Mike Elzinga · 30 December 2013
phhht · 30 December 2013
DS · 30 December 2013
fittest meme · 30 December 2013
phhht · 30 December 2013
apokryltaros · 30 December 2013
apokryltaros · 30 December 2013
DS · 30 December 2013
Mike Elzinga · 30 December 2013
Just Bob · 30 December 2013
Mike Elzinga · 30 December 2013
Scott F · 30 December 2013
Just Bob · 30 December 2013