Tag: Human Evolutionary Genomics

Reflections on the evolution at ASHG 2012

By Razib Khan | November 11, 2012 1:54 pm

As most readers know I was at ASHG 2012. I’m going to divide this post in half. First, the generalities of the meeting. And second, specific posters, etc.


- Life Technologies/Ion Torrent apparently hires d-bag bros to represent them at conferences. The poster people were fine, but the guys manning the Ion Torrent Bus were total jackasses if they thought it would be funny/amusing/etc. Human resources acumen is not always a reflection of technological chops, but I sure don’t expect organizational competence if they (HR) thought it was smart to hire guys who thought (the d-bags) it would be amusing to alienate a selection of conference goers at ASHG. Go Affy & Illumina!

- Speaking of sequencing, there were some young companies trying to pitch technologies which will solve the problem of lack of long reads. I’m hopeful, but after the Pacific Biosciences fiasco of the late 2000s, I don’t think there’s a point in putting hopes on any given firm.

- I walked the poster hall, read the titles, and at least skimmed all 3,000+ posters’ abstracts. No surprise that genomics was all over the place. But perhaps a moderate surprise was how big exomes are getting for medically oriented people.

- Speaking of medical/clinical people, I noticed that in their presentations they used the word ‘Caucasian‘ a lot. This was not evident in the pop-gen folks. It shows the influence of bureaucratic nomenclature in modern medicine, as they have taken to using somewhat nonsensical US Census Bureau categories.

- Twitter was a pretty big deal. There were so many interesting sessions that I found myself checking my feed constantly for the #ASHG2012 hashtag. It was also an easy way to figure out who else was at the same session (e.g., in my case, very often Luke Jostins).

- If you could track the patterns of movements of smartphones at the conference it would be interesting to see a network of clustering of individuals. For example, the evolutionary and population genomics posters were bounded by more straight-up informatics (e.g., software to clean your raw sequence data), from which there was bleed over. But right next to the evolution and population genomics sections (and I say genomics rather than genetics, because the latter has been totally subsumed by the former) you had some type of pediatric disease genetics aisles. I wasn’t the only one to have a freak out when I mistakenly kept on moving (i.e., you go from abstruse discussions of the population structure of Ethiopia, to concrete ones about the likely probability of death of a newborn with an autosomal dominant disorder, with photos of said newborn!).

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Crohn's disease is about barely keeping you alive

By Razib Khan | August 10, 2011 1:05 am

The Pith: Natural selection is a quick & dirty operator. When subject to novel environments it can react rapidly, bringing both the good and the bad. The key toward successful adaptation is not perfection, but being better than the alternatives. This may mean that many contemporary diseases are side effects of past evolutionary genetic compromises.

The above is a figure from a recent paper which just came out in Molecular Biology and Evolution, Crohn’s disease and genetic hitchhiking at IBD5. You probably have heard about Crohn’s disease before, there are hundreds of thousands of Americans afflicted with it. It’s an inflammatory bowel ailment, and it can be debilitating even to very young people. The prevalence also varies quite a bit by population. Why? It could be something in the environment (e.g., different diet) or genetic predisposition, or some combination. What the figure above purports to illustrate is the correlation between Crohn’s disease and the expansion of the agricultural lifestyle.

But don’t get overexcited Paleos! There are many moving parts to this story, and I need to back up to the beginning. The tens of thousands of genes which you inherited from your parents are embedded within the genome and aligned in a set of sequences, one after the other. On the one hand for the purposes of conceptualizing evolutionary dynamics, such as natural selection or random genetic drift, focusing on a single gene is useful. It has power to illustrate some basic and elementary principles.  But sometimes you need to take a more synoptic view, and look at genes in their broader context. In this post I’ll avoid molecular or statistical epistasis, gene-gene interaction. Rather, let’s just consider the static landscape of the genome, where genes are physical concrete entities which are embedded in a particular spatial relationship to other genes, upstream or downstream in the genetic code. These physical or statistical associations of genes can form a de facto supergene through linkage, and their variants combine to form haplotypes, sequences of markers across small stretches of the genome. But recall that these associations are counter-balanced by genetic recombination, which tears apart physical sequences and sows them to the opposite DNA strand.

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Why the human X chromosome is less diverse

By Razib Khan | July 25, 2011 12:37 am

The Pith: The human X chromosome is subject to more pressure from natural selection, resulting in less genetic diversity. But, the differences in diversity of X chromosomes across human populations seem to be more a function of population history than differences in the power of natural selection across those populations.

In the past few years there has been a finding that the human X chromosome exhibits less genetic diversity than the non-sex regions of the genome, the autosome. Why? On the face of it this might seem inexplicable, but a few basic structural factors derived from the architecture of the human genome present themselves.

First, in males the X chromosome is hemizygous, rendering it more exposed to selection. This is rather straightforward once you move beyond the jargon. Human males have only one copy of genes which express on the X chromosome, because they have only one X chromosome. In contrast, females have two X chromosomes. This is the reason why sex linked traits in humans are disproportionately male. For genes on the X chromosome women can be carriers of many diseases because they have two copies of a gene, and one copy may be functional. In contrast, a male has only a functional or nonfunctional version of the gene, because he has one copy on the X chromosome. This is different from the case on the autosome, where both males and females have two copies of every gene.

This structural divergence matters for the selective dynamics operative upon the X chromosome vs. the autosome. On the autosome recessive traits pay far less of a cost in terms of fitness than they do on the X chromosome, because in the case of the latter they’re much more often exposed to natural selection via males. In the rest of the genome recessive traits only pay the cost of their shortcomings when they’re present as two copies in an individual, homozygotes. A simple quasi-formal example illustrates the process.

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Evolution may explain why baby comes early

By Razib Khan | April 16, 2011 5:36 pm

Image credit

The Pith: In this post I review a paper which covers the evolutionary dimension of human childbirth. Specifically, the traits and tendencies peculiar to our species, the genes which may underpin those traits and tendencies, and how that may relate to broader public health considerations.

Human babies are special. Unlike the offspring of organisms such as lizards or snakes human babies are exceedingly helpless, and exhibit an incredible amount of neoteny in relation to adults. This is true to some extent for all mammals, but obviously there’s still a difference between a newborn foal and a newborn human. One presumes that the closest analogs to human babies are those of our closest relatives, the “Great Apes.” And certainly the young of chimpanzees exhibit the same element of “cuteness” which is appealing to human adults. Still there is a difference of degree here. As a childophobic friend observed human infants resemble “larvae.” The ultimate and proximate reason for this relative underdevelopment of human newborns is usually attributed to our huge brains, which run up against the limiting factor of the pelvic opening of women. If a human baby developed for much longer through extended gestation then the mortality rates of their mothers during childbirth would rise. Therefore natural selection operated in the direction it could: shortening gestation times. You might say that in some ways then the human newborn is an extra-uterine fetus.

ResearchBlogging.orgA new paper in PLoS Genetics attempts to fix upon which specific genomic regions might be responsible for this accelerated human gestational clock. An Evolutionary Genomic Approach to Identify Genes Involved in Human Birth Timing:

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To gain pallor is easier than losing it

By Razib Khan | September 28, 2010 5:21 pm


John Hawks illustrates what can be gained at the intersection of old data and analysis and new knowledge, Quote: Boyd on New World pigmentation clines:

I’m using some statistics out of William Boyd’s 1956 printing of Genetics and the Races of Man[1]. It gives a good accounting of blood group data known more than fifty years ago, which I’m using to illustrate my intro lectures. Meanwhile, there are some interesting passages, from the standpoint of today’s knowledge of the human genome and its variation.

On skin pigmentation – this is the earliest statement I’ve run across of the argument that the New World pigmentation cline is shallower than the Old World cline because of the relative recency of occupation….

Looking at what was said about pigmentation generations ago is of interest because it’s a trait which in many ways we have pegged. See Molecular genetics of human pigmentation diversity. Why humans vary in pigmentation in a deep ultimate sense is still an issue of some contention, but how they do so, and when the differences came about, are questions which are now modestly well understood. We know most of the genetic variants which produce between population variation. We also know that East and West Eurasians seem to have been subject to independent depigmentation events. We also know that some of the depigmentation was relatively recent, probably after the Last Glacial Maximum, and possibly as late as the advent of agriculture.

On the New World cline, which is clearly shallower than that of the Old World. The chart below from Signatures of positive selection in genes associated with human skin pigmentation as revealed from analyses of single nucleotide polymorphisms is useful:

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