The human brain utilizes about ~20% of the calories you take in per day. It’s a large and metabolically expensive organ. Because of this fact there are lots of evolutionary models which focus on the brain. In Richard Wrangham suggests that our need for calories to feed our brain is one reason we started to use fire to pre-digest our food. In Geoffrey Miller seems to suggest that all the things our big complex brain does allows for a signaling of mutational load. And in Robin Dunbar suggests that it’s social complexity which is driving our encephalization.
These are all theories. Interesting hypotheses and models. But how do we test them? A new preprint on bioRxiv is useful because it shows how cutting-edge methods from evolutionary genomics can be used to explore questions relating to cognitive neuroscience and pyschopathology, Polygenic selection underlies evolution of human brain structure and behavioral traits:
…Leveraging publicly available data of unprecedented sample size, we studied twenty-five traits (i.e., ten neuropsychiatric disorders, three personality traits, total intracranial volume, seven subcortical brain structure volume traits, and four complex traits without neuropsychiatric associations) for evidence of several different signatures of selection over a range of evolutionary time scales. Consistent with the largely polygenic architecture of neuropsychiatric traits, we found no enrichment of trait-associated single-nucleotide polymorphisms (SNPs) in regions of the genome that underwent classical selective sweeps (i.e., events which would have driven selected alleles to near fixation). However, we discovered that SNPs associated with some, but not all, behaviors and brain structure volumes are enriched in genomic regions under selection since divergence from Neanderthals ~600,000 years ago, and show further evidence for signatures of ancient and recent polygenic adaptation. Individual subcortical brain structure volumes demonstrate genome-wide evidence in support of a mosaic theory of brain evolution while total intracranial volume and height appear to share evolutionary constraints consistent with concerted evolution…our results suggest that alleles associated with neuropsychiatric, behavioral, and brain volume phenotypes have experienced both ancient and recent polygenic adaptation in human evolution, acting through neurodevelopmental and immune-mediated pathways.
The preprint takes a kitchen-sink approach, throwing a lot of methods of selection at the phenotype of interest. Also, there is always the issue of cryptic population structure generating false positive associations, but they try to address it in the preprint. I am somewhat confused by this passage though:
Paleobiological evidence indicates that the size of the human skull has expanded massively over the last 200,000 years, likely mirroring increases in brain size.
From what I know human cranial sizes leveled off in growth ~200,000 years ago, peaked ~30,000 years ago, and have declined ever since then. That being said, they find signatures of selection around genes associated with ‘intracranial volume.’
There are loads of results using different methods in the paper, but I was curious note that schizophrenia had hits for ancient and recent adaptation. A friend who is a psychologist pointed out to me that when you look within families “unaffected” siblings of schizophrenics often exhibit deviation from the norm in various ways too; so even if they are not impacted by the disease, they are somewhere along a spectrum of ‘wild type’ to schizophrenic. In any case in this paper they found recent selection for alleles ‘protective’ of schizophrenia.
There are lots of theories one could spin out of that singular result. But I’ll just leave you with the fact that when you have a quantitative trait with lots of heritable variation it seems unlikely it’s been subject to a long period of unidirecitional selection. Various forms of balancing selection seem to be at work here, and we’re only in the early stages of understanding what’s going on. Genuine comprehension will require:
– attention to population genetic theory
– large genomic data sets from a wide array of populations
– novel methods developed by population genomicists
– and funcitonal insights which neuroscientists can bring to the table
3 thoughts on “Quantitative genomics, adaptation, and cognitive phenotypes”
Re: “the human skull has expanded massively over the last 200,000 years“.. is it possible they just forgot a 0? 2,000,000 years is pretty much when massive skull expansion started. eg Brain size graph
By mentioning the 20% calories figure (~66% at age 4!), you imply one of the balancing factors is malthusian. If that is the main driver, then it’s possible cognitive efficiency is under constant selection, even if brain size itself goes up or down depending on local selective pressures at any given point in time/environment. Not clear to me from the paper if that might be detectable, but in principle it should be as neuroscientists make progress on genetic underpinnings of cognition.
The larger point on balancing selection is it implies plenty of standing variation for greater cognitive capability. You’d just need on margin slightly more food (if that were the main driver), which is no longer a constraint. This standing variation would show up in the polygenic IQ score papers now coming out. Supporting the Stephen Hsu view. If there weren’t balancing selection, that approach should still work, but far less well.
Not quite sure about logic of above two points. So am interested in comments if they are off base. Thanks.
All that goes on in the brain being multidimensional and all. . .
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