Evolution did not stop at the neck: APOE4

Apolipoprotein E (APOE) has a well established link to a general neuro-psychiatric disorder: Alzheimer’s disease.   Studies show  that subjects with one copy of the e4 APOE4 allele have 4 times the odds of getting AD and those with  two copies have 20 times the risk.  A diagnosis of Alzheimer's dementia is not subject to the vagaries of psychometrics.   APOE e4 is also associated with increased cognitive decline in non-demented adults (Fan 2008) and even  some decrease in the cognition of children has been observed (Bloss 2008). 

So is the incidence of the e4 allele the same in all human populations? Probably not, Eisenberg (2010) has the latest paper to find that the incidence varies by latitude.  Populations near the equator and the Arctic have a higher incidence than those at mid latitudes.   They found this curvilinear relationship even after adjusting for population structure supporting their hypothesis that the differences are driven by the natural selection instead of genetic drift. 

APOE4 is associated with high cholesterol levels and Eisenberg's hypothesis is that this was beneficial in warm or cold climates where humans had high metabolic rates.  Fortunately, human equality is a contingent fact of history so we can ignore these findings.