[ExI] Armchair Evolutionary Psychology: Larks vs Night Owls

[The Avantguardian]

After looking at the avilable evidence, I would concur with Emlyn that
this is indeed a genetic phenomenon. The PER3 (Period homolog 3) gene
is a transcription factor and one of perhaps about a dozen highly
conserved circadian rhythm genes. They were first discovered in
drosophila (fruit flies) so these genes are very basal showing up very
early in the evolutionary history of life. They are expressed in the
retina and are associated with the photoreceptors there.  

With regard to Lee's question regarding the population genetics and
equilibrium of the various alleles of PER3, the according to the Archer
et. al. paper that Emlyn cited:

"The Per3 polymorphism correlated significantly with extreme diurnal
preference, the longer allele associating with morningness and the
shorter allele with eveningness. The shorter allele was strongly
associated with the delayed sleep phase syndrome patients, 75% of whom
were homozygous." 

Now this is a fairly convincing demonstration that homozygousity of the
shorter PER3 allele correlates with delayed sleep phase syndrome
(DSPS). Now if one looks at prevelance studies for DSPS one finds that
the prevalence is 0.17% and 0.13% in Norway and Japan respectively.
This seems to demonstrate that night owls are a widespread phenomenon
and there appears to be no linkage between DSPS and any particular
ethnic group. This would suggest that there is a stable equilibrium at
an average ~0.15% for extreme night owls and that the mutation arose
well before the divergence of "racial" characteristics amongst humans. 


http://www.ncbi.nlm.nih.gov/pubmed/10607071
http://www.blackwell-synergy.com/doi/abs/10.1046/j.1440-1819.1999.00533.x
http://www.chestjournal.org/cgi/content/full/130/6/1915#B13

Another curious observation about DSPS is that it is of much higher
prevalence in teenagers than in adults, estimated at over 7% and skewed
toward males. But the ratio of DSPS equalizes between sexes during
adulthood. (Pelayo, R, Thorpy, MJ, Govinski, P Prevalence of delayed
sleep phase syndrome among adolescents [abstract].Sleep Res
1988;17,392)

So what do the above quoted numbers tell us about the population
genetics of larks and nightowls? Well lets try to estimate the
frequency of the short and long PER3 alleles that I will designate as O
(for owl) and L (for lark). Now if we have homozygous owl individuals
OO at 0.15% of the population. The assumption of Hardy Weinberg
equilibrium (a gross simplification but still informative) would mean
that LL^2 + 2LO + OO^2 = 1. This would entail that the overall
frequency of the O allele is about sqrt(0.0015)=.0387 while the L
allele would be at 1-.0387 or 0.9613 in the general population.

So this is where it gets interesting. It is fairly well accepted that
even a *fatal* recessive mutant allele will remain stable in a
population over time as long as there is some selective benefit to
being heterozygous for that mutation over being homozygous for the
dominant allele. (c.f. sickle cell anemia, cystic fibrosis, etc.) If we
calculate the frequency of heterozygotes for owlishness we have
2LO=0.0775 or about 7.75% of the population being heterozygotes. This
is very close to the published prevalence of DSPS in adolescents.

Going back to the "sneaky fucker" hypothesis, if these, heterozygotes
got to mate earlier in life because their DSPS allowed them to engage
in sex without getting trounced by the sleeping dominant males, it
would explain why such a deleterious mutation would remain stable in
the population. This also explains why the majority of adolescent DSPS
cases are male, since they have most to gain by being sneaky fuckers. 

While this may seem to imply some sort of sex-linkage for PER3 which is
*not* observed, it could easily be a case of incomplete penetrance or
epistasis on the part of the female heterozygotes for PER3. In response
Damien's concerns about insufficient cultural information regarding
sleeping and sexual arrangements in our ancestors, I would surmise that
variations in such a basal genetic system as circadian rhythm genes,
would predate the evolutionary split between us and chimpanzees.
Therefore the cultural details would be less relevant in this instance.
Of course the only way to find out for sure is to look for nightowls
among chimpanzees and find out if they are indeed sneaky fuckers.

BTW, Stephano, you may may find this interesting:

http://en.wikipedia.org/wiki/Non-24-hour_sleep-wake_syndrome

On a more general note, I tend to object to calling what amounts to
completely natural genetic variations syndromes, disorders, and
diseases. From my point of view they are simply variants. If we lived
underground, or in one of Spike's space ships, dwarfism would be the
norm. If people had complete knowledge of themselves at the genetic
level, I am certain that practically everyone would belong and identify
with one genetic minority group or another, irrespective of the
biological fallacy known as "race".

Stuart LaForge
alt email: stuart"AT"ucla.edu

"Life is the sum of all your choices."  
Albert Camus


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