Transcript Are Ashkenazi Jews an example of natural selection for increased

Ashkenazi Jews & Intelligence
Freud, Einstein, Mahler
Kristy Brady
8 February 2006
Outline
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Heterozygote advantage
Example: sickle-cell anemia & malaria
A brief history of Ashkenazi Jews
Observations on Ashkenazi Jew intelligence
& disease
• Hypothesis correlating intelligence &
disease
• Proposed evolutionary mechanisms
• Alternative hypotheses
Heterozygote Advantage
(aka: overdominance)
• A case in which heterozygotes have a higher
fitness than homozygotes at a given locus.
Maintains genetic polymorphism in a
population.
− e.g., sickle-cell allele & malaria tolerance
− e.g., cystic fibrosis allele & TB resistance
Distribution of sickle-cell anemia &
malaria
Green: regions with malaria
Red stripes: regions with high frequency of sickle-cell trait
Heterozygote Advantage
Genotypes (S = wild-type allele, s = sickle-cell allele)
SS: no sickle-cell trait, not malaria tolerant
Ss: no sickle-cell disease, malaria tolerant
ss: sickle-cell disease
sickle-cell
malaria
Ashkenazi Jews: Observations
• Have the highest average IQ test scores of any ethnic
group (for which there are data).
• During the 20th century, comprised ~3% of the U.S.
population, but won 27% of Nobel prizes awarded to
U.S. scientists.
• Represent over half of the world chess champions.
• High incidence of sphingolipid storage diseases: TaySachs, Gaucher, Niemann-Pick.
• High incidence of certain cancers (DNA repair cluster
mutations).
Ashkenazi Jews: A brief history
• 3 Jewish groups: Ashkenazi
(blue), Middle Eastern (green),
Sephardic (red)
• Ashkenazi Jews (from Hebrew
word for German) moved
north of Alps in 1st millennium
A.D. Settled in Rhineland
during 800s.
• In 12th & 13th centuries,
expelled from Western Europe.
Moved to Poland & Lithuania
where Ashkenazi center
remained for 5 centuries.
Ashkenazi Jews: A brief history
• In 19th & 20th centuries, large migration to W. Europe,
Americas, Australia, & South Africa
Sphingolipid Storage Diseases –
neurological disorders
Sphingolipids: A member of a class of lipids
derived from the aliphatic amino alcohol sphingosine.
They play an important role in signal transmission
and cell-cell recognition.
•Gaucher: accumulation of
sphingolipid that promotes
growth & branching of axons.
•Niemann-Pick & Tay-Sachs:
accumulation of sphingolipids
that promote growth of
dendrites.
Hypothesis
•Heterozygote advantage (e.g., Gaucher allele,
recessive)
– GG: wild-type
– Gg: no disease, moderate increased
linkage between brain cells yields
increased intelligence
– gg: Gaucher disease, increased linkage
between brain cells yields increased
intelligence (as measured by IQ tests)
•Similar scenarios for heterozygotes with
Niemann-Pick or Tay-Sachs alleles.
Evolutionary mechanisms
•Selective pressures
− money-related occupations (e.g., banking, tax
farming); today IQ and success are positively
correlated in these professions
•Assortative mating/no gene flow
– Ashkenazim tend to marry among themselves
Evolutionary mechanisms
•Fitness
– in Europe, prior to the 18th century, affluent
families tended to have more children surviving to
adulthood than poorer families
•Effective population size
– ~40% of Ashkenazi Jews
are descended from 4
women living sometime in
the last 2,000 yrs (Am. J. Hum.
Gen. 2006, 78:487-97)
Evolutionary mechanisms
•Heritability of IQ
– in youth “in impoverished families, 60% of the
variance in IQ is accounted for by the shared
environment, and the contribution of genes is
close to zero; in affluent families, the result is
almost exactly the reverse.” (Psych. Sci. 2003, 14:623-8)
– in youth “past experiences…influence today’s IQ
only because of their effect on past IQ and the
effect of past IQ on today’s environment” (Psych. Rev.
2001, 108:346-69)
– adult IQ largely affected by adult environment
(Psych. Rev. 2001, 108:346-69)
Alternatives to selection?
•Genetic drift
– evidence of genetic bottleneck
•Diet
– quantity of omega-3 fatty acids consumed by a
pregnant mother greatly influences a child’s
verbal IQ & social skills
•Culture
– education highly
valued
Potential Experiments
• IQ tests controlled for genotypes
• Environmental effect studies
– dietary controls