1. Population Migration and the Variation of Dopamine D4 Receptor (DRD4) Allele Frequencies Around the Globe
Moira Maxwell

2. Dopamine
The neurotransmitter dopamine plays a huge role in the body, and has been implicated in ADHD
D4 is a dopamine receptor gene that has 2-11 repeat units.
DRD4 in humans is linked to:
Novelty-seeking
Hyperactivity
Risk-taking behaviors
These are typical ADHD behaviors
Consistent with findings in animal studies:
Increase in exploratory behavior
Increase in speed and locomotion
These behaviors are adaptive for maximizing resources in a new environment

4. Hypotheses
Populations with a history of migration in the past 1, ,000 years will show high rates of DRD4 alleles.
Macro-migration: Populations that have migrated many thousands of miles will show a higher rate of DRD4 alleles than populations who have not migrated as far.
Micro-migration: Populations that use sedentary subsistence strategies will have a lower percentage of DRD4 alleles than nomadic populations.

5. Methods
12 studies with a total sample size of 2,320 individuals with no psychiatric disorders from 39 countries were analyzed.
4,640 alleles looking for the proportion of 7-repeats the proportion of long alleles as an index of genotype in a population

6. Macro-migration
Estimated in thousands of miles (kmiles) of population from homeland of its language family.
Based on the archeological record and historical linguistics
Macro-migrations occured 1,000-30,000 years ago

7. 6 Main Migration Routes
American Indians- NE Asia>North America>Central and South America
China>Japan/Taiwan/SE Asia/The Pacific
Jews>Ethiopia/Yemen/Europe
West Africa>Central/Eastern/Southern Africa
Black Sea>Europe(Indio Europeans)
Melanesians>SE Asia

8. Table 2 (highlight muskoke, guahibo, ticuna, roman jews, malay, bantu, swedes

9. Macro-migration hypothesis
A strong correlation of r =.85 (p<.001) was found between kmi traveled and DRD4 long allele frequency distributions
Supports hypothesis for macro-migration
used a solid p-value

10. Fig 1
C. Chen et al.
of correlations. A more accurate view is that autocorrelation increases the variance

11. Micro-Migration
Measures survival strategies used by the 39 societies in this analysis.
Societies were coded as either sedentary=0 (28 societies) or as nomadic=1 (11 societies)
Nomadic populations had a 10.4% higher rate of DRD4 long alleles than sedentary ones.

12. Micro-migration hypothesis
A correlation of r =.52 (p<.001) was found between percentage of long alleles and subsistence type in a population
Supports hypothesis for micro-migration

13. Macro and micro-migration
Correlation of macro and micro-migration was r =.39 (p<.05)
Regression analysis showed that macro-migration was a better indicator of DRD4 allele frequency than micro-migration
For every kmi away from homeland, the proportion of DRD4 long alleles increased by 4.3%

14. The confound of Auto-correlation
Occurs when sampling units are not independent
Leads to Type I error (rejecting null when it is actually true)
All societies in this study have had contact with other societies, making it hard to discern if there is an effect of auto-correlation or not. This is particularly true of American Indians who are known to share common ancestry with each other.
Researchers corrected for this by including all available data in order to increase sample size
After correction, hypothesis for macro-migration was still supported, but not as strongly for micro-migration as p-value dropped from p<.001 to p<.11

15. Does the bottleneck effect explain the results?
Populations that are isolated after migration experience a bottleneck effect because of a limited gene pool.
None of the other 128 alleles examined followed the migratory patterns of the populations

16. DRD4 allele frequency and migration
Correlation found? Yes
Causation? Three competing hypotheses
“Founder’s effects” or
“Natural selection/deselection”
Spontaneous mutation - can rule this out because r is too high to be by chance

17. Founder’s effects hypothesis
Populations migrated because of the genetic makeup of their members
More long alleles in population would increase exploratory behavior which may increase group migration
DRD4 allele frequencies seen in early populations would be the same in modern populations

18. Natural selection / deselection
Forced migration due to
War
Depletion of natural resources causing famine
Natural selection in populations that adapted to multiple migrations would cause an increase of DRD4 genes
In sedentary populations, lack of migration would cause a deselection of DRD4 long alleles.

19. How to test
Ideal solution: Compare genetic data from populations that is at least 15,000 years old (pre-agriculture) and compare that with modern genetic data on frequency of DRD4 alleles.
This is not possible yet, so the authors chose the proximate mechanism of testing DRD4 frequency in modern day immigrants and in their counterparts remaining in country of origin.

20. Table 3

21. Results
Table 3 shows a lack of support for the “Founder’s effects” hypothesis as DRD4 frequency rates are the same for immigrants and non-immigrants
Founder’s hypothesis and NS/dS hypothesis are competing, so therefore the “Natural selection/deselection” hypothesis is supported.

22. Natural selection/deselection
Humans began as nomadic foragers who migrated away from their places of origin repeatedly. Researchers believe there would have been strong pressure to select for genes that favored exploratory behaviors. Repeated migrations would continue to increase the frequency of these genes.
~15,000 years ago humans began to use agriculture rather than hunter-gathering. Populations who did not migrate far after agriculture would have deselected for migratory genes.
Deselection appears to explain the effects micro-migration on DRD4 frequencies.

23. ADHD
Thought to be a result of selective pressures on migratory behaviors.
Very adaptive then, but not in our (almost) exclusively sedentary modern societies.
Evolutionary time-lag: If society persists as sedentary for long enough, a decrease in ADHD will be seen as it will no longer be adaptive.

24. Further testing Testing on non-human primates
Are there high rates of DRD4 long alleles in other populations with macro-migration?

25. Global rates of ADHD
Worldwide rate of ADHD is 5.2% (Polanczyk et. al, 2007)
“The North American rate (6.2%) only slightly exceeded the European rate (4.6%). The highest rates emerged from Africa (8.5%) and South America (11.8%). Corroboration comes from a dimensional ADHD scale used in 21 countries. Japanese and Finnish children scored lowest, Jamaican and Thai children scored highest, and American children scored about average.” (Moffitt and Melchior, 2007)