1. Weak forces in Evolution
Dan Graur

2. 1. 2. Changes in allele frequencies are important.
Changes in genotype frequencies are not so important. 1. 2.

3. Mating

4. Deviation from randomness:
By genetic similarity:
Assortative mating
Disassortative mating
By genetic relatedness:
Inbreeding
Outbreeding

5. Skin color

6. disassortative
assortative

7. Human assortative mating: Rule #1

8. Strong assortative mating by skin color in the US
Percentage of non-hispanic whites: 80%
Percentage of blacks: 13%
Expected percentage of interracial marriages: 10.4%
Observed percentage of interracial marriages: 0.9%
Ratio of = 2:1
white female/black male
black female/white male

9. Assortative mating is very strong in humans even with respect to non-genetic traits.

10. Degrees of inbreeding found in nature:

12. Inbreeding is usually forbidden…
P = Ptolemy
C = Cleopatra
Inbreeding is usually forbidden…
P ix
P xii
C v
C vii
An exception!

13. The fish Rivulus marmoratus exhibits the most extreme form of inbreeding: Selfing

14. equilibrium allele frequencies
Random mating 
equilibrium allele frequencies

15. Assortative mating 
excess homozygotes

16. Disassortative mating
excess heterozygotes

17. extreme deviations from random mating allele frequencies remain
Even if
extreme
deviations
from random
mating
occur in all
generations,
allele
frequencies
remain
constant.

18. Mating pattern
is not an important
evolutionary force

19. Clinical effects of inbreeding
P.T. Barnum + Tom Thumb

20. Clinical effects of inbreeding

22. Migration

24. Migration will cause changes in the allele frequencies of each of the two subpopulations.

25. However, because of gene flow, the two subpopulations are, in fact, one population, in which allele frequencies do no change.

26. Genotype frequencies will change in a similar fashion to that in disassortative mating.

27. Migration
is not an important
evolutionary force

28. Mutation
Mutation:
A transmissible change in the genetic material

29. Mutations are the ultimate source of variation
Mutations are the ultimate source of variation. Only mutations can create genetic novelty.

30. Mutations arise all the time
Mutations arise all the time. Per definition, the initial frequency of a mutation in a diploid population is 1/2N.*
*N = population size

31. …resulting in a change in allele frequencies from 0 to 0.00000000007.
The human population on October 31, 2011 was estimated to be 7 billion people. The number of alleles at an autosomal locus is, therefore, 14 billion. A mutation arising today in the human population will have an initial frequency of about 7 × 10-11…
…resulting in a change in allele frequencies from 0 to

33. Mutation is the
ultimate source of
variability, but it is
not an important
evolutionary force.

34. For a mutation to become significant, it must increase its frequency, so that it becomes fixed* in the population.
*frequency of allele = 1.0

35. Two factors can lead to the fixation of a new mutation:
Selection
Random genetic drift