1. Mechanisms of Evolution

2. Changes in Gene Frequency
of the Gene Pool
of a Population

3. The Gene Pool
Definition: the frequency of alleles in a population B/B+b = Freq B b/B+b = Freq b

4. Genetic Variation in the Gene Pool Polygenic Traits in a Population
Fingers
and
Beans

5. Genetic Variation in the Gene Pool

6. Gene Frequency change over time:
Small Gene Pool - more variation
Selective Mating (Choice or Isolation)
Mutation
Migration/Emigration (Gene Flow)
Natural Selection - adaptation

7. 1. Small Gene Pool – Genetic Drift random fluctuations in gene frequency
Genetic drift refers to random fluctuations in the gene frequency of a population over time. This is more likely to occur in a small population. The smaller the population, the more the gene frequencies are likely to fluctuate from generation to generation.
In the example here, we start with the assumption that 50% of the genes are one allele and the other allele is 50%. The population size is small – 20 individuals. If we run the simulations on this small population we get a wide variation in the allele frequnecy over time but two observations stand out: 1. one of the alleles disappears from the population and 2. the disappearance is within a few generations. Neither allel is better or more fit than the other but random selection occurs and one of the two alleles is eliminated. This pattern is expected in small populations. In large populations, the alleles persist and remain relativity constant in frequency over many generations.

8. Large Gene Pool – Slight Genetic Drift
With a large population, n=1000; both alleles persist through many generations. We call this population equilibrium. When one observes this equilibrium, one knows that neither allele conveys an advantage and therefore there is no favoring of one allele over the other.

9. 2. Selective Mating
If one color or one pattern is more attractive to a mate, that pattern or color will persist and the others will decrease in frequency and may eventually disappear.

10. 3. Mutations

11. 4. Emigration / Immigration:

12. 5. Natural Selection
This lab illustrates the

13. What if natural selection is operating?
O r I g I n a l
O v e r T i m e
Stabilizing Selection
Directional Selection
Disruptive Selection

14. Stabilizing Selection
Range narrows
One phenotype dominates

15. Directional Selection
the mean shifts
One end of the distribution gains dominance

16. Directional Selection
One phenotype gains dominance
Allele Frequency
Generation 1
Generation 8

17. Directional Selection
and back again
p2 + 2pq + q2 = 1

18. Disruptive Selection
Splitting the mean
Two phenotypes dominate

19. Speciation barriers to gene flow
How new species arise
The process of forming new species is called speciation.

20. Isolation

21. Geographic Isolation

22. the Bottle Neck Effect Reduced population size
Reduced genetic diversity
Founder Population
rare alleles increase in frequency

23. Reproductive Isolation

24. Adaptive radiation
emergence of many species
from one original species

25. Darwin’s Finches

26. Survival of the Fittest Descent with modification
Adaptive Radiation and Beak Shape
Survival of the Fittest
Descent with modification

27. regulation of the Bmp4 protein is the principal way in which beak variation occurs in the finches

28. Gradualism or
Punctuated
Equilibrium?

29. Gradualism or
Punctuated
Equilibrium?

30. There is no
master plan