1. Genetic Variation - The fuel of natural selection
Campbell et al, chapter 23

2. Populations are polymorphic
Nature vs. Nurture
Source of variation
Maintenance of variation

3. Populations are polymorphic
Nature vs. Nurture
Source of variation
Maintenance of variation

4. Polymorphic populations
Example: Darwin finches on Galapagos

5. Polymorphic populations
Example: Lazuli bunting

6. Polymorphic populations
Example: Swallowtail butterfly

7. Populations are polymorphic
Nature vs. Nurture
Source of variation
Maintenance of variation

8. Nature vs. Nurture Genome (blueprint) Nature Phenotypic Expression
Environment
Nurture
Protein
I found the gene
that makes us
believe all
traits are based
on genes

9. Nature vs. Nurture
Gene
Chromosome
Genome
Genepool

10. Nature vs. Nurture Diploidy There are 2 copies of each gene
Father Mother
Offspring
2 identical copies =
homozygot
Father Mother
Offspring
2 different copies =
heterozygot

11. Nature vs. Nurture Genotype is more variable than phenotype
Only genetic variation counts for evolution

12. Nature vs. Nurture How to separate the two?
Example: Altitudinal gradient
Common Garden Exp.
Gene
Environment
Mixture of both

13. Populations are polymorphic
Nature vs. Nurture
Source of variation
Mutation
Recombination
Maintenance of variation

14. Source of Genetic Variation
1. Mutation
Change in: DNA sequence
Chromosome structure
Number of Chromosomes
Due to: Copying Errors
Environmental factors

15. Source of Genetic Variation
2. Recombination
Reshuffling of chromosomes during reproduction
Crossing over A B C A B C

16. Populations are polymorphic
Nature vs. Nurture
Source of variation
Maintenance of variation
Selection
Heterosis

17. Maintenance of Variation
1. Stabilising selection
Favours mean over tail
Loss of variation

18. Maintenance of Variation
1. Stabilising selection
Example: Darwin Finches
Bill size
Foraging efficiency

19. Maintenance of Variation
2. Directional selection
One tail is favoured over
the other
Trait is changing over time

20. Maintenance of Variation
2. Directional selection
Example: Heliconius butterfly on passion flower vines
Plant:
Toxin
Insect:
Enzymes
Plant:
False eggs
Leads to an arms race between plant and insect

21. Maintenance of Variation
2. Directional selection
The arms race idea lead to a more general hypothesis:
Red Queen Hypothesis
‘It takes you all the running to stay in place’

22. Maintenance of Variation
2. Directional selection
Selection may change in time
Example: Darwin Finches on Galapagos
El Nino
Bill size
wet dry
small large seeds
Time

23. Maintenance of Variation
2. Directional selection
Selection may change in space
Example: African Firefinches

24. Maintenance of Variation
3. Disruptive selection
Selection favours the two
tails over the mean
There are two forms

25. Maintenance of Variation
3. Disruptive selection
Example: Lazuli buntings
Showy
male
Average
male
Drab
male
Sneak in
attracted

26. Maintenance of Variation
4. Frequency-dependent selection
A mode of selection where a phenotype is only
favoured when it is either rare or common.

27. Maintenance of Variation
4. Frequency-dependent selection
Example: Swallowtail butterfly (Papillo dardanus)
Males
Swallowtail females mimic
these toxic species without
being toxic themselves
Toxic
species
Only works if the cheats are rare

28. Maintenance of Variation
5. Heterozygote advantage
Heterozygote individuals have higher fitness than either
homozygote individuals.
This is a common principle in plant and animal breeding

29. Maintenance of Variation
5. Heterozygote advantage
Example: Red blood cells - sickle cell disease
Homozygot (normal cells):
vulnerable to malaria
Homozygote (sickled cells):
lethal
Heterozygote:
non-lethal & resistant to
malaria

30. A large number of processes
create and maintain genetic
variation that is the base for
evolution...
…but does this lead to
new species? - next here