1. Modes of selection

2. Stabilizing Selection

3. Cumulative effects of disruptive selection can be large
Chose high and low oil plants to breed

4. Disruptive selection
Selected only flies with high or low number of bristles to reproduce

5. Selection Does Not Mean Evolution!!!
Evolution is a change in the frequencies of alleles in a population
Selection can lead to evolution if the difference in reproductive success is tied to genetic variation
Speed of change (evolution) depends on
Amount of variation
Selection differential (S)
Heritability (h2)

6. Evolutionary response to selection
Imagine a lake stocked with bluegills
Evolution only results if the difference in size is due to additive genetic differences between individuals
If difference in size is due to environment (temperature or crowding  competition) h2 = 0,
If only the larger ones reproduce
Offspring will still have same size distribution as parents
No evolution
Selection can be weak or strong – still no evolution

7. Selection differential measures the strength of selection
Fish example. Fig 7.9

8. Evolutionary response to selection
If h2 = 0, then selection will result in larger fish because breeding parents were larger etc.
Evolution occurs
R = h2 x S

9. High heritability results in larger change
Calculating the evolutionary response to selection
R = h2 x S

10. Key Concepts Evolution and selection are not the same
Selection can occur without evolution
The magnitude of change depends on:
Strength of selection (selection differential)
Heritability

11. Quantitative trait locus (QTL)analysis links traits with genes
Can use technique to map loci of interest – map genome (if desired)
Mate two purebred fish parents
Mate F1 x F1 to get many, many F2 fish
Identify large number of short segments of DNA
Single nucleotide polymorphisms SNPs
Simple sequence repeats
Transposable elements

12. QTL Analysis
Markers selected so that alleles at each one are different depending on whether they are inherited from male or female chromosome
Examine F2 offspring for these genetic markers and test whether the genotype at particular markers is associated with the phenotypic trait of interest.
Markers that have a statistically significant association with the expression of a phenotypic trait are called quantitative trait loci QTLs

13. QTL analysis of coat color in mice

14. QTL analysis of coat color in mice

15. Much of variation in coat color explained by differences in two genes
Epistatic interaction
Corin only explains a small amount of variation

16. Expression of Agouti during development influences coat color

17. Expression of Agouti during development influences coat color
Agouti is expressed in mouse embryo 12 days after conception
Mice are not making pigment this early,
Agouti lays down a “pre-pattern”
As embryo develops, certain cells in epithelium differentiate into pigment–producing melanocytes and migrate into hairs,
Agouti delays maturation of melanocytes.
Expression strongest on ventral side of embryonic mouse, weakest on dorsal side.
So, slow-maturing melanocytes on belly fail to reach hairs and cannot color them
Dorsal melanocytes turn the hairs brown

18. Genetic manipulation of dark mice makes them lighter
Engineered mainland mice to be homozygous for lite color of Agouti. Offspring showed movement of Agouti furthur towards the dorsal side.

19. Key Concepts
QTL analysis identifies regions of the genome associated with phenotypic variation

20. Environmental influences on quantitative traits
VP = VG + VE
Total phenotypic variance in population
Variance due to genetic differences
Variance due to environmental differences

21. Phenotypic plasticity
A single genotype produces different phenotypes de-pending on the environment

22. Reaction norm

23. Genotype x environment interaction
First two rows – difference due to phenotypic plasticity; no genetic effects
Phenotype determined by G and E
Genotype x environment interaction

24. Phenotypic plasticity in Caenorhabditis elegans
Shows genetic basis of Phenotypic Plasticity

25. Plasticity can evolve Mate biggest body size difference
Mate smallest body size difference
Random mating
Result
Plasticity evolved
Bigger difference
Smaller difference
S. Scheiner

26. Rapid change can lead to mismatch between plastic traits and environment

27. Key Concepts
When plasticity is heritable, the response can evolve