1. The plant of the day
Bristlecone pine - Two species Pinus aristata (CO, NM, AZ), Pinus longaeva (UT, NV, CA)
Thought to reach an age far greater than any other single living organism (~5000yrs)
Used by dendrochronologists to determine past climatic events
(back to 7K BC)
Inhabits harsh environments (arid, alkaline soil) free of competition (short growing season)
Slow growing
Dense wood (stops infection)
Long lived needles

2. Non-random mating, genetic drift, and population structure
Assortative mating – mating with individuals that are similar or dissimilar for a given trait.
Inbreeding – mating with a close relative. 2

3. Positive Assortative Mating
If the phenotype is under genetic control, Positive assortative mating increases homozygosity and decreases heterozygosity for the genes affecting the trait. 3

4. Positive Assortative Mating
in the genus Burmeistera, bats are more efficient at moving pollen between wide flowers, whereas hummingbirds excel at pollen transfer between narrow flowers. 4

5. Negative Assortative Mating
If the phenotype is under genetic control, Negative assortative mating increases heterozygosity and decreases homozygosity for the genes affecting the trait. 5

6. Negative Assortative Mating
Plant self-incompatibility systems lead to negative assortative mating.
Examples: Sunflowers Cocoa tree Blue bells Brassica rapa (field mustard) 6

7. Inbreeding Inbreeding: mating with a close relative
Biparental: two different individuals are involved 7

8. Extreme inbreeding
Intragametophytic selfing: mating between gametes produced from the same haploid individual
-100% homozygosity in one generation!
- some ferns and mosses 8

9. The effects of inbreeding on genotype and allele frequencies
Fewer heterozygotes and more homozygotes
No change in allele frequency 9

10. Inbreeding Inbreeding does NOT change allele frequency by itself
It does increase homozygosity
Inbreeding coefficient (F):
measures the extent to which populations depart from the expectation of 2pq (remember p² + 2pq + q² = 1)
He = Expected heterozygosity, HW (2pq)
Ho = Observed heterozygosity
F = (He-Ho)/He 10

11. Evolutionary Consequences of Inbreeding
In large, random mating populations, most individuals will not suffer from deleterious effects of recessive deleterious alleles
Under inbreeding, increased homozygosity for these recessive deleterious alleles results in reduced fitness 11

12. Genetic drift
Definition: Changes in allele frequency due to random sampling.
One of the requirements for the maintenance of stable allele
frequencies in populations is a very large population size.
Genetic drift is the consequence of finite population size. 12

13. Genetic drift Alleles that do not affect
fitness fluctuate randomly in frequency, which eventually results in the loss of alleles from populations. One allele
becomes fixed. 13

14. Genetic drift
Different populations will lose different alleles. The probability that a particular allele will be fixed in a population in the future equals the frequency of the allele in the population.
If a large
number of
populations is
considered,
each drifting,
the total
heterozygosity
overall will
decrease. 14

15. Genetic drift
Starting with a population size of N with two alleles in equal frequencies p and q, the likely magnitude of divergence from the initial frequencies increases with time. 15

16. Genetic drift After 2N generations, all
allele frequencies are equally likely.
The average time to fixation of one of the alleles is 4N generations. 16

17. Effective population size
Effective population size - number of individuals in the
population that successfully pass genes to the next generation.
-usually smaller than the actual population (census) size
-drift will occur more quickly in smaller populations 17

18. Effective population size and Drift18

19. Effective population size
The effective population size (Ne) is affected by biological
parameters other than the number of breeding individuals in the population. These include:
Variation in offspring number among individuals
A sex ratio other than 1:1
Natural selection
Inbreeding (reduces the number of different copies of a gene passed to the next generation)
Fluctuations in population size 19

20. Founder effects
When a small number of individuals from a source population establish a new population genetic variation can be lost. The loss of genetic variation due to such an extreme bottleneck is called the founder effect.
Simulations of founder effects
suggest that a small number
founders and a small population
growth rate (r) result in greater loss
of genetic diversity.
Eventually mutation will restore
genetic variation in a founding
population. 20

21. Genetic Drift: summary
Effects of Drift
Genetic Drift: summary
Within populations
Changes allele frequencies
Reduces variance
Does not cause deviations from HW expectations
Among populations (if there are many)
Does NOT change allele frequencies
Does NOT degrade diversity
Causes a deficiency of heterozygotes compared to Hardy-Weinberg expectations (if the existence of populations is ignored), like inbreeding. 21

22. Genetic drift: why is it important?
Effects of Drift
Genetic drift: why is it important?
Erodes genetic variation within populations
Causes population differentiation
Strength is dependant on population size
The demographic history of populations effects patterns of genetic variation
Can oppose selection- conservation implications
Provides a “neutral” model for evolutionary change and most molecular changes are effectively neutral 22

23. Sewall Wright Population structure
How do we measure population genetic structure?
Sewall Wright 23

24. Wright’s fixation index
Fixation index is a measure of genetic differentiation among populations
Compare heterozygosity at different hierarchical levels
FST=(HT-HS)/HT
HT: The overall expected HW heterozygosity for the total area
HS: The average expected HW heterozygosity among organisms within populations 24

25. Linanthus parryae population structure
What is the genetic divergence among sub populations FST?
What could be causing the divergence in flower colour among the sub populations?