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

Non-random mating, genetic drift, and population structure

Non-random mating

Assortative mating

– mating with individuals that are similar or dissimilar for a given trait.

Inbreeding

relative.

– mating with a close

Positive Assortative Mating

If the phenotype is under genetic control,

Positive assortative mating

increases homozygosity and decreases heterozygosity for the genes affecting the trait.

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.

Negative Assortative Mating

If the phenotype is under genetic control,

Negative assortative mating

increases heterozygosity and decreases homozygosity for the genes affecting the trait.

Negative Assortative Mating

Plant self-incompatibility systems lead to negative assortative mating.

Examples: Sunflowers Cocoa tree Blue bells Brassica rapa (field mustard)

Inbreeding

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

Extreme inbreeding

Intragametophytic selfing: mating between gametes produced from the same haploid individual -100% homozygosity in one generation!

- some ferns and mosses

The effects of inbreeding on genotype and allele frequencies

Fewer heterozygotes and more homozygotes No change in allele frequency

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 2

pq (remember p² + 2pq + q² = 1) He =

Expected heterozygosity, HW (2

pq

)

Ho =

Observed heterozygosity

F =

(

He-Ho

)/

He

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

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.

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.

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.

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.

Genetic drift

Genetic drift After 2N generations, all allele frequencies are equally likely.

The average time to fixation of one of the alleles is 4N generations.

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

Effective population size and Drift

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

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.

• 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.

• 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

Population structure How do we measure population genetic structure?

Sewall Wright

Wright’s fixation index Fixation index is a measure of genetic differentiation among populations Compare heterozygosity at different hierarchical levels

F ST =(H T -H S )/H T H T

: The overall expected HW heterozygosity for the total area

H S

: The average expected HW heterozygosity among organisms within populations

Linanthus parryae

population structure

What is the genetic divergence among sub populations

F ST

?

What could be causing the divergence in flower colour among the sub populations?