Evolution Lecture 12

End of Chapter 8 and beginning of Chapter 9

Topics for today

1. How variation arises by mutation?
2. What impact does it have on organisms?
3. Macromutation - Polyploidy
4. Variation through recombination

EvoBeaker: Sickle cell alleles

Rates of spontaneous can be estimated by visible polymorphisms

Estimates of mutation rates in functional parts of the genome

1. Mutation rates are slow

2. BUT must extrapolate across the whole genome

3. Lots of raw material for change over 1,000’s of generations

Mutation rates are sensitive to the environment

1. Do mutation rates of repetitive elements increase in baby mice when their parents are raised in different conditions?

How does mutation accumulation influence genetic variance and viability?

1. 1.7 million flies examined to estimate effects on viability of mutations on chromosome 2

2. Balanced by wild type chromosome

3. Represent 1/3 of genome

Do mutations have beneficial effects?

1. Single genetically identical line at generation zero

2. Samples of founders frozen

3. Mutations accumulate spontaneously each generation

4. Compared population growth rate of evolving population to the founding population

5. E. coli

Beneficial effects of mutations shown through experimental evolution

1. Elegant example of potential evolution in response to climate change

Limits of mutation

1. Mutation only alter preexisting traits

2. Many mutations may have the same phenotypic effects

a. Retinitis pigmentosa can be caused by mutations of genes on 8 of 23 of human chromosomes

3. Not all mutations are equally likely to occur

a. AT and GC pairings work

b. Purine-purine and pyrimidine-pyrimidine pairings don’t work

c. Experimental evolution in phage strains

d. Most mutations occur repeatedly at a small number of sites

e. Mutations match those found in natural populations

f. Limited number of pathways to adaptation

Polyploidy is a macromutation that doubles genome size

Does this provide greater evolutionary potential?

1. More gene products

2. Greater genetic diversity

3. Opportunity for duplicated genes to diverge in function

4. More gene interactions

Do polyploids evolve faster in response to climate change?

Etterson research: Artificial selection on drought tolerance on two different ploidy levels

Meiosis rapidly produces genetic variance by two mechanisms

1. Independent segregation of bivalents

The number of combinations equals 2n

-n = 36

# possibilities = 6.8710

-n = 82

# possibilities = 4.8424

Meiosis rapidly produces genetic variance by two mechanisms

2. Recombination

Genetic variance increases due to independent segregation & recombination

VP = VG + VE

Generation of genetic diversity

Mutation creates different alleles

Major diseases that cause human mortality

Leading causes of death

Ages 0-44

Sub-Saharan Africa, South Asia, and Southeast Asia

(World Health Organization)

Malaria caused by Plasmodium

1. Mosquito-borne protozoan parasite that has two host species

Regions with endemic malaria VS. Regions with sickle cell anemia

Is there a relationship?

Two hemoglobin alleles differ by one bp

1. Glutamic acid is hydrophilic but Valine is hydrophobic
2. Hydrophobic spot sticks to the hydrophobic spot of adjacent hemoglobin
3. Causes polymerization

Sickle cell anemia

1. HbS blood cells “sickle” after releasing oxygen
2. Cell membranes are rigid
3. Blocks small blood vessels
4. Causes pain and organ damage

Is there a relationship?

1. HbA/HbA are susceptible to malaria
2. HbA/HbS “sickle” when infected with malarial parasite
3. Body’s immune system recognizes and destroys infected cells
4. HbS/HbS blood cells often “sickle” leading to early mortality

Mortality due to sickle cell anemia in the U.S.

1. No advantage in regions without malaria
2. Heterozygote is resistant to malaria
3. Homozygotes for sickle cell allele have high mortality anywhere

EvoBeaker: Sickle Cell Alleles