evolution is: change in allele frequencies across generations. form of a gene proportions
TRANSCRIPT
Evolution is:
Change in allele frequencies across generations.
Form of a gene proportions
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FREQUENCIES ALWAYS SUM TO 1.
Frequency ofstudents who preferFoss
Frequency ofstudents who preferthe new Bobs
Frequency ofstudents who preferDana
We directly monitor genotype frequencies:
But we must also keep track of allele frequencies.
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A
a
A
a
aa
Aa
aa
Aa
p = frequency of one alleleq = frequency of the other allele
Hardy-Weinberg principle says:Given certain assumptions,
p and q will not change (no evolution)
and
Genotype frequencies will be in proportions:
p2:2pq:q2
Frequencies in sperm
F
requ
enci
es in
egg
s
q =
0.7
p =
0.3
Graphical demonstration of H-W predictions aboutgenotype frequencies
p = 0.3 q = 0.7
0.09 0.21
0.21 0.49
p2 = 0.09, 2pq = 0.42, q2 = 0.49
Assumptions of Hardy-Weinberg model:
•No selection
•No mutation
•Infinite population size
•No migration (movement between differing populations)
•Random mating
H-W serves as NULL MODEL. Allows us to study departures from assumptions.
Class simulation of H-W principles
1. Calculate genotype frequencies.
2. Make gametes by separating alleles.
3. Calculate allele frequencies.
4. Allow random mating.(did not do-could start here.)
5. Calculate genotype and allele frequencies.
6. Is population in H-W equilibrium?
Examine 1st H-W assumption: no selection
Genotype frequencies during and after DDT sprayingSpraying began in 1964 and stopped in 1968. R is the allele for resistance.
1967 1969
Genotype Frequency Genotype Frequency
++ 0 ++ .25
R+ .10 R+ .65
RR .90 RR .10
What are allele frequencies?Is there a selective cost associated withthe R allele in the absence of insecticide?
All human populations are polymorphic for the prion protein PrPC. Alleles have different codons at position 129: one encoding methionine; the other valine.
Prions naturally occur in mammals, as part of the nervous system. Rogue prions are bent out of shape. A prion can become a rogue prion by being exposed to other rogue prions.
MM MV VVUnexposed 0.221 0.514 0.264Survivors of feasts 0.133 0.767 0.100
Visually compare genotype frequencies for unexposed and survivors.How did natural selection operate in this population?
Heterozygote advantage in survival
Kuru in New Guinea; Prion disease spread by ritual mortuary cannibalism.
A bit of background for the next example:
HLA = MHC for humans (Human leucocyte antigens; on most nucleated cells)
Major Histocompatibility ComplexPresents antigens for recognition to immune system.
http://bio.classes.ucsc.edu/bio80j/Lecture%20Slides/20MHC_illustr.gif
Violations of which H-W assumptions could cause excess of heterozygotes at HLA loci?
Second assumption of H-W -- no mutation.
Mutation occurs at DNA level and at chromosomal level.
Errors in DNA replication can create new alleles of genes or change regulatory sequences.
Unequal crossing over in meiosis can lead to gene duplication,freeing up new gene copy for new function.
Source of all genetic variation but minimal effects in short term.
http://www.ntbest.org/PhotoGallery/photo2004/PSMallDay/Best%20Mutation/Mutation%20Award.JPG
Examine third H-W assumption: Infinite population size.
In finite populations, allele frequencies will change due tosampling error.
The smaller the population, the stronger “genetic drift”.
Genetic drift simulation:http://www.biology.arizona.edu/evolution/act/drift/dna_phenotype.html
Drift is powerful in small populations.
Pohnpei Landscape. Photo (c) FSM Visitors Board.
In 1775, 20 survivors of a typhoon on Pingelap Atoll.One survivor had a mutation in a gene needed for function of cone cells.
Typical human populations, q = 0.007, and q2 miniscule.Pingelap Atoll, q = 0.2, and 1/20 people cannot see color.
Drift can overwhelm selection in small populations;source of concern for conservation genetics.
Cheetahs alive today are genetically almost identical.Cannot respond to environmental change, disease, etc.
Drift of neutral DNA provides record of evolutionary history.
Go to:https://www3.nationalgeographic.com/genographic/atlas.html
And look at migration routes of genetic markers.
Y chromosome (paternally inherited)Mitochondrial DNA (maternally inherited)
Examine fourth H-W assumption: no migration
Migration is another name for gene flow among pops.
Will change allele frequencies within a pop if the sourcepop differs in frequencies.
Establishment of new populations following volcano eruption.
Gene flow can cause homogenization of populations.
EPA study finds that bent grass transgenes can travel as far as 13 miles.*Herbicide resistance could spread into natural populations.Can use pop. gen. models to predict rate of spread.
Current concern:Gene flow of genetically-engineered crop genes into wild plants.
*Chemical & Engineering News. September 27, 2004
Gene flow may explain why populations can’t adapt to conditions at margin of species range.
Lots of geneflow from bigcentral pop.
cold
hot
humiddry
Examine final H-W assumption: random matingDoes not directly change p, q.Does cause genotype freqs to depart from p2: 2pq: q2.
Non-random mating could be:
Disassortative (mates are moredifferent than random)
Assortative (mates are moresimilar than random)
Assortative mating includesinbreeding, which is matingof related individuals.
Disassortative Mating -- increases heterozygosity
Variation at MHC loci
may be maintained by
dissasortative mating.
Fish and mammals can
distinguish among MHC
genotypes by smell.
MHC = Major Histocompatibility Complex, called HLA in humans
Assortative mating-- If trait heritable, increases homozygosity.In extreme, could lead to speciation.
White-throated sparrow morphs (color of head stripe).
Inbreeding increases homozygosity.
Inbreeding is a form of non-random mating.
Inbreeding depression is reduction of fitness upon inbreeding.
Why does inbreeding often cause reduction of fitness?
Increase in homozygosity.
Deleterious recessive mutations revealed.
Swedish adders suffering from inbreeding produced stillborn offspring.
http://evolution.berkeley.edu/evolibrary/article/0_0_0/conservation_03
Mechanism Impact on Variation
Selection Maintain, increase, or reduce.Can produce adaptation.
Mutation Increase -- original source of all variation.
Drift Reduce
Gene flow Increase within-populationReduce among-population
Non-random Increase heterozygosity (disassortative)Mating or Increase homozygosity (assortative)