Transcript Biological Science 2/e

Evolution is:
Change in allele frequencies across generations.
Form of a gene
proportions
FREQUENCIES ALWAYS SUM TO 1.
Frequency of
students who prefer
Foss
1
2
3
Frequency of
students who prefer
Dana
Frequency of
students who prefer
the new Bobs
We directly monitor genotype frequencies:
Aa
Aa
1
2
1
2
3
3
aa
aa
But we must also keep track of allele frequencies.
A
A
1
1
2
2
3
3
a
a
p = frequency of one allele
q = 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
Graphical demonstration of H-W predictions about
genotype frequencies
Frequencies in sperm
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 spraying
Spraying 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 with
the R allele in the absence of insecticide?
Heterozygote advantage in survival
Kuru in New Guinea;
Prion disease spread by ritual mortuary cannibalism.
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.
Unexposed
Survivors of feasts
MM
0.221
0.133
MV
0.514
0.767
VV
0.264
0.100
Visually compare genotype frequencies for unexposed and survivors.
How did natural selection operate in this population?
A bit of background for the next example:
HLA = MHC for humans (Human leucocyte antigens; on white blood cells)
Major Histocompatibility Complex
Presents 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 to
sampling 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.
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.
Pohnpei Landscape. Photo (c) FSM Visitors Board.
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 source
pop differs in frequencies.
Establishment of new populations following volcano eruption.
Gene flow can cause homogenization of populations.
Current concern:
Gene flow of genetically-engineered crop genes into wild plants.
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.
*Chemical & Engineering News. September 27, 2004
Gene flow may explain why populations can’t adapt
to conditions at margin of species range.
cold
dry
Lots of gene
flow from big
central pop.
hot
humid
Examine final H-W assumption: random mating
Does not directly change p, q.
Does cause genotype freqs to depart from p2: 2pq: q2.
Non-random mating could be:
Disassortative (mates are more
different than random)
Assortative (mates are more
similar than random)
Assortative mating includes
inbreeding, which is mating
of 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 is a form of non-random mating.
Inbreeding increases homozygosity.
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 or reduce
Can produce adaptation.
Mutation
Increase -- original source of all variation.
Drift
Reduce
Gene flow
Increase within-population
Reduce among-population
Inbreeding
Increase heterozygosity (disassortative)
Increase homozygosity (assortative)