Evolution is: Change in allele frequencies across generations.

Evolution is:

Change in allele frequencies across generations.

Form of a gene proportions

1

2

3

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.

1

2

3

1

2

3

1

2

3

1

2

3

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 white blood 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 or reduceCan produce adaptation.

Mutation Increase -- original source of all variation.

Drift Reduce

Gene flow Increase within-populationReduce among-population

Inbreeding Increase heterozygosity (disassortative)Increase homozygosity (assortative)