Evolution is: Change in allele frequencies across generations. Form of a gene proportions

FREQUENCIES ALWAYS SUM TO 1. Frequency of students who prefer Foss Frequency of students who prefer the new Bobs Frequency of students who prefer Dana

We directly monitor genotype frequencies: But we must also keep track of allele frequencies. A a A a aa Aa aa Aa

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: p 2 :2pq:q 2

Frequencies in sperm Frequencies in eggs q = 0.7 p = 0.3 Graphical demonstration of H-W predictions about genotype frequencies p = 0.3q = p 2 = 0.09, 2pq = 0.42, q 2 = 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 R is the allele for resistance GenotypeFrequencyGenotypeFrequency R+.10R+.65 RR.90RR.10 What are allele frequencies? Is there a selective cost associated with the 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 VV Unexposed Survivors of feasts 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 Complex Presents antigens for recognition to immune system.

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.

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:

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 q 2 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: 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.

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 gene flow from big central pop. cold hot humid dry

Examine final H-W assumption: random mating Does not directly change p, q. Does cause genotype freqs to depart from p 2 : 2pq: q 2. 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 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.

MechanismImpact on Variation Selection Maintain, increase, or reduce. Can produce adaptation. MutationIncrease -- original source of all variation. DriftReduce Gene flowIncrease within-population Reduce among-population Non-randomIncrease heterozygosity (disassortative) Mating or Increase homozygosity (assortative)