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Welcome to Spring Semester 2013
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Introduction to Genetics and Evolution
Mohamed Noor
A whirlwind introduction to evolution and genetics, from basic principles to current applications, including how disease genes are mapped and how we leverage evolutionary concepts to aid humanity.
This week we will continue to look at issues of population genetics. These
are the class materials we'll work with this week.Problem
Set: This week is the sixth graded problem set: Population Genetics Problem
Set, Week 7.
This assignment should be completed at least once before the Nov. 26th, 12:00 pm EDT deadline. Please read the grading policy (if you haven't already done so) before completing the problem set, to better understand the requirements for the course.
There is also a set of practice problems available. The download links for the problems and their solutions is available to the right of lecture #6 in week 7. These problems are not graded nor are they required for understanding the assigned problem set. They are purely for the benefit of students that wish to have additional material to work through and check their understanding of the class topics.
Don't forget to check your graded problem set from Week 6, also in the wiki page for Problem Sets and Exams! Also, please post in the Problem Set Week 6 forum if you got a correct answer for one of the problems, in the threads provided (the "How Did You Correctly Solve..." series), so that all students can see the line of thinking that reached a correct answer!
Lecture: Natural selection: Fundamentals
This assignment should be completed at least once before the Nov. 26th, 12:00 pm EDT deadline. Please read the grading policy (if you haven't already done so) before completing the problem set, to better understand the requirements for the course.
There is also a set of practice problems available. The download links for the problems and their solutions is available to the right of lecture #6 in week 7. These problems are not graded nor are they required for understanding the assigned problem set. They are purely for the benefit of students that wish to have additional material to work through and check their understanding of the class topics.
Don't forget to check your graded problem set from Week 6, also in the wiki page for Problem Sets and Exams! Also, please post in the Problem Set Week 6 forum if you got a correct answer for one of the problems, in the threads provided (the "How Did You Correctly Solve..." series), so that all students can see the line of thinking that reached a correct answer!
Lecture: Natural selection: Fundamentals
- Darwin and Wallace emphases
- Reiterate-- mathematical inevitability & 3 conditions
- Simulating selection at a single locus
- Dominance matters
- Concept of “relative fitness”
- Effects on genotype and allele frequencies
- Dominance matters
- Directional selection
- Different types, vary with
dominance
- Leads to loss of one allele eventually
- Different types, vary with
dominance
- Alleles maintained at predictable equilibrium frequency
- “Unstable” equilibrium
- Alleles will probably be lost in real population
- Selection changes based on frequency of alleles
- Alleles maintained at predictable equilibrium frequency
- Directional selection (not to be confused with single-locus version)
- Changes mean phenotype of population
- Stabilizing selection
- No change in mean phenotype, but loss of variance
- Disruptive selection
- No change in mean phenotype, but increase in variance
- Final thoughts-- Fisher’s fundamental theorem of natural selection
- Selection reduces variation in fitness
- Rate of increase in fitness is proportional to the genetic variance in fitness
- Selection reduces variation in fitness
- Larger samples maintain representation
- Larger population sizes more constant in allele frequencies
- In one generation, alleles about equally likely to increase or decrease in
frequency
- Unlikely to stay at “exactly” same frequency
- Average change in allele frequency = (pq)/2N
- In long term, drift always leads to fixation or loss of alleles variable at
a locus eventually
- Probability of eventual fixation equals
allele frequency
- Probability of eventual loss equals one minus allele frequency
- Same principle for multiple populations-- average across many stays same
- If drift is strong, can overpower selection and make somewhat bad allele fix
- If mutations arise at a roughly constant rate, can predict
- Probability of mutation arising in large population size directly cancels
with probability of fixation so rate of arising & fixing of neutral
mutations is independent of population size
- If have mutation rate, can estimate time to common ancestor
- Must divide by two (since mutations arise in both lineages)
- Sample calculations
- Must divide by two (since mutations arise in both lineages)
- Evolutionary Analysis, 4/e, Freeman & Herron, pages 182-207,
232-241, 346-350
- Introduction to Genetic Analysis, 10/e Griffiths et al, pages 661-666