Population Genetics


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Population Genetics

  1. 1. Population Genetics Reconciling Darwin & Mendel
  2. 2. Darwin <ul><li>Darwin’s main idea (evolution), was accepted </li></ul><ul><li>But not the mechanism (natural selection) </li></ul><ul><ul><li>Scientists did not understand Darwin’s mechanism because there was no understanding of genetics </li></ul></ul><ul><li>Even once scientists grasped Mendel, genetics was viewed as an either/or </li></ul><ul><ul><li>didn’t understand many traits are polygenic </li></ul></ul><ul><li>So how do you get the variation on which selection works? </li></ul>
  3. 3. Ideas About Evolution <ul><li>Orthogenesis </li></ul><ul><ul><li>1920’s </li></ul></ul><ul><ul><li>saw evolution as a predictable progression to more & more elite forms of life </li></ul></ul><ul><li>Population Genetics </li></ul><ul><ul><li>1930’s </li></ul></ul><ul><ul><li>reconciled Darwin & Mendel </li></ul></ul>
  4. 4. Genetics of Populations <ul><li>Population </li></ul><ul><ul><li>a localized group of individuals belonging to same species </li></ul></ul><ul><ul><li>The definition of a species not always clear   </li></ul></ul><ul><li>Gene pool = The total genes in a population </li></ul><ul><li>Evolution on the smallest scale occurs when the relative frequency of alleles in a population changes over a succession of generations = microevolution   </li></ul>
  5. 5. Genetics of a Non-evolving Population <ul><li>The gene pool is in stasis </li></ul><ul><li>This is described by Hardy-Weinberg Theorem: </li></ul><ul><li>The frequencies of alleles in a population’s gene pool remain constant over the generations unless acted on by agents other than sexual recombination </li></ul><ul><li>i.e. shuffling the deck has no effect on the overall genetic make-up of the population </li></ul>
  6. 6. The Hardy-Weinberg Theorem <ul><li>Example </li></ul><ul><li>In pink flowers ( A) , is dominant over white flowers ( a) </li></ul><ul><li>2 alleles for at this locus </li></ul><ul><li>Sample 500 plants: </li></ul><ul><li>20 white flowers ( aa ) </li></ul><ul><li>480 pink [320 (AA); 160 ( Aa )] </li></ul><ul><li>Therefore there are 1000 genes for flower color in the population </li></ul>
  7. 7. Example (Continued) <ul><li>The dominant allele accounts for 800 of these: </li></ul><ul><ul><li>[(320 x 2) + (160 x 1)] </li></ul></ul><ul><li>Therefore: </li></ul><ul><ul><li>the frequency of A in the population = 80% </li></ul></ul><ul><ul><li>the frequency of a = 20% </li></ul></ul>
  8. 8. Predicting Change <ul><li>How will genetic recombination during sexual reproduction affect the frequencies in the next generation? </li></ul><ul><li>If mating is random: </li></ul><ul><ul><li>the probability of picking 2 AA = (0.8 x 0.8) = .64 </li></ul></ul><ul><ul><li>the probability of picking 2 aa = (0.2 x 0.2) = .04 </li></ul></ul><ul><ul><li>and of heterozygotes = 2(0.8 x 0.2) = 0.32 </li></ul></ul><ul><ul><ul><li>there 2 heterozygote combinations: aA & Aa </li></ul></ul></ul><ul><ul><ul><li>sperm or egg </li></ul></ul></ul>
  9. 9. Hardy-Weinberg Equilibrium <ul><li>This shows that the alleles are present in the gene pool in the same frequencies as they were in the previous generation </li></ul><ul><ul><li>A: [0.64 + (0.32  2)] = 0.8 </li></ul></ul><ul><ul><li>a: [0.04 + (0.32  2)] = 0.2 </li></ul></ul><ul><li>The gene pool is at equilibrium </li></ul><ul><li>This is called Hardy-Weinberg equilibrium </li></ul>
  10. 10. The Hardy-Weinberg Equation <ul><li>This example is the simplest case: </li></ul><ul><ul><li>2 alleles, one is dominant </li></ul></ul><ul><li>For this case: </li></ul><ul><ul><li>if p = frequency of one allele </li></ul></ul><ul><ul><li>q the frequency of the other </li></ul></ul><ul><li>Then: p + q = 1 </li></ul><ul><ul><ul><ul><ul><li>probability of AA = p 2 </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>probability of aa = q 2 </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>probability of Aa = 2 pq </li></ul></ul></ul></ul></ul><ul><li>Therefore: p 2 + 2 pq + q 2 = 1 </li></ul>
  11. 11. Uses of Hardy-Weinberg <ul><li>Thus you can calculate the frequency of a gene in a population if you know the frequency of the genotypes </li></ul><ul><li>This is important in genetic disease counseling </li></ul>
  12. 12. Relevance to Evolution <ul><li>A population at genetic equilibrium does not evolve </li></ul><ul><li>Hardy-Weinberg tells us what to expect in non-evolving populations </li></ul><ul><li>Therefore it is a baseline for comparing actual populations where gene pools may be changing. </li></ul><ul><li>Can determine if the population is evolving </li></ul>
  13. 13. Genetic Equilibrium <ul><li>Hardy-Weinberg equilibrium is maintained only if the population meets all 5 of the following criteria: </li></ul><ul><ul><li>Very large population size </li></ul></ul><ul><ul><li>Isolation from other populations </li></ul></ul><ul><ul><ul><li>migration can effect the gene pool </li></ul></ul></ul><ul><ul><li>No net mutations </li></ul></ul><ul><ul><li>Random matings </li></ul></ul><ul><ul><li>No natural selection </li></ul></ul><ul><ul><ul><li>no difference in reproductive success) </li></ul></ul></ul><ul><li>Describes an ideal that never exists in nature </li></ul>
  14. 14. Altering Genetic Equilibrium <ul><li>For evolution to take place something must upset the genetic equilibrium of the population: </li></ul><ul><li>Factors that change genetic equilibrium are: </li></ul><ul><ul><li>Genetic drift </li></ul></ul><ul><ul><li>Migration (Gene flow) </li></ul></ul><ul><ul><li>Non-randon mating (Isolation) </li></ul></ul><ul><ul><li>Mutation </li></ul></ul><ul><ul><li>Natural selection </li></ul></ul>
  15. 15. Genetic Drift <ul><li>Changes in gene frequency of a very small population due to chance </li></ul><ul><li>Controlled by the laws of probability & chance </li></ul><ul><li>Bottleneck effect </li></ul><ul><ul><li>Chance sampling error due to small population </li></ul></ul><ul><li>Founder’s effect </li></ul><ul><ul><li>a few individuals colonize a remote spot </li></ul></ul><ul><ul><li>causes drift </li></ul></ul>
  16. 16. Illustrating Genetic Drift
  17. 17. The Bottleneck Effect
  18. 18. Gene Flow (Migration) <ul><li>Movement of organisms into or out of a population </li></ul><ul><li>Takes their genes out of the gene pool </li></ul><ul><li>Most populations are not completely closed </li></ul><ul><ul><li>gain & lose alleles </li></ul></ul>
  19. 19. Non-random Mating <ul><li>More apt to mate with close neighbors </li></ul><ul><li>Promotes inbreeding </li></ul><ul><li>Assortive mating </li></ul><ul><ul><li>seek mate like self (i.e. size) </li></ul></ul>
  20. 20. Isolation
  21. 21. Mutation <ul><li>A change in a gene </li></ul><ul><li>An alteration of DNA </li></ul><ul><li>The original source of variation </li></ul><ul><li>Raw material on which natural selection works </li></ul>
  22. 22. Natural Selection <ul><li>If one type produces more offspring than another, upsets the balance of equilibrium </li></ul><ul><li>There are three types of natural selection: </li></ul><ul><ul><li>Stabilizing Selection </li></ul></ul><ul><ul><li>Disruptive Selection </li></ul></ul><ul><ul><li>Directional Selection </li></ul></ul>