Genes And Variation


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Genes And Variation

  1. 1. Genes and Variation<br />
  2. 2. In the 1930’s, evolutionary biologists began to connect Mendel’s work to Darwin’s theory.<br />With the discovery of DNA’s structure in 1953 by Watson and Crick, the gene became the focus of evolution. [Scientists began speaking about evolution in genetic terms.]<br />
  3. 3. In genetic terms, evolution is any change in the relative frequency of alleles in the genepool of a population.<br />Points to make:<br />1. Gene pool  all genes (alleles) that are present in a population.<br />2. Frequency  the number of times that an allele occurs in a gene pool.<br /><ul><li>Gene pools are important to evolutionary theory, because evolution changes populations over time.</li></ul>3. Evolution is studied and understood in populations.<br />
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  5. 5. Why do allele frequencies change?<br />1. Mutations in the DNA<br />Mutations are rare and many are harmful.<br />Is it likely that mutations are solely responsible for shifting allele frequencies?<br />No…<br />
  6. 6. 2. Migration – Gene Flow<br />Immigration of new individuals with new/different alleles.<br />
  7. 7. 3. Genetic Drift<br />A change in allele frequency caused by a chance event. (e.g. massive death – survival is unrelated to genotype – contradicts Darwin’s “survival of the fittest”)<br />
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  9. 9. 4. Natural Selection<br />Differential survival and reproduction of traits controlled by genes.<br />
  10. 10. Natural Selection by the numbers…<br />Gene frequency in a population of 100 organisms: 25 AA / 50 Aa / 25 aa(Generation 1)<br />(100 A’s)/200 total alleles = 50% A<br />(100 a’s)/200 total alleles = 50% a <br />Nature selects against A and causes a 20% of those expressing A to die. Resulting in…<br />20 AA / 40 Aa / 25 aa(Generation 2)<br />(80 A’s)/170 total alleles = 47% A<br />(90 a’s)/170 total alleles = 53% a<br />Generation 3 16 AA / 32 Aa / 25 aa<br />(64 A’s)/146 total alleles = 43% A<br />(57 a’s)/146 total alleles = 56% a<br />
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  12. 12. Are there any conditions under which evolution will not occur?<br />To clarify how evolutionary change operates, scientists often find it helpful to determine what happens when no change takes place.<br />If allele frequencies do not change, the population will not evolve. This is called genetic equilibrium.<br />What has to occur for a population to reach genetic equilibrium? (Hardy-Weinberg equilibrium)<br />
  13. 13. Five conditions required to maintain genetic equilibrium (no evolution)<br />RANDOM MATING – All the members of the population must have an equal opportunity to produce offspring. Nonrandom mating would cause certain genes to be selected over others.<br />LARGE POPULATION SIZE - Genetic Drift has little effect on large populations.<br />NO GENE FLOW – No movement into or out of the population.<br />NO MUTATIONS<br />NO NATURAL SELECTION<br />
  14. 14. Mutations<br />+ Gene Flow<br />+ Genetic Drift<br />+ Natural Selection<br />Evolution<br />(Microevolution)<br />
  15. 15. Moving beyond microevolution…<br />Mutation, chance events, and natural selection changes the allele frequency within a population…<br />…but how (when) do you get a new species?<br />That depends on how you define the term ‘species’…<br />…’species’ two organisms that can interbreed and produce fertile offspring.<br />
  16. 16. Same species?<br />Yes…Why?<br />They all interbreed with each other.<br />Theridiongrallator.<br />
  17. 17. What helps new species form? (Speciation)<br />Isolation<br />Two types of isolation…<br />
  18. 18. Geographic Isolation<br />Two populations are separated by geographic barriers such as rivers, mountains, oceans…<br />
  19. 19. The Kaibab(left) squirrel evolved from the Abert(right) squirrel when the Colorado river split the species (Abert) into two separate populations, thus two separate gene pools. <br />Natural selection worked separately on each group and led to the formation of two distinct species of squirrels.<br />
  20. 20. Behavioral Isolation<br />When two populations are capable of interbreeding but don’t since they have differences in reproductive strategies that involve behavior. (Eastern/Western Meadowlark)<br />They use different songs to attract mates.<br />
  21. 21. Peter and Rosemary Grant<br />
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  23. 23. Mutation/Gene Flow/Genetic Drift/Natural Selection<br />+ 3.8 billion years<br />Evolution<br />(Macroevolution)<br />