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Course information

Course information. Revision of important ideas and vocabulary from the first year What is evolution?

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Course information

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  1. Course information

  2. Revision of important ideas and vocabulary from the first year • What is evolution? • Darwin used the phrase “Descent with modification” to describe evolution. This course is about how evolution gives rise to different taxa (genera, species, sub-species etc) and genetic diversity within species. A profound understanding of evolution requires a knowledge of the genetic processes underlying evolution • and the nature of the genetic diversity. • This course aims to introduce you to these topics. The course is designed for both ecology students with an interest in the processes underlying evolution and genetics students with an interest in the evolutionary consequences of genetic variation.

  3. Texts If you find you are not familiar with the material that we revise in the first week, you should as a matter of urgency carry out someremedial reading • Look up the topics that we cover in: • Genetics • Robert F. Weaver, Philip W. Hedrick. • 2nd ed. Dubuque, Iowa : W.C. Brown, c1992. • In the library of there are 14 one week loan copies & 3 short loan • Evolution : a biological and palaeontological approach • [edited by] Peter Skelton.Wokingham : Addison-Wesley, 1993. • A newer text is • Evolution and the Ecology of the Organism • Rose RR & Mueller LD • NJ: Pearson/Prentice Hall, 2004

  4. Library

  5. . • Some revision questions: assess yourself • Distinguish between the terms gene, allele & locus

  6. Other dominant alleles are actually selected against- a dominantly inherited condition called synpolydactyly What does it mean if a trait is dominant/ recessive. Is a dominant allele the most frequent in the population? Is the dominant allele the fitter?

  7. What is allele frequency? What is fixation? What is the proportion of homozygotes in a population with an allele frequency of 0, of 1, of 0.5. How many alleles are there at a locus in any one individual? How many alleles can there be in a population?

  8. Genetic Drift: what is it

  9. Fitness: What is it?

  10. Fitness can be considered to be a property of genotypes (not genes): the expectation of the relative number of descendant genes at the same stage of the life cycle in the next generation. It is has many components, for example AA and BB here differ in both viability and reproductive success. In the case above fitness AA = 10/2 =5 fitness BB = 4/2 =2 AB

  11. relative fitness are WAA=1 (by convention) and WBB=2/5. Notice BB actually left as many genes as it had in the first generation, it is the relative number that is important. There are no BBs in the next generation but the fitness is non-zero. It is the descendant genes not genotypes that count. AB

  12. relative fitness are WAA=1 (by convention) and WBB=2/5. Notice BB actually left as many genes as it had in the first generation, it is the relative number that is important. There are no BBs in the next generation but the fitness is non-zero. It is the descendant genes not genotypes that count. AB

  13. Key graph for understanding how genotype frequencies change with allele frequency Proportion of copies of the allele that occur in heterozygotes Allele frequency

  14. Selection against heterozygotes Waa=Wbb>Wab eg. Podisma pedestris chromosomal fusion.

  15. The rare allele is selected against. Why? Calculations show that selection of only 0.05 (I.e. hybrids are 99.5% as fit as chromosomal homozygotes) is enough to maintain the 800m wide zone if grasshoppers move 20m in a generation. How could genetic drift help the new form become established?

  16. Allele frequency changes First example Waa=Wab>Wbb E.g. Biston betularia in the regions of blackened tree trunks. As the a allele is dominant both of aa & ab express the black colouration- hence Waa=Wab

  17. Initially the increase in a frequency accelerates as a becomes more common because a greater proportion of the allele occur in homozygotes (hence the advantage is not shared with the b allele). As a approaches fixation the rate of increase slows because the recessive b allele is rare and occurs predominantly in heterozygotes, hence it is masked and escapes some of the selection. Segregation will throw up bb combinations each generation which will suffer the selective consequences.

  18. % population with HbS allele 14 10 6 2 For example if the fitnesses are Wab=1, Waa=0.84, Wbb=0.01 Then we could explain an equilibrium frequency of 14%. Why do both alleles increase in frequency when rare?

  19. % population with HbS allele 14 10 6 2 For example if the fitnesses are Wab=1, Waa=0.84, Wbb=0.01 Then we could explain an equilibrium frequency of 14%. Why do both alleles increase in frequency when rare?

  20. Mutation could Introduce one red allele It could spread locally through drift (good luck) through selection (local adaptation) and then through gene flow

  21. Explaining polymorphism (the raw material for evolution) and differentiation Four fundamental processes Origin: Mutation Spread or loss within a population: Genetic drift and Selection Movement from one population to another: Gene flow

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