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  1. 1. The Evolution of Populations Chapter 23 BCOR 012 February 03 & 05, 2010
  2. 2. Outline: The Evolution of Populations (Chapter 23) February 03, 2010 Introduction Evolution is a population-level phenomenon Linking Darwinian evolution and mendelian inheritance The Modern Synthesis Population Genetics A population’s gene pool is defined by its allele frequencies Hardy-Weinberg theorem Manipulating the H-W equation Example from health science – Cystic fibrosis Assumptions of H-W Microevolution Causes Drift Bottleneck Founder Effect Natural Selection - the English Peppered Moth
  3. 3. Darwin’s arguments that life has evolved were accepted more readily than his contention that natural selection was the mechanism. This was partly because it was not known how characteristics were passed from generation to generation.
  4. 4. Theodosius Dobzhansky Ernst Mayr G. Ledyard Stebbins Sewell Wright George Gaylord Simpson Botany Genetics Paleontology Systematics Population Genetics
  5. 5. The smallest unit that can evolve is the population ... So what is a population? A population is a set of individuals of the same species that live close enough together to interbreed.
  6. 6. Gene pool: the total collection of alleles present in a popu- lation is that population’s gene pool. A population’s gene pool is defined by its allele frequencies.
  7. 7. Example: • Flower color in Phlox is determined by alternative alleles at the color locus. • R is dominant to r, and results in red color. The rr genotype yields white color. • In one population, the frequency of R has been determined to be 0.8, whereas the frequency of r is 0.2. (Note that allele frequencies sum to 1.0)
  8. 8. Hardy-Weinberg Theorem The Hardy-Weinberg theorem states that, in a non-evolving population, allele and genotype frequencies remain constant through time.
  9. 9. If a population is in Hardy-Weinberg equilibrium, genotype frequencies are given by: p2 + 2pq + q2 where p is the frequency of one allele and q is the frequency of the other.
  10. 10. RR = .64 Rr = .32 rr = .04 So, if our Phlox population is in Hardy- Weinberg equilibrium, what are the expected genotype frequencies? Q. In a population of 500 plants, how many will have the white flowered phenotype? A. 20 plants (0.04 X 500) R = 0.8; r = 0.2 (p2) (q2) (2pq)
  11. 11. A Real Example: Frequency of cystic fibrosis phenotype in the caucasian population = 1/2500. What proportion of the population are carriers? (The CF allele is recessive.) q2 = 1/2500 = 0.0004 q = √.0004 = 0.02 p = 0.98 2pq = 0.0392 or about 1 in 25 people
  12. 12. Free BCOR 012 tutoring will be available this FRIDAY, 2/5 (today) from 4 - 5:30pm in L/L A161 for drop-in tutoring. The Sunday evening tutoring session on 2/7 is canceled and will resume on Sunday, 2/14 at 7:30 pm.
  13. 13. In tufted titmice, crest height is controlled by a pair of alleles at the crest locus. A tall crest is determined by the dominant allele T. Individuals with the tt genotype have a short crest. In a population of 100 titmice, 9 birds are short-crested. What proportion of the tall-crested birds are homozygous at the crest locus? A. 91% B. 49% C. 21% D. a little over half E. 42% Clicker Question
  14. 14. Assumptions of Hardy-Weinberg: • No mutations • Large population size • No migration • No natural selection (i.e., all members survive and reproduce) • Random mating These conditions are almost never met in nature. Thus HW is an ideal case.
  15. 15. Q. So how do populations evolve? A. Through genetic drift and natural selection
  16. 16. Genetic drift can alter population allele frequencies.
  17. 17. Two situations can shrink a population to a size small enough for genetic drift to operate: • The bottleneck effect • The founder effect
  18. 18. The bottleneck effect
  19. 19. The African cheetah population experienced one (perhaps two) bottlenecks.
  20. 20. Founder effect is the establishment of a new population by a few original founders which carry only a small fraction of the total genetic variation of the source population.
  21. 21. The Juan Fernandez Islands, located some 700 km west of Santiago in Chile. There and nowhere else in the world exists Thyrsopteris elegans, a genus of tree ferns endemic to the islands.
  22. 22. Q. What permits novel life forms like Thyrsopteris elegans and Darwin’s finches to evolve on islands? A. Genetic change in small populations as a result of founder effect, and adaptation to the new environmental conditions.
  23. 23. Natural Selection as the Mechanism of Adaptive Evolution Modes of Selection Directional Disruptive Stabilizing Sexual Constraints on Natural Selection Microevolution Natural Selection - the English Peppered Moth Outline: The Evolution of Populations (Chapter 23) February 5
  24. 24. Five premises underlying Darwin’s theory of Evolution by Natural Selection: • Variability: Populations of organisms are variable • Heritability: Some of the variable traits are passed from generation to generation • Overproduction: More individuals are produced in a population than will survive to reproduce • Competition: Individuals compete for limited resources
  25. 25. Five premises underlying Darwin’s theory: • Differential Survival: Those individuals better suited to their environment will leave more descendents than less well suited individuals. This is natural selection!
  26. 26. • Species A species is a set of populations that are reproductively isolated from other such population sets. • Populations A population is a set of conspecific individuals living close enough together to interbreed. The population is the smallest unit of evolution • Individuals Selection acts upon the individual Individuals, Populations, and Species are Hierarchically Related
  27. 27. Biston betularia, the peppered moth melanistic and normal forms Allele frequencies change in response to natural selection Reference: Kettlewell, H. B. D. 1961. The phenomenon of industrial melanism in Lepidoptera. Ann. Rev. of Entomol. 6: 245 - 262.
  28. 28. The peppered moths satisfy the conditions for natural selection: • the population is variable • color pattern is inherited • the different forms have different fitnesses
  29. 29. Genetic Variation and Natural Selection Variation is the raw material of evolution
  30. 30. Genetic variation occurs both within populations and between populations Shell color variation within a population of marine snail Plant height varies between yarrow populations in the Sierra Nevada.
  31. 31. Sources of genetic variation: I. Mutation, including: • Point mutations Sickle cell anemia results from a point mutation. Just one nucleotide substitution in the hemoglobin gene resulted in an amino acid substitution that causes the sickle cell disease. • Chromosomal rearrangements Humans have 46 chromosomes, whereas chimpanzee, gorilla, and orangutan have 48. This major karyotypic difference was caused by the fusion of two ancestral chromosomes to form human chromosome 2. (Yunis and Prakash 1982)
  32. 32. II. Crossing Over, Independent Assortment, and Sexual Recombination Sources of genetic variation:
  33. 33. A closer look at natural selection as the mechanism of adaptive evolution
  34. 34. Two measures of fitness: • Darwinian fitness - the contribution an individual makes to the next generation relative to the contribution of other individuals. • Relative fitness - the contribution a particular genotype makes to the next generation compared to the contributions of alternative genotypes at the same locus.
  35. 35. Relative fitness, an example: Consider our population of red- or white-flowered Phlox. Let’s say the red-flowered plants are visited preferentially by pollinating insects and so are more successful at producing offspring. The white- flowered plants, by contrast, leave only 75% of the descendents that red-flowered plants do. We arbitrarily assign a fitness value of 1.0 to the more successful geno- types; thus the fitness of RR and Rr is 1.0 and that of rr is 0.75.
  36. 36. Natural selection acts on the phenotype. As particular variants are selected, favorable genotypes are maintained or increased. The unit of selection is the individual.
  37. 37. Natural selection can be directional, diversifying, or stabilizing.
  38. 38. In this diagram, the white arrow indicates natural selection working against the lighter-colored phenotypes. Under directional selection, the average fur color darkens in the population in response.
  39. 39. Under diversifying selection, both the lighter and the darker phenotypes are favored over the medium ones. Thus both lighter and darker coats will increase in frequency.
  40. 40. Under stabilizing selection, the average phenotype is favored. More extreme variants decrease in frequency in response.
  41. 41. Which of the three selection types would result in black or white snuzzles but no gray ones? A. Directional Selection B. Stabilizing Selection C. Diversifying Selection D. None E. All Clicker Question
  42. 42. Sexual selection has led to the evolution of sexual dimorphism in many animal species.
  43. 43. Constraints on Evolution: • Natural selection can operate only on existing features; it can’t fashion new ones de novo • Adaptations are often compromises - what is an optimal solution for one challenge is not necessarily optimal for another • Not all evolution is adaptive