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AP Biology Chapter 23 The Evolution of Populations

AP Biology Chapter 23 The Evolution of Populations. Campbell and Reece 10 th Edition. Individuals do not evolve, populations do over time. Individuals do not evolve , populations do. Medium Ground Finch from island of Daphne Major in Galápagos Islands

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AP Biology Chapter 23 The Evolution of Populations

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  1. AP BiologyChapter 23The Evolution of Populations Campbell and Reece 10th Edition

  2. Individuals do not evolve, populations do over time • Individuals do not evolve, populations do • Medium Ground Finch from island of Daphne Major in Galápagos Islands • Long period of drought altered their food supply to mostly larger nuts & over the years those individuals with larger beaks were more successful

  3. Overall size of bird b/4 & after drought years

  4. Average beak size & size of individual birds larger after the drought so…. • The medium ground finch population had evolved by natural selection

  5. Genetic VariationasCause of Evolution • Darwin reasoned that natural selection acted on genetic variation of populations • He knew nothing about genes • Few yrs later: Mendel’s paper on inheritance in pea plants: stage set for understanding variation

  6. Genetic Variation • Genotype inheritable, phenotypes are not • Example: moth, Nemoriaarizonaria, appears very different eating oak flowers vs. oak leaves

  7. Phenotype is result of : genotype + environmental influence

  8. In general, only the genetically determined part of a phenotype can affect evolution • Discrete characters = “either/or” ( Mendel’s pea) = single gene • Most heritable variations involve quantitative characters: vary along a continuum ≥ 2 genes

  9. Average Heterozygosity • way to quantify gene variability • average % of loci that are heterozygous • can calculate average: turns out if the average heterozygosity is 14% there is enough genetic variation for natural selection to act  evolutionary change

  10. Gel Electrophoresis used to calculate heterozygosity

  11. Gel Electrophoresis • does not show silent mutations (DNA changes but still codes for same a.a.)

  12. Gene Variation Between Populations Geographic Variation • Differences in genetic composition of separate populations

  13. Cline: a graded change in a character along a geographic axis

  14. SourcesofGenetic Variation • Mutation • Gene Duplication • Sexual Reproduction • Other process that results in new alleles or new genes

  15. Sources of genetic variation • Organisms with short life spans new genetic variants arise fairly rapidly

  16. Formation of New Alleles • Mutations • can’t predict where in genome or what type mutation • for multicellular organisms only mutations in gametes cell line passed to new generations (most are in somatic cell line) • most point mutations silent or only slightly harmful, rarely are they beneficial

  17. Altering Gene # or Position • Chromosomal changes that delete, disrupt, or rearrange usually lethal or harmful • If genes left intact they may be neutral changes • Translocation: • Part of 1 chromosome breaks off & attaches to another chromosome

  18. Translocations

  19. Duplications of Chromosomes • If large segments duplicated usually harmful • Duplications of small pieces may be beneficial • mutations accumulate over time • eventually that duplication takes on new role • end result: expanded genome

  20. Rapid Reproduction • Average mutation rate in plants & animals is considered low ~ 1 mutation in every 100,000 genes / generation

  21. Prokaryotic Mutation Rates • Shorter generation spans allows for generation of genetic variation in a population • Virus populations, especially retroviruses process is fastest

  22. HIV • Single stranded RNA: • less complicated to duplicate • fewer RNA repair mechanisms in host cells

  23. HIV • Most effective treatment for a quickly mutating retrovirus has been combination protocols

  24. Sexual Reproduction • Most of genetic variation due to crossing over and independent assortment of chromosomes in meiosis and fertilization

  25. Hardy-Weinberg Equation can tell you if a Population is Evolving • presence of genetic variation does not guarantee that population is evolving • 1 of 3 factors that cause evolution must be at work in a population • Population: group of same species in same area that interbreed, with fertile offspring

  26. Populations • Examples of isolated populations: • Islands • Lakes • Even populations not strictly isolated members tend to breed with own population so are genetically closer to them than other groups

  27. Gene Pools • consists of all copies of every allele at every locus in all members of a population

  28. Fixed Genes • if there is only 1 allele for a locus that allele is said to be fixed in the gene pool; entire population is homozygous for that gene • If there are 2 or more alleles for a locus then individuals may be homozygous or heterozygous

  29. Each allele has a frequency in the population

  30. The Hardy-Weinberg Principle • to test whether natural selection is acting on a particular locus: • Determine what the frequency would be if it were not evolving • Then compare that calculation with what you measure in the population • No difference: not evolving • Difference: evolving

  31. Hardy-Weinberg Principle 1908 • Hardy • Weinberg

  32. Hardy-Weinberg Principle • states that the frequencies of alleles & genotypes in a population will remain constant from generation to generation, provided that only Mendelian segregation & recombination of alleles are at work • If that is true the population is said to be in HARDY-WEINBERG EQUILIBRIUM

  33. Hardy-Weinberg Equilibrium

  34. Hardy-Weinberg Equilibrium

  35. assumes random mating

  36. Hardy-Weinberg Problems • http://nhscience.lonestar.edu/biol/hwe/q1d.htmlhttp://www.phschool.com/science/biology_place/labbench/lab8/intro.html • Problem 2: • If 9% of an African population is born with a severe form of sickle cell anemia (ss) what % of the population will be more resistant to malaria because they are heterozygous (Ss) for the sickle-cell gene?

  37. Answer to problem 2 • 2pq = 2 (.7 x .3) = .42 = 42% of the population are heterozyotes (carriers)

  38. Conditions for Hardy-Weinberg Equilibrium • No Mutations • Random Mating • No Natural Selection • Extremely Large Populations • No Gene Flow

  39. Departure from any of the 5 conditions usually results in evolutionary changes • A population may be evolving at some gene loci and in Hardy-Weinberg Equilibrium at other loci

  40. Applying the Hardy-Weinberg Principle • Can be used to estimate the frequency of a gene causing inherited disease in a population • Must assume: • No new mutations • Random mating • Ignore any effects of differential survival & reproductive success • No genetic drift

  41. How Allele Frequencies are Altered in a Population

  42. Conditions Necessary for H-W Equilibrium • No Mutations: not usually significant unless mutation produces new alleles that have a strong influence in a (+) or (-) way • Random Mating: not usually significant • No Natural Selection cause most • Extremely Large Populations evolutionary • No Gene Flow change

  43. NATURAL SELECTION • is based on differential success in survival & reproduction • if NS consistently favoring some alleles over others , NS can cause adaptiveevolution (dfn: evolution that results in a better match between organisms & their environment)

  44. GENETIC DRIFT • process in which chance events cause unpredictable fluctuations in allele frequencies from one generation to the next • the smaller the population the more pronounced the effect

  45. GENETIC DRIFT

  46. 2 Examples of Genetic Drift • Founder Effect genetic drift that occurs when a few individuals become isolated from a larger population & form a new population whose gene pool composition is not reflective of original population

  47. Founder Effect

  48. Founder EffectTristan da Cunha15 British colonist in 1814

  49. Tristan da Cunha • 1 colonist carried recessive allele for retinitis pigmentosa

  50. Tristan da Cunha • By late 1960’s, there were 240 descendants of the original founders • 4 had retinitis pigmentosa • This frequency is 10x higher than frequency of retinitis pigmentosa in England

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