AP Biology Chapter 23 The Evolution of Populations

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