1. Ch 23 Evolution of Populations Changes in allele frequency within populations drive evolution. Microevolution considers mechanisms that cause generation-to-generation changes in allele frequency within populations.

2. Populations, Allele Frequency Change, and Microevolution A population is a group of interbreeding organisms present in a specific location at a specific time. Allele frequency is the frequency of a particular allele in the population. The population, not the species or individual, is the fundamental unit of evolution.

3. Populations Are the Units of Evolution

4. The Genetic Basis of Evolution For evolution to occur, genetic differences must at least partially account for phenotypic differences.

5. What Drives Evolution? There are 5 forces of change. Only natural selection makes a population better adapted (more fit) to its environment.

6. Mutations Provide Raw Material For Evolution One type of mutation at the level of the gene. One type of mutation at the level of the chromosome. Mutations are usually neutral or harmful in their effects; only rarely are they beneficial.

7. Mutations “Just Happen” Mutations occur at random without regard to whether they have a beneficial, neutral or harmful effect. For this reason, mutations are a randomly acting evolutionary force.

8. Mutation Mutation is the only source of new alleles in a species. Mutation acting alone works too slowly to drive evolution. With an average mutation rate, it takes ~ 70,000 generations, far more than the number of generations of modern humans, to reduce allele frequency by 50%. Loss of an allele due to mutation

9. Gene Flow or Migration Gene flow makes separate populations more similar genetically. The effects of gene flow are seen in many human populations, including the U.S. population. Gene flow in plants – wind-dispersed pollen moving between Monterey pines.

10. Genetic Drift Genetic drift is random fluctuation in allele frequency between generations. The effects of genetic drift are pronounced in small populations.

11. A Genetic Bottleneck is a Form of Genetic Drift Once again, small bottlenecked populations = big effect. In a genetic bottleneck, allele frequency is altered due to a population crash.

12. Genetic Bottleneck – A Historical Case Other animals known to be affected by genetic bottlenecks include the cheetah and both ancient and modern human populations. Note: A genetic bottleneck creates random genetic changes without regard to adaptation. A severe genetic bottleneck occurred in northern elephant seals.

13. Endangered Species Are in the Narrow Portion of a Genetic Bottleneck and Have Reduced Genetic Variation

14. The Effect of Genetic Drift is Inversely Related to Population Size Large populations = small effects. Small populations = large effects.

15. The Founder Effect is Another Variation of Genetic Drift A founder effect occurs when a small number of individuals from one population found a new population that is reproductively isolated from the original one. Migration from England

16. The Founder Effect is Another Variation of Genetic Drift The South Atlantic island of Tristan da Cunha was colonized by 15 Britons in 1814, one of them carrying an allele for retinitis pigmentosum. Among their 240 descendents living on the island today, 4 are blind by the disease and 9 others are carriers.

17. The Founder Effect Old Order Amish populations are derived from a few dozen colonists who escaped religious persecution in Germany in 1719 to settle in Pennsylvania. The community is closed. Allele and genetic disease frequencies in Amish are significantly different from the German ancestral and the surrounding local populations.

18. Polydactyly in Pennsylvania Old Order Amish (founding population approximately 200 people, almost always marry within the group) Johns Hopkins University Press

19. The Founder Effect

20. Non-Random Mating Non-random mating occurs when there is a bias for or against mating with related individuals. Inbreeding is preferential mating with relatives. Inbreeding increases the frequency of homozygosity relative to random mating, elevating the frequency of recessive genetic disorders. Cute, but prone to genetically-based disorders. Inbreeding is a common form of non-random mating.

21. Non-Random Mating The high frequency of particular recessive genetic disorders seen in many closed communities is a consequence of the founder effect and inbreeding. Remember that inbreeding includes matings of distant relatives – the Amish have never practiced marriage between sibs or other immediate relatives.

22. Natural Selection Natural selection leads to adaptation – an increase in the fitness of a population in a particular environment. Natural selection works because some genotypes are more successful in a given environment than others. Successful (adaptive) genotypes become more common in subsequent generations, causing an alteration in allele frequency over time that leads to a consequent increase in fitness. It’s not natural – but this is one outcome of strong selection.

23. Three Forms of Natural Selection

24. Directional Selection Hominid Brain Size

25. Galapagos Finch: Subject of a Classic Study of Evolution in Action Peter and Mary Grant and their colleagues observed how beak depth, a significant trait for feeding success, varied in populations experiencing climatic variations.

26. Beak Depth Changed in a Predictable Way in Response to Natural Selection Significantly, beak depth is a genetically determined trait. Type of Natural Selection?

27. Human Birth Weight Is Under Stabilizing Selection Modern medicine relaxes this and other forms of selection. Type of Selection?

28. Stabilizing Selection for the Sickle Cell Allele In heterozygous form, the sickle cell allele of  -globin confers resistance to malaria. Therefore, the allele is maintained, even though it’s harmful in homozygous form.

29. Changing Selection With Changes in Human Culture?

31. Hardy-Weinberg Theorem  Serves as a model for the genetic structure of a nonevolving population (equilibrium)  5 conditions:  1- Very large population size;  2- No migration;  3- No net mutations;  4- Random mating;  5- No natural selection

32. Hardy-Weinberg Equation  p=frequency of one allele (A);  q=frequency of the other allele (a);  p+q=1.0 (p=1-q & q=1-p) (p=1-q & q=1-p)  Hardy-Weinberg Equation:  p 2 + 2pq + q 2 = 1.0  p 2 =frequency of AA genotype;  2pq=frequency of Aa plus aA genotype;  q 2 =frequency of aa genotype;

33. AP Lab Hardy-Weinberg Populations or how Hardy and Weinberg grew to love and frequently count on each other.  Introduction: In 1908 Hardy and Weinberg suggested a scheme whereby evolution could be viewed as changes in the frequency of alleles. In 1908 Hardy and Weinberg suggested a scheme whereby evolution could be viewed as changes in the frequency of alleles. The 5 postulates of the Hardy-Weinberg Theorem are: The 5 postulates of the Hardy-Weinberg Theorem are: The Hardy-Weinberg Equation is: The Hardy-Weinberg Equation is:

34. Hardy-Weinberg Lab  Exercise A: Estimating Allele Frequencies for a Specific Trait within a Sample Population. The ability to taste bitter PTC is evidence of the presence of a dominant allele in either the homozygous (AA) or heterozygous (Aa) condition. The ability to taste bitter PTC is evidence of the presence of a dominant allele in either the homozygous (AA) or heterozygous (Aa) condition. Tape in the PTC data table and record class data. Calculate the Allele Frequency based on H-W. Tape in the PTC data table and record class data. Calculate the Allele Frequency based on H-W.

35. Hardy-Weinberg Lab  8B: Case Study I: Ideal Hardy-Weinberg The entire class will represent a breeding population. Assume that gender and genotype are irrelevant to mate selection. The entire class will represent a breeding population. Assume that gender and genotype are irrelevant to mate selection. Every person will start as Aa. Every person will start as Aa. What is the frequency of A? What is the frequency of A? What is the frequency of a? What is the frequency of a? Follow instruction for mating procedures Follow instruction for mating procedures Random, but how to recognize each other??? Random, but how to recognize each other??? Record class data. Analysis will come later. Record class data. Analysis will come later.

36. Hardy-Weinberg Lab  8B: Case Study II: Selection This simulation is modified to be more realistic. In the natural environment, not all genotypes have the same rate of survival; that is, the environment might favor some genotypes while selecting against others. Sickle-Cell Anemia is such a case. This simulation is modified to be more realistic. In the natural environment, not all genotypes have the same rate of survival; that is, the environment might favor some genotypes while selecting against others. Sickle-Cell Anemia is such a case. Start with all Aa Start with all Aa aa does not survive to reproduce. aa does not survive to reproduce. Five generations, record Five generations, record

37. Hardy-Weinberg Lab  8B: Case Study III: Heterozygote Advantage Many human populations show an unexpectedly high frequency of the sickle-cell allele. It turns out that individuals who are heterozygous are slightly more resistant to a deadly form of malaria than homozygous dominant individuals. Many human populations show an unexpectedly high frequency of the sickle-cell allele. It turns out that individuals who are heterozygous are slightly more resistant to a deadly form of malaria than homozygous dominant individuals. Keep everything the same except if the offspring is AA flip a coin. Heads does NOT survive, Tails survives. Remember, aa never survives. Keep everything the same except if the offspring is AA flip a coin. Heads does NOT survive, Tails survives. Remember, aa never survives. Five Rounds at least. Five Rounds at least.

38. Hardy-Weinberg Problems  Hardy-Weinberg Problem 1 (step by step)  p 2 + 2pq + q 2 = 1 p + q = 1  Assumptions for Hardy-Weinberg Equilibrium:  1- Very large population size;  2- No migration;  3- No net mutations;  4- Random mating;  5- No natural selection

39. Problem: 1 in 1700 US Caucasian newborns have cystic fibrosis. C for normal is dominant over c for cystic fibrosis. 1. When counting the phenotypes in a population why is cc the most significant?  CC and Cc look phenotypically the same, so can’t be counted separately. Only one phenotype for cc, so CAN count  2. What percent of the above population have cystic fibrosis (cc or q 2 )?  q 2 = 1/1700 =.00059 =.059%

40. Problem: 1 in 1700 US Caucasian newborns have cystic fibrosis.  ALLELE FREQUENCY CALCULATIONS:  Why calculate "q" first?  You know q 2 so q = square root q 2  (frequency)c = q = square root.00059 =.024 = 2.4%  Why is it now easy to find "p"?  p + q = 1 so…  1 – q = p so…  (f)C = p so…  1-.024 =.976 or 97.6%

41. GENOTYPE FREQUENCY CALCULATIONS: (f)CC: Normal homozygous dominant = p 2 = (.976 2 ) =.953 = 95.3% population CC  (f)Cc: carriers of cystic fibrosis =  2pq = 2(.976 x.024) =.0468 or 4.68% Cc  6. How many of the 1700 of the population are homozygous Normal?

42. Population Estimates  6. How many of the 1700 of the population are homozygous Normal? .953 x 1700 = ~1620.1 are normal CC  7. How many of the 1700 in the population are heterozygous (carrier)? .0468 x 1700 = ~79.56 are carriers Cc

43. What Hardy-Weinberg Assumption Broken?  8. It has been found that a carrier is better able to survive diseases with severe diarrhea. What would happen to the frequency of the "c" if there was a epidemic of cholera or other type of diarrhea producing disease? Would "c" Increase or would it Decrease?  Cc survives over CC, so reduces #C’s in total population, therefore increases percentage of c.  Broken Assumption:  No Natural Selection

44. Hardy-Weinberg Problem 2 If 9% of an African population is born with a severe form of sickle-cell anemia (ss), what percentage of the population will be more resistant to malaria because they are heterozygous(Ss) for the sickle-cell gene?  9% =.09 = ss = q2  (f)s = q = Square root of.09 =.3  p = 1 -.3 =.7  2pq = 2 (.7 x.3) =.42 = 42% of the population are heterozyotes (carriers)

45. Hardy-Weinberg Problem 3  This is a classic data set on wing coloration in the scarlet tiger moth (Panaxia dominula). Coloration in this species had been previously shown to behave as a single-locus, two-allele system with incomplete dominance. Data for 1612 individuals are given below:  White-spotted (AA) =1469 Intermediate (Aa) = 138 Little spotting (aa) =5  Calculate the following frequencies: A, a, AA, Aa, aa  (f)AA = 1469/1612 =.911 or 91.1%

46. Hardy-Weinberg Problem 4A  After graduation, you and 19 friends (wish I had 19 friends!) build a raft, sail to a deserted island, and start a new population, totally isolated from the world. Two of your friends carry (that is, are heterozygous for) the recessive cf allele, which in homozygotes causes cystic fibrosis.  A. Assuming that the frequency of this allele does not change as the population grows, what will be the instance of cystic fibrosis on your island?  There are 40 total alleles of the 20 people of which 2 alleles are cystic fibrosis causing.  2/40 =.05 (f) for the cystic fibrosis allele  thus cc or q 2 = (.05) 2 =.0025 or.25% of the population will be born with cystic fibrosis

47. Hardy-Weinberg Problem 4B  B. Cystic fibrosis births on the island is how many times greater than the original mainland. The frequency of births on the mainland is.059%.  0.25/.059 = about 4 times greater occurrence  Which HW Assumption is stretched here?  What “effect” at play here? Founder Effect Founder Effect