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Introduction: Is population size a critical component to evolution? Are there differences in the evolutionary pressures on small populations versus larger.

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Presentation on theme: "Introduction: Is population size a critical component to evolution? Are there differences in the evolutionary pressures on small populations versus larger."— Presentation transcript:

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2 Introduction: Is population size a critical component to evolution? Are there differences in the evolutionary pressures on small populations versus larger populations? How does genetic drift or natural selection affect the rate of evolution on a population? Do alleles which are low in frequency within a population disappear over time? This experiment will try to answer these questions. Materials: approximately 100 red colored and 100 blue colored straw pieces. large opaque bin (to contain your sample population).

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4 Part A: Initial population has equal number of dominant and recessive alleles in the gene pool. 1. Count out 80 blue straw pieces (blue is the dominant allele – B) and 80 red straw pieces (red is the recessive allele – b) and place them in your large opaque bin. This represents a population with 80 individuals (genotypes undetermined). 2. Shake the container and thoroughly mix the alleles (‘gene pool’) in the bin. 3. Place the bin above your head and pick out 20 individuals only (40 alleles in total out of the total 160 in the gene pool). 4. Determine and record the number of each genotype (for all experiments) in a table similar to Table 1. Remember that each individual has two alleles (BB – (2 blue straws) – Homozygous dominant; Bb – (1 blue and 1 red straw) – Heterozygous; bb (2 red straws) – Homozygous recessive). 5. Record the F1 allelic frequencies as decimal values (total number of dominant alleles (B) divided by the total number of alleles in your population of 20 individuals; total number of recessive alleles (b) divided by the total number of alleles in your population of 20 individuals). 6. Place the next generation of dominant and recessive alleles in the same proportion as recorded in the “offspring” (e.g.: x 160 = 68 B alleles (blue straws); x 160 = 92 b alleles (red straws)). 7. Repeat steps 2 – 6 for four additional generations (F2, F3, F4, and F5 generations).

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7 Part B. Initial population has an unequal number of dominant and recessive alleles in the gene pool. Each student will begin with a different proportions of dominant (B) versus recessive (b) alleles. 1. Count out blue straw pieces (blue is the dominant allele – B) and red straw pieces (red is the recessive allele – b) in the numbers given below and place them in your large opaque bin. This represents a population with 80 individuals (genotypes undetermined). i.MA B; 10 bvii.LM - 20 B; 140 b ii.SG B; 20 bviii.LH - 15 B; 145 b iii.JM B; 25 bix.RC - 10 B; 150 b iv.RM B; 35 b v.BT B; 40 b vi.SR - 30 B; 130 b 2. Repeat steps 2 – 7 from Part A above.

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9 Part C: Selective pressures within the environment are acting upon certain genotypes. A.Follow the following scenarios: 1. Count out 80 blue straw pieces (blue is the dominant allele – B) and 80 red straw pieces (red is the recessive allele – b) and place them in your large opaque bin. This represents a population with 80 individuals (genotypes undetermined). 2. Repeat steps 2 – 7 from Part A above. i.MA - Out of the 20 individuals, 2 homozygous recessive were eaten and removed each generation; ii.RC - Out of the 20 individuals, 3 homozygous dominant were eaten and removed each generation; iii.LH - Out of the 20 individuals, 2 homozygous dominant were eaten and removed each generation; B. Follow the following scenarios: 1. Count out 120 blue straw pieces (blue is the dominant allele – B) and 40 red straw pieces (red is the recessive allele – b) and place them in your large opaque bin. This represents a population with 80 individuals (genotypes undetermined). 2. Repeat steps 2 – 7 from Part A above. iv.SG - Out of the 20 individuals, 3 homozygous recessive were eaten and removed each generation; v.RM - Out of the 20 individuals, 4 homozygous recessive were eaten and removed each generation; vi.LM - Out of the 20 individuals, 4 homozygous dominant were eaten and removed each generation. C. Follow the following scenarios: 1. Count out 40 blue straw pieces (blue is the dominant allele – B) and 120 red straw pieces (red is the recessive allele – b) and place them in your large opaque bin. This represents a population with 80 individuals (genotypes undetermined). 2. Repeat steps 2 – 7 from Part A above. vii.JM - Out of the 20 individuals, 3 homozygous recessive were eaten and removed each generation; viii.SR - Out of the 20 individuals, 4 homozygous recessive were eaten and removed each generation; ix.BT - Out of the 20 individuals, 4 homozygous dominant were eaten and removed each generation.

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14 1. Plot graphs for each section (Part A, B & C) of allelic frequency versus generation. Use two different symbols to graph your data on the same graph. [Must be graphed using a computer.] 2. Based on your results, was evolution occurring in any of the sections (Part A, B & C). Give an explanation for each Part whether evolution was occurring or not. If evolution was occurring, which of the five conditions of the Hardy-Weinberg principle were not met? 3. For part C, if evolution was occurring, describe the type of selective evolution that was occurring. Draw a basic graph [Number of Individuals in the Population versus Genotype] that you would expect to see over time. 4. Read the following passage: Industrial melanism in the peppered moth (Biston betularia) during the industrial revolution in England. The moths fly by night and rest during the day on lichen covered tree trunks where they are preyed upon by birds. Prior to the industrial revolution most of the moths were light colored and well camouflaged. A few dark (melanistic) were occasionally noted. During the industrial revolution soot began to blacken the trees and also cause the death of the lichens. The light colored moths were no longer camouflaged so their numbers decreased quite rapidly. With the blackening of the trees the numbers of dark moths rapidly increased. The frequency of the dark allele increased from less than 1% to over 98% in just 50 generations. Since the 1950's attempts to reduce industrial pollution in Britain have resulted in an increase in numbers of light form. What type of selection is taking place in the peppered moth, Biston betularia? Draw a series of graphs depicting Number of Individuals in the Population versus Genotype for the following time periods: 1:Pre-industrial revolution; 2:During the Industrial revolution; 3:Post-industrial revolution.

15 Table 1: Numbers of Individuals in each Generation and Their Allelic Frequencies under Different Evolutionary Pressures Generation Different Evolutionary Pressures Random Mating, No Natural Selection (B = b in initial population) Random Mating, No Natural Selection (B ≠ b in initial population Natural Selection Occurring Number of Individuals Allelic Frequencies (decimal) Number of Individuals Allelic Frequencies (decimal) Number of Individuals Allelic Frequencies (decimal) F1F1 BB = Bb = bb = B = b = BB = Bb = bb = B = b = BB = Bb = bb = B = b = F2F2 BB = Bb = bb = B = b = BB = Bb = bb = B = b = BB = Bb = bb = B = b = F3F3 BB = Bb = bb = B = b = BB = Bb = bb = B = b = BB = Bb = bb = B = b = F4F4 BB = Bb = bb = B = b = BB = Bb = bb = B = b = BB = Bb = bb = B = b = F5F5 BB = Bb = bb = B = b = BB = Bb = bb = B = b = BB = Bb = bb = B = b =


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