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Lecture 9 Microevolution 3 – Random Drift Assumptions of Hardy-Weinberg Equilibrium

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Lecture 9 Microevolution 3 – Random Drift Assumptions of Hardy-Weinberg Equilibrium Lecture 9 Microevolution 3 – random drift Assumptions of Hardy-Weinberg equilibrium 1. Mating is random 2. No selection 3. No mutation 4. No migration 5. Population size is infinite (i.e., no genetic drift) The Neutral Theory of Evolution Motoo Kimura 1924 -1994 Random genetic drift Definition: random changes in the frequencies of neutral alleles from generation to generation caused by “accidents of sampling” Random genetic drift white = p = 0.50 red = q = 0.50 Random genetic drift white = p = 0.50 red = q = 0.50 ↓ Sample 40,000,000 balls Random genetic drift white = p = 0.50 red = q = 0.50 ↓ 20,000,167 white 19,999,833 red Random genetic drift white = p = 0.50 red = q = 0.50 ↓ 20,000,167 white 19,999,833 red now p = 0.5000042 q = 0.4999958 Random genetic drift white = p = 0.50 red = q = 0.50 ↓ Sample 200 balls Random genetic drift white = p = 0.50 red = q = 0.50 ↓ 94 white 106 red Random genetic drift white = p = 0.50 red = q = 0.50 ↓ 94 white 106 red now p = 0.470 q = 0.530 Some properties of random genetic drift Some properties of random genetic drift 1. Magnitude inversely proportional to effective population size (Ne). Some properties of random genetic drift 1. Magnitude inversely proportional to effective population size (Ne). 2. Ultimately results in loss of variation from natural populations. Some properties of random genetic drift 1. Magnitude inversely proportional to effective population size (Ne). 2. Ultimately results in loss of variation from natural populations. 3. The probability of fixation of a neutral allele is equal to its frequency in the population. Some properties of random genetic drift 1. Magnitude inversely proportional to effective population size (Ne). 2. Ultimately results in loss of variation from natural populations. 3. The probability of fixation of a neutral allele is equal to its frequency in the population. 4. Will cause isolated populations to diverge genetically. Some properties of random genetic drift 1. Magnitude inversely proportional to effective population size (Ne). 2. Ultimately results in loss of variation from natural populations. 3. The probability of fixation of a neutral allele is equal to its frequency in the population. 4. Will cause isolated populations to diverge genetically. 5. Is accentuated during population bottlenecks and founder events. What is effective population size? What is effective population size? • in any one generation, Ne is roughly equivalent to the number of breeding individuals in the population. What is effective population size? • in any one generation, Ne is roughly equivalent to the number of breeding individuals in the population. • this is equivalent to a contemporary effective size. What is effective population size? • in any one generation, Ne is roughly equivalent to the number of breeding individuals in the population. • this is equivalent to a contemporary effective size. • Ne is also strongly influenced by long-term history. What is effective population size? • in any one generation, Ne is roughly equivalent to the number of breeding individuals in the population. • this is equivalent to a contemporary effective size. • Ne is also strongly influenced by long-term history. • this is equivalent to a species’ evolutionary effective size. Factors affecting effective population size Factors affecting effective population size 1. Fluctuations in population size Factors affecting effective population size 1. Fluctuations in population size - here, Ne is equal to the harmonic mean of the actual population numbers: Factors affecting effective population size 1. Fluctuations in population size - here, Ne is equal to the harmonic mean of the actual population numbers: 1/Ne = 1/t(1/N1 + 1/N2 + 1/N3 + … 1/Nt) Factors affecting effective population size 1. Fluctuations in population size - here, Ne is equal to the harmonic mean of the actual population numbers: 1/Ne = 1/t(1/N1 + 1/N2 + 1/N3 + … 1/Nt) Example: Over 3 generations, N = 2000, 30, 2000 Factors affecting effective population size 1. Fluctuations in population size - here, Ne is equal to the harmonic mean of the actual population numbers: 1/Ne = 1/t(1/N1 + 1/N2 + 1/N3 + … 1/Nt) Example: Over 3 generations, N = 2000, 30, 2000 Arithmetic mean = 1343.3 Factors affecting effective population size 1. Fluctuations in population size - here, Ne is equal to the harmonic mean of the actual population numbers: 1/Ne = 1/t(1/N1 + 1/N2 + 1/N3 + … 1/Nt) Example: Over 3 generations, N = 2000, 30, 2000 Arithmetic mean = 1343.3 Harmonic mean = 87.4 Factors affecting effective population size 2. Unequal numbers of males and females Factors affecting effective population size 2. Unequal numbers of males and females • let Nm = No. of males, Nf = No. of females: Factors affecting effective population size 2. Unequal numbers of males and females • let Nm = No. of males, Nf = No. of females: 4NmNf Ne = Nm + Nf Factors affecting effective population size 2. Unequal numbers of males and females • let Nm = No. of males, Nf = No. of females: 4NmNf Ne = Nm + Nf Example: Breeding populations of northern elephant seals: Factors affecting effective population size 2. Unequal numbers of males and females • let Nm = No. of males, Nf = No. of females: 4NmNf Ne = Nm + Nf Example: Breeding populations of northern elephant seals: • 15 alpha males each controlling a harem of 20 females: Factors affecting effective population size 2. Unequal numbers of males and females • let Nm = No. of males, Nf = No. of females: 4NmNf Ne = Nm + Nf Example: Breeding populations of northern elephant seals: • 15 alpha males each controlling a harem of 20 females: census size (N) = 315 Factors affecting effective population size 2. Unequal numbers of males and females • let Nm = No. of males, Nf = No. of females: 4NmNf Ne = Nm + Nf Example: Breeding populations of northern elephant seals: • 15 alpha males each controlling a harem of 20 females: census size (N) = 315 effective size (Ne) = 57.1 Factors affecting effective population size 3. Large variance in reproductive success • reduces Ne because a small number of individuals have a disproportional effect on the reproductive success of the population. Genetic Bottlenecks • genetic bottlenecks refer to severe reductions in effective population size. Ne Time Genetic Bottlenecks • genetic bottlenecks refer to severe reductions in effective population size. Genetic Bottlenecks • genetic bottlenecks refer to severe reductions in effective population size. Examples: the northern elephant seal (Mirounga angustirostis) and the cheetah (Acinonyx jubatus). Founder effects • occur when a new population is founded from a small number of individuals. Founder effects • occur when a new population is founded from a small number of individuals. Consequences: Founder effects • occur when a new population is founded from a small number of individuals. Consequences: 1. New population has a fraction of genetic variation present in the ancestral population. Founder effects • occur when a new population is founded from a small number of individuals. Consequences: 1. New population has a fraction of genetic variation present in the ancestral population. 2. Initial allele frequencies differ because of chance. Founder effects • occur when a new population is founded from a small number of individuals. Consequences: 1. New population has a fraction of genetic variation present in the ancestral population. 2. Initial allele frequencies differ because of chance. Example: the silvereye, Zosterops lateralis Founder effects in the silvereye, Zosterops lateralis Timing of island hopping… … and reductions in allelic diversity The interplay between drift, migration, and selection 1. Gene flow vs. drift The interplay between drift, migration, and selection 1. Gene flow vs. drift • random drift and gene flow act in opposition to each other! The interplay between drift, migration, and selection 1. Gene flow vs. drift • random drift and gene flow act in opposition to each other! random drift → allow genetic divergence of two populations. The interplay between drift, migration, and selection 1. Gene flow vs. drift • random drift and gene flow act in opposition to each other! random drift → allow genetic divergence of two populations. gene flow → prevent divergence. The interplay between drift, migration, and selection 1. Gene flow vs. drift • random drift and gene flow act in opposition to each other! random drift → allow genetic divergence of two populations. gene flow → prevent divergence. if Nem > 1, gene flow overrides drift and prevents divergence The interplay between drift, migration, and selection 1. Gene flow vs. drift • random drift and gene flow act in opposition to each other! random drift → allow genetic divergence of two populations. gene flow → prevent divergence. if Nem > 1, gene flow overrides drift and prevents divergence if Nem < 1, random drift can lead to genetic divergence. The interplay between drift, migration, and selection 2. Gene flow and selection The interplay between drift, migration, and selection 2. Gene flow and selection • gene flow and selection usually act in opposition. The interplay between drift, migration, and selection 2. Gene flow and selection • gene flow and selection usually act in opposition. selection → favor different alleles in different populations (“local adaptation”). The interplay between drift, migration, and selection 2. Gene flow and selection • gene flow and selection usually act in opposition. selection → favor different alleles in different populations (“local adaptation”).
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