chapter 23: the evolution of populations. essential knowledge u 1.a.1 – natural selection is a...
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- Chapter 23: The Evolution of Populations
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- Essential Knowledge u 1.a.1 Natural selection is a major mechanism of evolution (23.2). u 1.a.2 Natural selection acts on phenotypic variations in populations (23.1 & 23.4). u 1.a.3 Evolutionary change is also driven by random processes (23.3). u 2.c.1 Changes in genotype can result in changes in phenotype (23.4). u 4.c.3 The level of variation in a population affects population dynamics (23.1 23.3). u 4.c.4 The diversity of species within an ecosystem may influence the stability of the ecosystem (23.2).
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- Question? u Is the unit of evolution the individual or the population? u Answer while evolution affects individuals, it can only be tracked through time by looking at populations.
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- So what do we study? u We need to study populations, not individuals. u We need a method to track the changes in populations over time. u This is the area of Biology called population genetics.
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- Population Genetics u The study of genetic variation in populations. u How do populations change, genetically, over time? u Represents the reconciliation of Mendelism and Darwinism.
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- Population u A localized group of individuals of the same species. u Must produce viable offspring
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- Species u A group of similar organisms. u A group of populations that could interbreed (successfully) u Populations are animals of the same species that are isolated due to geography
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- Gene Pool u The total aggregate of genes in a population. u All alleles at all gene loci in all individuals u If evolution is occurring, then changes must occur in the gene pool of the population over time.
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- Microevolution u Changes in the relative frequencies of alleles in the gene pool. u Micro = small u Microevolution is how we study evolution at the genetics level
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- Hardy-Weinberg Theorem u Developed in 1908. u Use as a benchmark to study evolutionary change in a population u Mathematical model of gene pool changes over time.
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- H-W Theorem u States: u The frequencies of alleles and genotypes in a populations gene pool remain constant (in a population that is NOT evolving)
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- Basic Equation u p + q = 1 u p = %/frequency of dominant allele u q = %/frequency of recessive allele
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- Expanded Equation u p + q = 1 u (p + q) 2 = (1) 2 u p 2 + 2pq + q 2 = 1 u We expand the equation to fit all three types of genotypes (Ex: AA, Aa, aa)
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- Genotypes u p 2 = Homozygous Dominant frequency 2pq = Heterozygous frequency q 2 = Homozygous Recessive frequency
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- Example Calculation u Lets look at a population where: u A = red flowers u a = white flowers
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- Starting Population u N = 500 u Red = 480 (320 AA+ 160 Aa) u White = 20 u Total Genes/Alleles = 2* x 500 = 1000 *2 alleles per genotype (hence the 2 in the equation)
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- Dominant Allele u A = (320 x 2) + (160 x 1) = 800 = 800/1000 = 0.8 = 80% u 320 = AA pop # (2 = # of dominant alleles in that AA genotype); u 160 = Aa pop # (1 = # of dominant alleles in Aa genotype); u 1000 = total genes 2 = # of times the dom allele is present in homozy dom genotype 1 = # of times the dom allele is present in heterozy genotype
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- Recessive Allele u a = (160 x 1) + (20 x 2) = 200 = 200/1000 =.20 = 20% u 20 = aa pop # (2 = # of recessive alleles in that aa/white genotype); u 160 = Aa pop # (1 = # of recessive alleles in Aa genotype); u 1000 = total genes 1 = # of times the rec allele is present in heterozy genotype 2 = # of times the rec allele is present in homozy rec genotype
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- Importance of Hardy-Weinberg u Yardstick to measure rates of evolution. u Predicts that gene frequencies should NOT change over time as long as the H-W assumptions hold. u Way to calculate gene frequencies through time.
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- Example u What is the frequency of the PKU allele? u PKU is expressed only if the individual is homozygous recessive (aa).
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- PKU Frequency u PKU is found at the rate of 1/10,000 births. u PKU = aa = q 2 q 2 =.0001 q =.01 (frequency of recessive alleles)
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- Dominant Allele u p + q = 1 p = 1- q p = 1-.01 p =.99
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- Expanded Equation u p 2 + 2pq + q 2 = 1 (.99) 2 + 2(.99x.01) + (.01) 2 = 1.9801 +.0198 +.0001 = 1 Freq of Homozy Dom genotype Freq of Heterozy genotype Freq of Homozy Rec genotype
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- Final Results u All we did is convert the frequencies (decimals) to % (by multiplying frequencies by 100%) u Normals (AA) = 98.01% u Carriers (Aa) = 1.98% u PKU (aa) =.01%
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- AP Problems Using Hardy-Weinberg u Solve for q 2 (% of total) u Solve for q (equation) u Solve for p (1- q) u H-W is always on the national AP Bio exam
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- Hardy-Weinberg Assumptions 1. Large Population 2. Isolation 3. No Net Mutations 4. Random Mating 5. No Natural Selection
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- If H-W assumptions hold true: u The gene frequencies will not change over time. u Evolution will not occur u Evolution will not occur. u How likely will natural populations hold to the H-W assumptions?
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- Microevolution u Caused by violations of the 5 H-W assumptions.
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- Causes of Microevolution 1. Genetic Drift 2. Gene Flow 3. Mutations 4. Nonrandom Mating 5. Natural Selection
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- Genetic Drift u Changes in the gene pool of a small population by chance. u Types: u 1. Bottleneck Effect u 2. Founder's Effect
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- By Chance
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- Bottleneck Effect u Loss of most of the population by disasters. u Surviving population may have a different gene pool than the original population. u Results: Some alleles lost, others are over-represented, genetic variety is decreased
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- Importance u Reduction of population size may reduce gene pool for evolution to work with. u Ex: Cheetahs
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- Founder's Effect u Genetic drift in a new colony that separates from a parent population. u Ex: Old-Order Amish u Results: Genetic variety reduced, some alleles increase while other lost
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- Importance u Very common in islands and other groups that don't interbreed.
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- Gene Flow u Movement of genes in/out of a population. u Ex: u Immigration u Emigration u Result: change in gene frequency
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- Mutations u Inherited changes in a gene.
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- Result u May change gene frequencies (small population). u Source of new alleles for selection. u Often lost by genetic drift.
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- Nonrandom Mating u Failure to choose mates at random from the population.
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- Causes u Inbreeding within the same neighborhood. u Assortative mating (like with like).
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- Result u Increases the number of homozygous loci. u Does not in itself alter the overall gene frequencies in the population.
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- Natural Selection u Differential success in survival and reproduction. u Result - Shifts in gene frequencies.
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- Comment u As the environment changes, so does natural selection and gene frequencies.
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- Result u If the environment is "patchy", the population may have many different local populations.
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- Genetic Basis of Variation 1. Discrete Characters 1. Discrete Characters Mendelian traits with clear phenotypes. 2. Quantitative Characters 2. Quantitative Characters Multigene traits with overlapping phenotypes.
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- Polymorphism u The existence of several contrasting forms of the species in a population. u Usually inherited as Discrete Characteristics.
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- Examples Garter Snakes Gaillardia
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- Human Example u ABO Blood Groups u Morphs = A, B, AB, O
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- Quantitative Characters u Allow continuous variation in the population. u Result u Geographical Variation u Clines: a change along a geographical axis
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- Yarrow and Altitude
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- Sources of Genetic Variation u Mutations. u Meiosis - recombination though sexual reproduction. u Crossing-over u Random fertilization
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- Comment u Population geneticists believe that ALL genes that persist in a population must have had a selective advantage at one time. u Ex Sickle Cell and Malaria, Tay-Sachs and Tuberculosis
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- Fitness - Darwinian u The relative contribution an individual makes to the gene pool of t
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