Chapter 23: The Evolution of Populations

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Chapter 23: The Evolution of Populations. Essential Knowledge. 1.a.1 Natural selection is a major mechanism of evolution (23.2). 1.a.2 Natural selection acts on phenotypic variations in populations (23.1 & 23.4). 1.a.3 Evolutionary change is also driven by random processes (23.3). - PowerPoint PPT Presentation



Chapter 23: The Evolution of PopulationsEssential Knowledge1.a.1 Natural selection is a major mechanism of evolution (23.2).1.a.2 Natural selection acts on phenotypic variations in populations (23.1 & 23.4).1.a.3 Evolutionary change is also driven by random processes (23.3). 2.c.1 Changes in genotype can result in changes in phenotype (23.4).4.c.3 The level of variation in a population affects population dynamics (23.1 23.3). 4.c.4 The diversity of species within an ecosystem may influence the stability of the ecosystem (23.2).Question?Is the unit of evolution the individual or the population?Answer while evolution affects individuals, it can only be tracked through time by looking at populations.So what do we study?We need to study populations, not individuals.We need a method to track the changes in populations over time. This is the area of Biology called population genetics.Population GeneticsThe study of genetic variation in populations.How do populations change, genetically, over time?Represents the reconciliation of Mendelism and Darwinism.PopulationA localized group of individuals of the same species.Must produce viable offspring

SpeciesA group of similar organisms.A group of populations that could interbreed (successfully)Populations are animals of the same species that are isolated due to geographyGene PoolThe total aggregate of genes in a population.All alleles at all gene loci in all individualsIf evolution is occurring, then changes must occur in the gene pool of the population over time.MicroevolutionChanges in the relative frequencies of alleles in the gene pool.Micro = smallMicroevolution is how we study evolution at the genetics levelHardy-Weinberg TheoremDeveloped in 1908. Use as a benchmark to study evolutionary change in a populationMathematical model of gene pool changes over time.H-W TheoremStates: The frequencies of alleles and genotypes in a populations gene pool remain constant (in a population that is NOT evolving)Basic Equationp + q = 1p = %/frequency of dominant alleleq = %/frequency of recessive alleleExpanded Equationp + q = 1(p + q)2 = (1)2p2 + 2pq + q2 = 1We expand the equation to fit all three types of genotypes (Ex: AA, Aa, aa)Genotypesp2 = Homozygous Dominant frequency2pq = Heterozygous frequencyq2 = Homozygous Recessive frequencyExample CalculationLets look at a population where: A = red flowers a = white flowers

Starting PopulationN = 500Red = 480 (320 AA+ 160 Aa)White = 20Total Genes/Alleles = 2* x 500 = 1000

*2 alleles per genotype (hence the 2 in the equation)

Dominant AlleleA = (320 x 2) + (160 x 1) = 800 = 800/1000 = 0.8 = 80%

320 = AA pop # (2 = # of dominant alleles in that AA genotype);160 = Aa pop # (1 = # of dominant alleles in Aa genotype);1000 = total genes

2 = # of times the dom allele is present in homozy dom genotype1 = # of times the dom allele is present in heterozy genotypeRecessive Allelea = (160 x 1) + (20 x 2) = 200 = 200/1000 = .20 = 20%

20 = aa pop # (2 = # of recessive alleles in that aa/white genotype);160 = Aa pop # (1 = # of recessive alleles in Aa genotype);1000 = total genes

1 = # of times the rec allele is present in heterozy genotype2 = # of times the rec allele is present in homozy rec genotypeImportance of Hardy-WeinbergYardstick to measure rates of evolution.Predicts that gene frequencies should NOT change over time as long as the H-W assumptions hold.Way to calculate gene frequencies through time.Example What is the frequency of the PKU allele?PKU is expressed only if the individual is homozygous recessive (aa).PKU FrequencyPKU is found at the rate of 1/10,000 births.PKU = aa = q2 q2 = .0001 q = .01 (frequency of recessive alleles)Dominant Allelep + q = 1 p = 1- q p = 1- .01 p = .99Expanded Equationp2 + 2pq + q2 = 1(.99)2 + 2(.99x.01) + (.01)2 = 1.9801 + .0198 + .0001 = 1Freq of Homozy Dom genotypeFreq of Heterozy genotypeFreq of Homozy Rec genotypeFinal ResultsAll we did is convert the frequencies (decimals) to % (by multiplying frequencies by 100%)

Normals (AA) = 98.01%Carriers (Aa) = 1.98%PKU (aa) = .01%AP Problems Using Hardy-WeinbergSolve for q2 (% of total)Solve for q (equation)Solve for p (1- q)H-W is always on the national AP Bio examHardy-Weinberg Assumptions1. Large Population2. Isolation3. No Net Mutations4. Random Mating5. No Natural SelectionIf H-W assumptions hold true:The gene frequencies will not change over time.Evolution will not occur.How likely will natural populations hold to the H-W assumptions?

MicroevolutionCaused by violations of the 5 H-W assumptions.Causes of Microevolution1. Genetic Drift2. Gene Flow3. Mutations4. Nonrandom Mating5. Natural SelectionGenetic DriftChanges in the gene pool of a small population by chance.Types:1. Bottleneck Effect2. Founder's Effect

By Chance

Bottleneck EffectLoss of most of the population by disasters.Surviving population may have a different gene pool than the original population.Results: Some alleles lost, others are over-represented, genetic variety is decreased

ImportanceReduction of population size may reduce gene pool for evolution to work with. Ex: Cheetahs

Founder's EffectGenetic drift in a new colony that separates from a parent population.Ex: Old-Order AmishResults: Genetic variety reduced, some alleles increase while other lost

ImportanceVery common in islands and other groups that don't interbreed.Gene FlowMovement of genes in/out of a population.Ex:ImmigrationEmigrationResult: change in gene frequencyMutationsInherited changes in a gene.

ResultMay change gene frequencies (small population).Source of new alleles for selection.Often lost by genetic drift.Nonrandom MatingFailure to choose mates at random from the population.

CausesInbreeding within the same neighborhood.Assortative mating (like with like).ResultIncreases the number of homozygous loci.Does not in itself alter the overall gene frequencies in the population.Natural SelectionDifferential success in survival and reproduction.Result - Shifts in gene frequencies.CommentAs the environment changes, so does natural selection and gene frequencies.

ResultIf the environment is "patchy", the population may have many different local populations.Genetic Basis of Variation1. Discrete Characters Mendelian traits with clear phenotypes.2. Quantitative Characters Multigene traits with overlapping phenotypes.PolymorphismThe existence of several contrasting forms of the species in a population.Usually inherited as Discrete Characteristics.Examples Garter SnakesGaillardia

Human ExampleABO Blood GroupsMorphs = A, B, AB, OQuantitative CharactersAllow continuous variation in the population.Result Geographical VariationClines: a change along a geographical axis

Yarrow and Altitude

Sources of Genetic VariationMutations.Meiosis - recombination though sexual reproduction.Crossing-overRandom fertilizationCommentPopulation geneticists believe that ALL genes that persist in a population must have had a selective advantage at one time.Ex Sickle Cell and Malaria, Tay-Sachs and Tuberculosis

Fitness - DarwinianThe relative contribution an individual makes to the gene pool of the next generation.How likely is it that an organism will survive and reproduce in a given environment?Relative FitnessContribution of one genotype to the next generation (when compared to other genotypes)Rate of SelectionDiffers between dominant and recessive alleles.Selection pressure by the environment/nature.

Modes of Natural Selection1. Stabilizing2. Directional3. Diversifying4. SexualStabilizingSelection toward the average and against the extremes.Ex: birth weight in humansDirectional SelectionSelection toward one extreme.Ex: running speeds in race animalsEx. Galapagos Finch beak size and food source

Diversifying(Disruptive)Selection toward both extremes and against the norm.Ex: bill size in birds

CommentDiversifying Selection - can split a species into several new species if it continues for a long enough period of time and the populations dont interbreed.

Sexual Mate selectionMay not be adaptive to the environment, but increases reproduction success of the individual.ResultSexual dimorphism.Secondary sexual features for attracting mates.

CommentsFemales may drive sexual selection and dimorphism since they often "choose" the mate.

QuestionDoes evolution result in perfect organisms?No!?CompromisesChance occurrences

SummaryRecognize the modern synthesis Theory of Evolution.Identify and use the Hardy-Weinberg Theorem for population genetics.Identify the Hardy-Weinberg assumptions and how they affect evolution of populations.Recognize causes and examples of microevolution.Identify modes of natural selection.Recognize why evolution does not produce "perfect" organisms.