Chapter 23: The Evolution of Populations
Post on 04-Jan-2016
DESCRIPTIONChapter 23: The Evolution of Populations. Question?. Is the unit of evolution the individual or the population ? Answer while evolution effects individuals, it can only be tracked through time by looking at populations. So what do we study?. - PowerPoint PPT Presentation
Chapter 23:The Evolution of Populations
Question?Is the unit of evolution the individual or the population?Answer while evolution effects 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.Represents the reconciliation of Mendelism and Darwinism.
Modern SynthesisUses population genetics as the means to track and study evolution.Looks at the genetic basis of variation and natural selection.
PopulationA localized group of individuals of the same species.
SpeciesA group of similar organisms.A group of populations that could interbreed.
Gene PoolThe total aggregate of genes in a population.If 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.
Hardy-Weinberg TheoremDeveloped in 1908. Mathematical model of gene pool changes over time.
Basic Equationp + q = 1p = % dominant alleleq = % recessive allele
Expanded Equationp + q = 1(p + q)2 = (1)2p2 + 2pq + q2 = 1
Genotypesp2 = Homozygous Dominants 2pq = Heterozygous q2 = Homozygous Recessives
Example CalculationLets look at a population where: A = red flowers a = white flowers
Starting PopulationN = 500Red = 480 (320 AA+ 160 Aa)White = 20Total Genes = 2 x 500 = 1000
Dominant AlleleA = (320 x 2) + (160 x 1) = 800 = 800/1000 A = 80%
Recessive Allelea = (160 x 1) + (20 x 2) = 200/1000 = .20 a = 20%
A and a in HW equationCross: Aa X AaResult = AA + 2Aa + aaRemember: A = p, a = q
Substitute the values for A and ap2 + 2pq + q2 = 1(.8)2 + 2(.8)(.2) + (.2)2 = 1.64 + .32 + .04 = 1
Dominant AlleleA = p2 + pq = .64 + .16 = .80 = 80%
Recessive Allelea = pq + q2 = .16 + .04 = .20 = 20%
ResultGene pool is in a state of equilibrium and has not changed because of sexual reproduction.No Evolution has occurred.
Importance of Hardy-WeinbergYardstick to measure rates of evolution.Predicts that gene frequencies should NOT change over time as long as the HW 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
Dominant Allelep + q = 1 p = 1- q p = 1- .01 p = .99
Expanded Equationp2 + 2pq + q2 = 1(.99)2 + 2(.99x.01) + (.01)2 = 1.9801 + .0198 + .0001 = 1
Final ResultsNormals (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 exam (but no calculators are allowed).
Hardy-Weinberg Assumptions1. Large Population2. Isolation3. No Net Mutations4. Random Mating5. No Natural Selection
If H-W assumptions hold true:The gene frequencies will not change over time.Evolution will not occur.But, 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 Selection
Genetic DriftChanges in the gene pool of a small population by chance.Types:1. Bottleneck Effect2. Founder's Effect
Bottleneck EffectLoss of most of the population by disasters.Surviving population may have a different gene pool than the original population.
ResultSome alleles lost.Other alleles are over-represented.Genetic variation usually lost.
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 Amish
ResultGenetic variation reduced.Some alleles increase in frequency while others are lost (as compared to the parent population).
ImportanceVery common in islands and other groups that don't interbreed.
Gene FlowMovement of genes in/out of a population.Ex:ImmigrationEmigration
ResultChanges in gene frequencies.
MutationsInherited 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, O
Quantitative CharactersAllow continuous variation in the population.Result Geographical VariationClines: a change along a geographical axis
Yarrow Plants and Altitude
Sources of Genetic VariationMutations.Recombination though sexual reproduction.Crossing-overRandom fertilization
Preserving Genetic Variation1. Diploidy - preserves recessives as heterozygotes.2. Balanced Polymorphisms - preservation of diversity by natural selection.
ExampleHeterozygote Advantage - When the heterozygote or hybrid survives better than the homozygotes. Also called Hybrid vigor.
ResultCan't bred "true and the diversity of the population is maintained.Ex Sickle Cell Anemia
CommentPopulation 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.
Relative FitnessContribution of one genotype to the next generation compared to other genotypes.
Rate of SelectionDiffers between dominant and recessive alleles.Selection pressure by the environment.
Modes of Natural Selection1. Stabilizing2. Directional3. Diversifying4. Sexual
StabilizingSelection toward the average and against the extremes.Ex: birth weight in humans
Directional SelectionSelection toward one extreme.Ex: running speeds in race animals.Ex. Galapagos Finch beak size and food source.
DiversifyingSelection 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?
Answer - No1. Historical Constraints2. Compromises3. Non-adaptive Evolution (chance)4. Available variations
SummaryKnow the difference between a species and a population.Know that the unit of evolution is the population and not the individual.
SummaryKnow the H-W equations and how to use them in calculations.Know the H-W assumptions and what happens if each is violated.
SummaryIdentify various means to introduce genetic variation into populations.Know the various types of natural selection.