mechanisms of evolution: how evolution happens to populations

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Three female African Swallowtail Butterflies ( Papillio dardanus ) from the same population. Mechanisms of Evolution: How evolution happens to populations. Lecture Outline -African Swallowtail butterflies 1. Introductory terms and concepts 2. Darwin, Mendel and the Modern Synthesis - PowerPoint PPT Presentation


  • Lecture Outline-African Swallowtail butterflies1. Introductory terms and concepts2. Darwin, Mendel and the Modern Synthesis-Sexual reproduction and Mendelian genetics -Selection, fitness and adaptation -The modern synthesis3. Genetic Variation within Populations-

  • 1. Introductory terms and concepts

  • Concepts and TermsSpecies: A group of populations whose individuals have the potential to interbreed and produce fertile offspring in naturePopulation: Localized group of individuals belonging to the same speciesDeme: Locally interbreeding group within a populationGene Flow: Consequence of migration between populations, followed by breeding.

  • Genotype frequencies: BB 36% Bb 48% bb 16%In a population of 500 individuals, how many individuals have genotype:BB 500 (.36) = 180 individualsBb 500 (.48) = 240 individualsbb 500 (.16) = 80 individualsIn a population of 500 individuals, how many copies of the B allele are there?360 alleles from the BB individuals (all the alleles from BB cats) + 240 alleles from the Bb individuals (1/2 the alleles from Bb cats) = 600 copies of the B allele

  • Genotype frequencies: BB 36% Bb 48% bb 16%In a population of 500 individuals, how many individuals have genotype:BB 500 (.36) = 180 individualsBb 500 (.48) = 240 individualsbb 500 (.16) = 80 individualsIn a population of 500 individuals, how many copies of the B allele are there? 360 from the BB individuals + 140 from the Bb individuals = 500 copies of the B allele(500)(.36)+

  • Consider a population with 500 diploid cats and the single gene locus that controls fur color.How many copies of the gene for fur color are there? 1000Consider our genotype frequencies:BB 36% Bb 48% bb 16%In a population of 500 individuals, how many individuals are:BB 500 (.36) = 180 individualsBb 500 (.48) = 240 individualsbb 500 (.16) = 80 individualscopies of the B allele are there?(500)(.36)+

  • Review terms:Diploid organismHomologous chromosomesGene LocusAllele

  • 30 individuals in populationtotal of 60 alleles at locus under investigation, of three forms; X1 (n=12), X2 (n=30), X3 (n=18)

  • 2. Darwin, Mendel and the Modern Synthesis

  • R.A. FisherJ.B.S. HaldaneSeawall WrightSome architects of the modern synthesis. The contributions of population geneticists and researchers in other fields helped reconcile Mendelian theory of heredity with Darwins theory of natural selectionOur contemporary, comprehensive theory of evolution was forged in the 30s and 40s and came to be known as the modern synthesis -- synthetic in that it integrated contributions from many fields including population genetics, paleontology, systematics and biogeography.

    The work of population geneticists was particularly important in leading to the reconciliation of Medelism and Darwinism and to meaningful understanding of the genetic basis of variation and natural selection

    Especially important contributions from the fledgling field of Population Genetics were that populations typically harbor extensive genetic variation and the importance of quantitative characters for gradual evolutionary change

  • Ernst Mayr, on the right, on an ornithological expedition in New Guinea in 1928, with his Malay assistant

  • Fundamental Perspectives of the Modern Synthesis

    The units of Evolution: Populations are the fundamental units of evolution

    The mechanism of Evolution. Natural Selection is a particularly important mechanism of evolution in that it alone results in adaptive evolutionary change

    The tempo of evolution: Gradualism -- large change can and does evolve as an accumulation of small changes over long periods of time.Note -- well study a complementary view on the tempo of evolutionpunctuated equilibrium, in our discussions of speciation and macroevolution.

  • Relating concepts of Evolution, Selection and Fitness

    Natural Selection Differential success in the reproduction of different phenotypes resulting from interactions of organisms with their environment. Evolution Intergenerational change in genetic constitution of a population (narrow sense -- ie, microevolution) Adaptive Evolution involves random chance in the forms of mutation and sexual recombination and probabilistic sorting in terms of the sifting action of natural selection Individual Fitness Relative contribution of individual to gene pool of the next generation -- relative to contribution of other individuals in population (Alternatively can measure lifetime fitness lifetime contribution to the gene pool) Quantifying Individual Fitness; Relative scale from 0.0 to 1.0, where individual with greatest contribution in population has fitness of 1.0.

    Genotype Fitness Relative contribution of genotype at a given locus to the next generation relative to contribution of other genotypes in population

    Dont be concerned for now about how to genotype fitness yet, but make sure you understand the concept of genotype fitness.

  • 3. Genetic variation within populationsExamine the instantaneous genetic structure of a sexually reproducing diploid population.Allele frequenciesGenotype frequenciesExamine the intergenerational behavior of genetic structure in a sexually reproducing diploid population that is not evolvingHardy-Weinberg EquilibriumConsider the causes of microevolution (non-equilibrium)Natural selection MutationGenetic driftGene flowNon-Random mating

  • Determining the Genetic Structure of a Population (at one locus)Genotype FrequenciesAllele Frequencies

  • The genetic (allelic) structure of a population may remain relatively constant over time, ie stay in equilibrium alternatively..Allele frequencies may change intergenerationally; this is evolution, and one or more processes may cause that changeMicroevolution refers to evolutionary change within and among populations ie, within species. One or more of five processes may drive microevolution:mutation, selection, drift (chance), nonrandom mating, migration

  • Hardy-Weinberg Equilibrium TheoremMathematical theorem that shows that frequencies of alleles and genotypes in a population's gene pool remain constant over generations unless acted on by agents other than sexual recombinationRecombination of alleles, via gametes, due to meiosis and random fertilization has no effect on overall genetic structure of a populationProbability of two events co-occurring equals the probability of each event occurring independently

  • Assumptions of Hardy-Weinberg Equilibrium of Genetic Structure of Populations: No Mutation No Random changes in genetic materialNo Migration [Gene Flow]No Movement of individuals among populations and subsequent breedingNo Genetic Drift No Changes in genetic structure due to chance (dumb luck)No Non-Random Mating Gametes (eggs and sperm) are not combined in a completely random fashionNo Natural Selection Differential reproduction

  • p2 + 2 pq + q 2 = 1 Freq. Freq. Freqof AA of Aa of aa

  • Hardy-Weinberg Theorem -- Its Importance in the Study of EvolutionH-W is a model that defines a pattern in nature; the line of reasoning is grounded in mathematics -- probability theory and algebraH-W has predictive valueH-W tells us what genetic structure to expect if a population is not evolving or can tell us about the extent to which a population is evolving

  • Using the Hardy-Weinberg EquationFrequency of allele for an inherited disease; the recessive allele coding for PKU in the U.S. population (A=normal, a=pku)p2 + 2pq + q2 = 1Freq. AA Freq. Aa Freq. aa 100%

    PKU genotype = aa Carrier genotype = Aa Freq. of PKU genotype = q2 Freq. Carrier genotype = 2pqOccurrence of PKU = 1 in 10,000 births = .0001Freq. of q2 = .0001 Freq. of q = (.0001)1/2 = 0.01Freq. of p = 1 - q = 1 - .01 = .99Freq. of carriers (heterozyotes) = 2pq = (2)(.99)(.01) = .0198 (=~2%)

  • Allele frequencies may change intergenerationally; this is evolution, and one or more processes may cause that changemutation, selection, drift (chance), nonrandom mating, migration

  • Mutations are Agents of Evolutionary Change

    Mutations are heritable changes in genetic information; may occur in somatic or germ cells. The origin of genetic variation is germ-line mutations. Various forms of point mutation and chromosomal mutation occur. (note error in definition of point mutation on preview image on class website) Rate at which mutations occur vary among nucleotide sites, among loci, among lineages..Mutation events are relatively rare for most loci studied; on the order of one mutation per locus per million zygotes per generation is somewhat typicalRates of mutation at single loci are usually so low that mutations in and of themselves usually result in only a very small deviation from H-W equilibrium.

  • Of evolutionary importance are point mutations and other mutations in germ line cells

  • Thought Experiment: What is your expectation regarding the probability of getting heads when you flip a coin? Tails? Why? Flip a coin 4 times, record results. Flip a coin 4000 times, record results. What is your expectation for both sets of trials?Which set of trials would you expect to be closer to your expectation?Why?

    Genetic Drift Can Cause EvolutionGenetic Drift:Random change in genetic structure of a population; due to chance

  • Sampling Error and Genetic Drift Sampling Error. Chance events (random departures from expectations based on underlying probabilities) are more likely to occur in small populations than in lar