Chapter 23: The Evolution of Populations. Important Point:

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  • Chapter 23:The Evolution ofPopulations

  • Important Point:

  • Gene PoolsOne species, but members are more likely to mate within their herd than the otherA gene pool is the sum of alleles at all loci within a population

  • PolymorphismA polymorphism is more than one allele present at a given locus within a single population of organismsPopulation genetics is essentially the study of allele and genotype frequencies within populations of organisms

  • Mendel meets H.W.Recall Mendelian geneticsHardy-Weinberg Equilibrium means genotype frequencies stay the same

  • Hardy-Weinberg Theorem32020x2

  • Hardy-Weinberg Theorem

  • Hardy-Weinberg Theorem

  • Hardy-Weinberg TheoremNote sameThe triumph of Darwinism occurred with the Modern synthesis, the integration of the mechanics of Darwinian evolution with those of Mendelian genetics (1930s)

  • H.W. EquilibriumHardy-Weinberg means that both genotype and allele frequencies stay the same over time

  • H.-W. Frequencies (2 alleles)Note how genotype frequencies are 100% a function of previous-generation allele frequencies.This is precisely what the H.W. equation tells us.It is the default evolutionary assumption(i.e., no evolution is occurring)Calculated H.W. frequencies,1 locus, 2 allelesFixed allele

  • H.-W. AssumptionsTo assume Hardy-Weinberg equilibrium all of the following must be true:The population must be very large (no sampling error/genetic drift)There must be no net mutationThere must be no natural selection (though as we will see that this assumption can be temporarily suspended in the course of using the Hardy-Weinberg theorem)No migration between populationsRandom mating (equivalent to mixing all sperm and eggs in population into a common bucket to foster fertilization)In other words, no mechanisms that can affect genetic structurei.e., allele or genotype frequenciesmay be operating

  • Eggs & Milt (Sperm) in Buckethttp://wdfw.wa.gov/wildwatch/salmoncam/hatchery.html

  • Non-Random MatingAnything that interferes with the random mating between individuals is nonrandom mating Nonrandom mating results in deviations from a Hardy-Weinberg generation of genotypes from a given frequency of alleles

  • H.-W. EquilibriumIf no mechanisms that can affect genetic structure are operating, thenHardy-Weinberg genotype frequencies will be established in a single generationAnd these frequencies will persist indefinitely(I.e., so long as there are no mechanisms operating that can affect genetic structure)Remember that an organism can be homozygous for a given allele even if within the population is polymorphic (meaning that more than one allele exists)Indeed, three alleles can exist within a population, even if only at best two can exist within a single individual

  • Chalk discussion of H.W. theorem, including, especially, p2 + 2pq + q2 = 1

  • Solving H.-W. ProblemsWork with Decimals, not percentages, not fractions, not absolute numbersConvert Phenotypes to Genotypes, whenever you are given phenotype information you should be pondering (i) how can I convert phenotypes to genotypes? and (ii) how can I convert known phenotype frequencies to genotype frequencies?Convert Genotypes to Alleles, once you know genotype frequencies it should be trivial to convert to allele frequencies: dont let this step trip you upConvert Alleles to Genotypes, if you know allele frequencies, but not genotype frequencies, then chances are you will need to figure out the latterIncorporating Selection, usually selection only operates at the diploid stage make sure frequencies always add up to onePractice, Practice, Practice, Practice, Practice!

  • Working with DecimalsConvert percentages to decimals (I.e., by dividing by 100): 25% 0.25Convert fractions to decimals (I.e., by dividing by the denominator): 0.25Convert absolute numbers to decimals (I.e., by dividing number by total): 60/240 0.25Many a Hardy-Weinberg solution has been tripped up by not employing decimals, i.e., by not employing frequenciesE.g., 25% x 25% = 625%! (which is incorrect)E.g., 0.25 x 0.25 = 0.0625! (which is correct)Yes, 25/100 x 25/100 = 625/100/100 = 0.0625But isnt that absurdly complicating???

  • Phenotype GenotypePhenotype to Genotype conversions are going to depend on the genetics of your locusAlways in these problems genotypes will be diploidIf alleles have a dominance-recessive relationship, then the heterozygote will have the same phenotype as the dominant homozygoteTherefore, if the relationship is dominant-recessive you will know with certainty only the genotypes of recessive homozygotes If the relationship is codominant or incomplete dominant, however, then there will be a one-to-one mapping of genotype to phenotypeThat is, for the latter (& only for the latter) genotype frequencies will be the same as phenotype frequencies

  • Dominant GenotypesIf a population is in Hardy-Weinberg equilibrium then the frequency of all genotypes, even dominant genotypes, may be estimatedStart with the frequency of the recessive homozygote this equals q2q therefore is equal to the square root of the frequency of the recessive homozygotep, the frequency of the dominant allele, therefore (if 2 alleles) can be assumed to be equal to 1 qThe dominant homozygote therefore can be assumed to have a frequency of (1 q)2The heterozygote therefore can be assumed to have a frequency simply of 2*p*qAlways assume Hardy-Weinberg equilibrium unless you have a compelling reason not to

  • Genotype AlleleOnce you know genotype frequencies, going from genotype frequencies to allele frequencies is easyDont let it trip you up!There are two formulas one can use and which one you use depends on whether you are working with absolute numbers versus genotype frequenciesf(A) = [2*f(AA) + 1*f(Aa) + 0*f(aa)] / 2[note that 2 = 2*f(AA) + 2*f(Aa) + 2*f(aa) since all frequencies should add up to 1]Note that this is just a ratio of number of alleles of a one type to total number of alleles present in a populationAlternatively, with X= # AA, Y= # Aa, & Z= # aa:f(A) = (2*X + 1*Y + 0*Z) / 2*(X + Y + Z)Note also that f(A) = 1 f(a) (for 2 allele system)[for ABO (3-allele) system, f(IA) = 1 - f(IB) - f(i)]

  • Allele GenotypeGenotype frequencies can be estimated from allele frequenciesFirst, you must assume Hardy-Weinberg equilibriumThen simply calculate genotype frequencies from allelic frequencies using the Hardy-Weinberg theorem(recall that p and q are allele frequencies)If you had 70 A alleles and 120 a alleles, then what are the expected frequencies of AA, Aa, and aa?f(A) = 70 / (70 + 120) = 0.37 f(a) = 0.63f(AA) = 0.372 = 0.14; f(aa) = 0.632 = 0.40; f(Aa) = 2 * 0.37 * 0.63 = 0.47;Check your answer 0.14 + 0.40 + 0.47 = 1.01, which is pretty close to 1.0 (rounding error?)

  • Non-Darwinian EvolutionGenerally natural selection is the evolutionary force most closely associated with Darwinism (i.e., Darwinian evolution)Keep in mind, though, that selection cannot operate without genetic variationGenetic variation, in turn, ultimately is a consequence of mutationNon-Darwinian mechanisms generally are not adaptive and include:Genetic driftMutationMigrationNon-Random mating

  • Non-Darwinian EvolutionGenerally natural selection is the evolutionary force most closely associated with Darwinism (i.e., Darwinian evolution)Keep in mind, though, that selection cannot operate without geneticGenetic variation, in turn, ultimately is a consequence of mutationNon-Darwinian mechanisms generally are not adaptive and include:Genetic driftMutationMigrationNon-Random mating

  • Non-Random MatingRandom mating violates statistical independence, which would complicate our mathRecall the Rule of Multiplication from Chapter 14How do we determine the chance that two or more independent events will occur together in some specific combination? The solution is in computing the probability for each independent event, then multiplying these individual probabilities to obtain the overall probability of the two events occurring together. (p. 254 C & R, 2002)It is because matings are random that the odds, e.g., of one A allele (from mom) being paired with another A allele (from dad) is p * p or p2If matings were not random then the probability of the above pairing could be >p2 or
  • Non-Darwinian EvolutionGenerally natural selection is the evolutionary force most closely associated with Darwinism (i.e., Darwinian evolution)Keep in mind, though, that selection cannot operate without geneticGenetic variation, in turn, ultimately is a consequence of mutationNon-Darwinian mechanisms generally are not adaptive and include:Genetic driftMutationMigrationNon-Random mating

  • Sampling Error: Genetic DriftErrors get bigger (as fraction of sample) as samples get smaller!

  • Non-Darwinian EvolutionGenerally natural selection is the evolutionary force most closely associated with Darwinism (i.e., Darwinian evolution)Keep in mind, though, that selection cannot operate without genetic variationGenetic variation, in turn, ultimately is a consequence of mutationNon-Darwinian mechanisms generally are not adaptive and include:Genetic drift BottleneckMutationMigrationNon-Random mating

  • Sampling Error: BottleneckWhen a population is reduced in size randomly, sampling error results in the allele frequencies of the new population not likely matching what were the allele frequencies in the old population

  • Cheetah, Product of BottleneckThe longer a population remains at a reduced size, the greater the effect of genetic drift on allele frequency

  • Non-Darwinian EvolutionGenerally natural selection is the evolutionary force most closely associated with Darwinism (i.e., Darwinian evolution)Keep in mind, though, that selection cannot operate without genetic variationGenetic variation, in turn, ultimately is a consequence of mutationNon-Darwinian mechanisms generally are not adaptive and include:Genetic drift Founder effectMutationMigrationNon-Random mating

  • Sampling Error: Founder EffectNote that the alleles lost are not necessarily the same alleles as may have been lost due to natural selection Genetic drift is sampling errorNew population

  • Products of Genetic DriftIsolated populations by chance fixed different karyotypesA locus for which only a single allele exists for an entire gene pool is considered to be fixed, i.e., a fixed locus

  • Non-Darwinian EvolutionGenerally natural selection is the evolutionary force most closely associated with Darwinism (i.e., Darwinian evolution)Keep in mind, though, that selection cannot operate without genetic variationGenetic variation, in turn, ultimately is a consequence of mutationNon-Darwinian mechanisms generally are not adaptive and include:Genetic driftMutationMigrationNon-Random mating

  • Mutation & Neutral VariationNote change in allele frequencies

  • Mutation (1/2)Mutation (or, at least, net mutation) also automatically changes allele frequencyFor example, a mutation involves the conversion of one allele into another alleleTypically mutation does not play a big, direct role in changing allele frequency because mutation rates per locus tend to be lowHowever, indirectly mutation is absolutely essential to microevolutionary processes because all allelic variation ultimately has a mutational originMutations represent random changes in highly evolved (i.e., information laden) nucleotide sequences, so often give rise to losses in gene function (thus most mutations are recessive)

  • Mutation (2/2)"Organisms are the refined products of thousands of generations of past selection, and a random change is not likely to improve the genome any more than firing a gunshot blindly through the hood of a car is likely to improve engine performance.Every now and then, though, a mutational change is adaptive (and even less often, both adaptive and dominant or codominant), i.e., novel functions or novel expression of old functions"On rare occasions, however, a mutant allele may actually fit its bearer to the environment better and enhance the reproductive success of the individual. This is not especially likely in a stable environment, but becomes more probable when the environment is changing and mutations that were once selected against are now favorable under the new conditions." your text

  • Non-Darwinian EvolutionGenerally natural selection is the evolutionary force most closely associated with Darwinism (i.e., Darwinian evolution)Keep in mind, though, that selection cannot operate without genetic variationGenetic variation, in turn, ultimately is a consequence of mutationNon-Darwinian mechanisms generally are not adaptive and include:Genetic driftMutationMigrationNon-Random mating

  • Migration (Gene Flow)Migration (movement of individuals) makes allele frequencies become more similar

  • Non-Darwinian EvolutionGenerally natural selection is the evolutionary force most closely associated with Darwinism (i.e., Darwinian evolution)Keep in mind, though, that selection cannot operate without genetic variationGenetic variation, in turn, ultimately is a consequence of mutationNon-Darwinian mechanisms generally are not adaptive and include:Genetic driftMutationMigrationNon-Random mating

  • Natural Selection (1/2)Make sure that you understand thatNatural selection acts on phenotypesGenotypes underlie phenotypesAlleles underlie genotypesTherefore, natural selection ultimately acts on allele frequencies, though selection occurs through the filter of both phenotype and genotype"An organism exposes its phenotypeits physical traits, metabolism, physiology, and behaviornot its genotype, to the environment. Acting on phenotypes, selection indirectly adapts a population to its environment by increasing or maintaining favorable genotypes in the gene pool." your text

  • Natural Selection (2/2)Natural selection can act during the haploid or diploid stageThe effect of natural selection is to reduce (not to increase) the absolute number of genotypes or allelesThat is, mutation places alleles into a gene pool, other microevolutionary forces can serve to increase the frequency of the allele, but selection acts to selectively remove maladaptive alleles (mutation in, selection out)In the absence of natural selection an organism contributes x gametes to the next generation; in the presence of natural selection an organism contributes
  • Incorporating SelectionRecall, for example, that we are diploid, and assume that natural selection is acting only at the diploid stage

  • Chalk discussion of effect of natural selection on H.W. frequencies

  • Selection for Toxin ResistanceSeeds that drift onto mine tailings die unless they are genetically predisposed toward heavy-metal resistant"The modern synthesis emphasizes the importance of populations as the units of evolution, the central role of natural selection as the most important mechanism of evolution, and the idea of gradualism to explain how large changes can evolve as an accumulation of small changes occurring over long periods of time." your text

  • Darwinian FitnessDarwinian fitness is the contribution an individual makes to the gene pool of the next generation relative to the contributions of other individuals. p. 457, Campbell & Reece, 2002Darwinian fitness is the allelic contribution an individual makes to the next generationDarwinian fitness is a quantity equal to the average reproductive output associated with a given genotype The more likely an individual is to survive and reproduce (i.e., to contributes its alleles to the next generation), the higher that individual's Darwinian fitnessDarwinian fitness is often simply called fitnessPeople typically consider Darwinian fitness on a locus-by-locus basis

  • Relative FitnessIn a more quantitative approach to natural selection, population geneticists define relative fitness as the contribution of a genotype to the next generation compared to the contributions of alternative genotypes for the same locus The relative fitness of the most reproductively successful variants is set at 1 as a basis for comparison. pp. 458-459, Campbell & Reece, 2002Restatement: Typically the genotype with the highest Darwinian fitness is given a relative fitness of 1.0 All other genotypes, i.e., those with lower than the highest Darwinian fitness, then have relative fitness values of less than 1.0If one genotype produces on average 4 progeny per generation and another produces on average 1 progeny per generation, then what is the relative fitness of the latter genotype? The former?

  • Modes of Selection

  • Stabilizing SelectionStabilized populations tend to be reasonably well adapted to their environments Stabilizing selection eliminates phenotypic extremes within a population, thus increasing the frequency of genotypes underlying intermediate phenotypes

  • Directional SelectionDirectional selection is natural selection against only one phenotypic extremeDirectional selection is what people typically think of when they think of natural selection

  • Disruptive SelectionIn disruptive selection the intermediate is selected againstDisruptive selection can result in balanced polymorphisms

  • Sickle-Cell PrevalenceSelection by malaria exposure

  • Directional Selection (in macroevolution)Note: This example is Macroevolutionary, not Micro"Of all the causes of microevolution, only natural selection generally adapts a population to its environment. The other agents of microevolution are sometimes called non-Darwinian because of their usually non-adaptive nature." your text

  • Sexual Selection

  • Sexual SelectionSexual selection are forces that impact on mate procurementIf you dont mate, you dont make babiesMate procurement involves competing with same gender individuals (e.g., other males) and attracting other-gender individualsIntrasexual selection is a consequence of direct competition (e.g., fighting) with ones own genderIntersexual selection (mate choice) is competition for the other genders eyeHow these mechanisms operate can differ greatly from gender to genderBasically, for some species (e.g., us), procuring a mate can be a very complicated experience

  • Sexual Selection

  • Cost of Sex (Why bother?)

  • Sexual DimorphismAmmonite sexual dimorphismNyala sexual dimorphismIn sexual dimorphism, males and females differ phenotypically in addition to their possessing different sexual organs

  • Sexual Dimorphism (elephant seals)Hey, I was bottlenecked, too!

  • Genetic PolymorphismGenetic polymorphism is the presence of multiple alleles at a given locus within a gene poolIn general, there is a lot more genetic polymorphism in populations than meets the eyeThis in part is because of hidden recessive alleles, and also because different alleles do not necessarily give rise to different phenotypesHeritable variation within a population is synonymous with polymorphism Therefore, the raw material of natural selection are polymorphisms

  • Genetic Polymorphism

  • Balanced PolymorphismBalanced polymorphisms are stably maintained multiple alleles at a given locusHeterozygous advantage, a.k.a., balancing selectionE.g., Sickle cell anemia but otherwise probably not too importantHybrid Vigor a product of heterozygous advantage and the masking of deleterious allelesE.g., Hybrid corn, but can this maintain polymorphisms in the wild?Frequency-dependent selection selection for alleles because they are rare, e.g., Major Histocompatibility ComplexNeutral variation selection not strong enough to remove alleles (unless environment changes)There is more neutral variation in larger populations due reduced strength of genetic drift

  • Environmental Variation: A Cline

  • Temporal Phenotypic Variation

  • Why no Perfect Organisms?"An organism's phenotype is constrained by its evolutionary history"Adaptations are often compromises"Not all evolution is adaptiveIt takes too much energy to optimize everything so much of most organisms is simply good enough to get the job done (a.k.a., the principle of allocation) "Selection can only edit variations that existEven if a perfect organism existed, it would only remain perfect so long as its environment remained unchangedTo make matters worse, environments even change over single individual's life spans

  • The End

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