Presentation on theme: "Option D: Evolution D4: The Hardy- Weinberg Principle."— Presentation transcript:
1 Option D:EvolutionD4: The Hardy-WeinbergPrinciple
2 Population Genetics = Foundation for studying evolution D 4.1 Explain how the Hardy-Weinberg equation is derived.Population Genetics = Foundation for studying evolutionDarwin’s could not explain how inherited variations are maintained in populations - not “trait blending”A few years after Darwin’s “Origin of Species”, Gregor Mendel proposed his hypothesis of inheritance: Parents pass on discrete heritable units (genes) that retain their identities in offspring
3 Hardy-Weinberg Theorem: D 4.1 Explain how the Hardy-Weinberg equation is derived.Hardy-Weinberg Theorem:Frequencies of alleles & genotypes in a population’s gene pool remain constant from generation to generation unless acted upon by agents other than sexual recombination (gene shuffling in meiosis)Equilibrium = allele and genotype frequencies remain constant
4 Hardy-Weinberg Theorem: D 4.1 Explain how the Hardy-Weinberg equation is derived.Hardy-Weinberg Theorem:Hypothetical, non-evolving populationpreserves allele frequenciesServes as a model (null hypothesis)natural populations rarely in H-W equilibriumuseful model to measure if forces are acting on a populationmeasuring evolutionary changeG.H. Hardy (the English mathematician) and W. Weinberg (the German physician) independently worked out the mathematical basis of population genetics in Their formula predicts the expected genotype frequencies using the allele frequencies in a diploid Mendelian population. They were concerned with questions like "what happens to the frequencies of alleles in a population over time?" and "would you expect to see alleles disappear or become more frequent over time?"G.H. HardymathematicianW. Weinbergphysician
5 Hardy-Weinberg theorem D 4.1 Explain how the Hardy-Weinberg equation is derived.Hardy-Weinberg theoremCounting Allelesassume 2 alleles = B, bfrequency of dominant allele (B) = pfrequency of recessive allele (b) = qfrequencies must add to 1 (100%), so:p + q = 1BBBbbb
6 Hardy-Weinberg theorem BBBbbbD 4.1 Explain how the Hardy-Weinberg equation is derived.Hardy-Weinberg theoremCounting Individualsfrequency of homozygous dominant: p x p = p2frequency of homozygous recessive: q x q = q2frequency of heterozygotes: (p x q) + (q x p) = 2pqfrequencies of all individuals must add to 1 (100%), so:p2 + 2pq + q2 = 1
7 Hardy-Weinberg theorem D 4.1 Explain how the Hardy-Weinberg equation is derived.Hardy-Weinberg theoremAlleles: p + q = 1Individuals: p2 + 2pq + q2 = 1BbBBBbbbBBBbbb
8 q2 (bb): 16/100 = .16 q (b): √.16 = 0.4 p (B): 1 - 0.4 = 0.6 D 4.2 Calculate allele, genotype and phenotype frequencies for two alleles of a gene, using the Hardy-Weinberg equation.population: 100 cats84 black, 16 whiteHow many of each genotype?q2 (bb): 16/100 = .16q (b): √.16 = 0.4p (B): = 0.6p2=.362pq=.48q2=.16BBBbbbMust assume population is in H-W equilibrium!What are the genotype frequencies?
9 Assuming H-W equilibrium D 4.2 Calculate allele, genotype and phenotype frequencies for two alleles of a gene, using the Hardy-Weinberg equation.p2=.362pq=.48q2=.16Assuming H-W equilibriumBBBbbbNull hypothesisp2=.20p2=.742pq=.642pq=.10q2=.16q2=.16Sampled data 1:Hybrids are in some way weaker.Immigration in from an external population that is predomiantly homozygous BNon-random mating... white cats tend to mate with white cats and black cats tend to mate with black cats.Sampled data 2:Heterozygote advantage.What’s preventing this population from being in equilibrium.bbBbBBSampled dataHow do you explain the data?How do you explain the data?
10 to verify that the PRESENT population is in genetic equilibrium D 4.2 Calculate allele, genotype and phenotype frequencies for two alleles of a gene, using the Hardy-Weinberg equation.Using the calculated gene frequency to predict the EXPECTED genotypic frequencies in the NEXT generationORto verify that the PRESENT population is in genetic equilibrium
11 Assuming all the individuals mate randomly D 4.2 Calculate allele, genotype and phenotype frequencies for two alleles of a gene, using the Hardy-Weinberg equation.Assuming all the individuals mate randomlySPERMSBAB 0.43A 0.57EGGSAAABp*p= p2p*qABBBq*q= q2p*q
12 Close enough for us to assume genetic equilibrium D 4.2 Calculate allele, genotype and phenotype frequencies for two alleles of a gene, using the Hardy-Weinberg equation.Close enough for us to assume geneticequilibriumGenotypesExpected frequenciesObserved frequenciesAAp2 = 0.32233 747 = 0.31AB2pq =0.50385 747 = 0.52BBq2 =0.18129 747 = 0.17
13 Application of H-W principle Sickle cell anemiainherit a mutation in gene coding for hemoglobinoxygen-carrying blood proteinrecessive allele = HsHsnormal allele = Hblow oxygen levels causes RBC to sicklebreakdown of RBCclogging small blood vesselsdamage to organsoften lethal
14 Sickle cell frequency High frequency of heterozygotes 1 in 5 in Central Africans = HbHsunusual for allele with severe detrimental effects in homozygotes1 in 100 = HsHsusually die before reproductive ageSickle Cell:In tropical Africa, where malaria is common, the sickle-cell allele is both an advantage & disadvantage. Reduces infection by malaria parasite.Cystic fibrosis:Cystic fibrosis carriers are thought to be more resistant to cholera:1:25, or 4% of Caucasians are carriers CcWhy is the Hs allele maintained at such high levels in African populations?Suggests some selective advantage of being heterozygous…
15 Single-celled eukaryote parasite (Plasmodium) spends part of its life cycle in red blood cells Malaria123
16 Heterozygote Advantage In tropical Africa, where malaria is common:homozygous dominant (normal)die or reduced reproduction from malaria: HbHbhomozygous recessivedie or reduced reproduction from sickle cell anemia: HsHsheterozygote carriers are relatively free of both: HbHssurvive & reproduce more, more common in populationHypothesis:In malaria-infected cells, the O2 level is lowered enough to cause sickling which kills the cell & destroys the parasite.Frequency of sickle cell allele & distribution of malaria
17 Conditions for Hardy-Weinberg Equilibrium: D 4.3 State the assumptions made when the Hardy-Weinberg equation is used.Conditions for Hardy-Weinberg Equilibrium:Hardy-Weinberg Theorem describes a non-evolvingpopulation.Extremely large population size (no genetic drift).No gene flow (isolation from other populations).No mutations.Random mating (no sexual selection).No natural selection.
18 D 4.3 State the assumptions made when the Hardy-Weinberg equation is used. If any of the Hardy-Weinberg conditions are not met microevolution occursMicroevolution = generation to generation change in a population’s allele frequencies
19 Main Causes of Microevolution MutationGene FlowNon-random matingGenetic DriftSelection
20 Not every mutation has a visible effect. 1. Mutation & VariationMutation creates variationnew mutations are constantly appearingMutation changes DNA sequencechanges amino acid sequence?changes protein?changes structure?changes function?changes in protein may change phenotype & therefore change fitnessEvery individual has hundreds of mutations1 in 100,000 bases copied3 billion bases in human genomeBut most happen in introns, spacers, junk of various kindNot every mutation has a visible effect.Some effects on subtle.May just affect rate of expression of a gene.
21 2. Gene Flow Movement of individuals & alleles in & out of populations seed & pollen distribution by wind & insectmigration of animalssub-populations may have different allele frequenciescauses genetic mixing across regionsreduce differences between populations
22 Human evolution today Are we moving towards a blended world? Gene flow in human populations is increasing todaytransferring alleles between populationsAre we moving towards a blended world?
24 4. Genetic drift Effect of chance events founder effect bottleneck small group splinters off & starts a new colonybottlenecksome factor (disaster) reduces population to small number & then population recovers & expands againWarblerfinchTree finchesGround finches1 family has a lot of children & grandchildren therefore has a greater impact on the genes in the population than other familiesGenghis Khan tracked through Y chromosome.
25 Founder effectWhen a new population is started by only a few individualssome rare alleles may be at high frequency; others may be missingskew the gene pool of new populationhuman populations that started from small group of colonistsexample: colonization of New WorldSmall founder group, less genetic diversity than AfricansAll white people around the world are descended from a small group of ancestors100,000 years ago(Chinese are white people!)
26 Bottleneck effectWhen large population is drastically reduced by a disasterfamine, natural disaster, loss of habitat…loss of variation by chance eventalleles lost from gene poolnot due to fitnessnarrows the gene pool
27 Cheetahs All cheetahs share a small number of alleles 2 bottlenecks less than 1% diversityas if all cheetahs are identical twins2 bottlenecks10,000 years agoIce Agelast 100 yearspoaching & loss of habitat
28 Conservation issuesPeregrine FalconBottlenecking is an important concept in conservation biology of endangered speciesloss of alleles from gene poolreduces variationreduces adaptabilityBreeding programs must consciously outcrossGolden Lion Tamarin
29 5. Natural selectionDifferential survival & reproduction due to changing environmental conditionsclimate changefood source availabilitypredators, parasites, diseasestoxinscombinations of alleles that provide “fitness” increase in the populationadaptive evolutionary change