Lecture #2 – Evolution of Populations

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Lecture #2 Evolution of Populations. Image of a population of penguins. Key Concepts:. The Modern Synthesis Populations and the Gene Pool The Hardy-Weinberg Equilibrium Micro-evolution Sources of Genetic Variation Natural Selection Preservation of Genetic Variation. Review definitions. - PowerPoint PPT Presentation

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1Lecture #2 Evolution of PopulationsImage of a population of penguins2Key Concepts:The Modern SynthesisPopulations and the Gene PoolThe Hardy-Weinberg EquilibriumMicro-evolutionSources of Genetic VariationNatural SelectionPreservation of Genetic Variation3Images species, population, communityReview definitionsSpecies individual organisms capable of mating and producing fertile offspring Population a group of individuals of a single speciesCommunity a group of individuals of different species4The Modern Synthesisintegrates our knowledge about evolutionDarwins natural selectionMendels hereditary patternsParticulate transfer (chromosomes)Structure of the DNA moleculeAll explain how the genetic structure of populations changes over time5KEY POINT

Environmental factors act on the individual to control the genetic future of the population

Individuals dont evolve..populations do* * * * * * * * * * * * * * * * * * * * *** * * * * * *

5The environment is the wall; natural selection is the gate6Image population of irisPopulation = a +/- localized group of individuals of one species7Critical ThinkingHow do we determine the boundaries of a population???8Critical ThinkingHow do we determine the boundaries of a population???Boundaries are scale dependentSome sub-populations overlapSome are more isolatedWe can look at populations at many different scales micro to meta9Recall basic genetic principles:In a diploid species (most are), every individual has two copies of every geneOne copy came from each parentMost genes have different versions = allelesDiploid individuals are either heterozygous or homozygous for each geneHeterozygous = AaHomozygous = AA or aa10Recall basic genetic principles:The total number of alleles for any gene in a population is the number of individuals in the population x 2If the population has 10 individuals, there are 20 copies of the A gene some A alleles and some a allelesAll these alleles comprise the gene pool11Hardy-Weinberg TheoremGene pool = all alleles in a populationAll alleles have a frequency in the populationThere is a percentage of A and a percentage of a that adds up to 100%Hardy-Weinberg Theorem demonstrates that allele frequencies dont change through meiosis and fertilization alone12Hardy-Weinberg TheoremA simple, mathematical modelShows that repeated random meiosis and fertilization events alone will not change the distribution of alleles in a populationEven over many generationsp2 + 2pq + q2 = 1

we will not focus on the math youll work on this in labHands OnThe equation: p2 + 2pq + q2 = 1 (Page 2)p = the frequency of one alleleq = the frequency of the other allelep + q MUST = 1 = 100% of the gene pool

13Hands OnIf the allele frequencies are known, the HW equilibrium can be demonstrated by a Punnett square

14Assume that we know there are T and t allelesMaternal ParentTt

Paternal Parent

T

TTTtt

TtttHands OnWhat are the frequencies of each allele in the F1 generation?15Assume that we know there are T and t allelesMaternal ParentTt

Paternal Parent

T

TTTtt

TtttHands On ResultsWhat are the frequencies of each allele in the F1 generation?16Assume that we know there are T and t allelesMaternal ParentTt

Paternal Parent

T

TT.5x.5=.25Tt.5x.5=.25t

Tt.5x.5=.25tt.5x.5=.25Hands On Results17p2 + 2pq + q2 = 1.52 + 2x.5x.5 + .52 = 125% TT : 50% Tt : 25% tt50% T and 50% t = no changeAssume that we know there are T and t allelesMaternal ParentTt

Paternal Parent

T

TT.5x.5=.25Tt.5x.5=.25t

Tt.5x.5=.25tt.5x.5=.2517Hands On Results18p2 + 2pq + q2 = 125% TT : 50% Tt : 25% tt50% T and 50% t = no changeThis goes on generation after generationThe phenotype remains 75% dominant and 25% recessiveAssume that we know there are T and t allelesMaternal ParentTt

Paternal Parent

T

TT.5x.5=.25Tt.5x.5=.25t

Tt.5x.5=.25tt.5x.5=.25Hands OnHow do you determine the allele frequencies???How do you find p and q???In this example, how do you know the percentage of T and the percentage of t???19Hands On ResultsRemember that the recessive phenotype is ttIf you know the percentage of the population that expresses the recessive phenotype, then t (q) is the square root of that number20p2 + 2pq + q2 = 125% TT : 50% Tt : 25% tt75% express dominant; 25% express recessive.25 = .5; determine p by subtractionHands OnClasp your hands21

Hands OnCount right thumbs up vs. left thumbs upRight thumb up is the recessive condition!Determine the distribution of T and t in our class populationType up a summary of your results and turn in tomorrow22Hands OnCount tasters and non-tasters in our class using PTC paperDetermine the distribution of T and t in our class populationType up a summary of your results and turn in tomorrow2324Hardy-Weinberg TheoremMeiosis and fertilization randomly shuffle alleles, but they don't change proportionsLike repeatedly shuffling a deck of cardsThe laws of probability determine that the proportion of alleles will not change from generation to generationThis stable distribution of alleles is the Hardy-Weinberg equilibriumDoesnt happen in nature!!!25Conditions for H-W Equilibrium:No natural selectionLarge population sizeIsolated populationRandom matingNo mutation

Doesnt happen in nature!!!The violation of each assumption acts as an agent of microevolution26The value of H-W???It provides a null hypothesis to compare to what actually happens in natureAllele frequencies DO change in natureBUT, they change only under the conditions of microevolutionIn nature, all the H-W assumptions are violatedResult populations DO evolve

27Critical ThinkingWhat are the limitations of the Hardy-Weinberg theorem???28Critical ThinkingWhat are the limitations of the Hardy-Weinberg theorem???The H-W model considers just one trait at a time, and assumes that just one gene with 2 alleles (one completely dominant) controls that traitRecall your basic genetics is this realistic???29Critical ThinkingReality is much more complex for most traits in most organismsIncomplete dominance or codominanceMore than 2 alleles for many genesPleiotropy one gene affects multiple traitsPolygenic traits multiple genes affect one traitEpistasis one gene affects expression of another geneEnvironmental effects on phenotypic expressionReproductive success depends on the way all genes and phenotypic traits interact30Individuals Do Not EvolveIndividuals vary, but populations evolveNatural selection pressures make an individual more or less likely to survive and reproduceBut, it is the cumulative effects of selection on the genetic makeup of the whole population that results in changes to the speciesThe environment is a wall; natural selection is a gate31The environment is the wall; natural selection is the gate* * * * * * * * * * * * * * * * * * * * *** * * * * * *

***** *****

?32Image natural variation in flower color; same image for all these summary slidesMicro-evolution:population-scale changes in allele frequenciesNatural SelectionGenetic DriftGene FlowSelective MatingMutation33Cartoon beaver with chainsaw paws natural selection does not grant organisms what they needNatural Selection the essence of Darwins theoryMore on this later.More on this later.Differential reproductive success is the only way to account for the accumulation of favorable traits in a population34Micro-evolution:population-scale changes in allele frequenciesNatural SelectionGenetic DriftGene FlowSelective MatingMutation35Reproductive events are samples of the parent populationGenetic Drift random changes in allele frequency from generation to generation

Larger pop = ~29% blueSmaller pop = 100% blue

Parent pop = 10% blueLarger samples are more representative than smaller samples (probability theory)36Genetic Drift random changes in allele frequency from generation to generationMore pronounced in smaller and/or more segregated populationsBottleneck effectFounder effect

Segregated pop = ~29% blueSegregated pop = 100% blue

Parent pop = 10% blue36Drift always influences evolution of populations = violation of large pop size in H-W; greater affect with smaller popsBottlenecking = extreme genetic drift37Diagram bottlenecking38Critical ThinkingWhat events could cause a bottleneck???39Critical ThinkingWhat events could cause a bottleneck??? Bottlenecks occur when there is an extreme and indiscriminate reduction in the reproducing populationDiseaseHerbivoryMalnutritionMajor disturbance (flood, fire)Human intervention40Image cheetahConservation implications cheetahs are a bottlenecked species41Maps historic and current range of cheetahsExtreme range reduction due to habitat destruction and poaching+Cheetahs were naturally bottlenecked about 10,000 years ago by the last major ice age (kinked tail)

The species is at risk of extinction41One web site says that all cheetahs are more genetically similar than identical twins!42Images bottlenecked and now endangered speciesAustralian Flame Robin, California Condor, Mauritian Kestrel..and many more, all driven nearly to extinction..Some colorful results of a quick web search on bottlenecked species43Founder Effect = extreme genetic driftOccurs when a single individual, or small group of individuals, breaks off from a larger population to colonize a new habitatIslandsOther side of mountainOther side of a riverThis small group may not represent the allele distribution of the parent population 44

Founder Effect45

46

47Image a founding population of seeds; possibly also the bird if its a gravid femaleLong distance dispersal events can lead to the founder effect48Critical ThinkingWhat do you think follows long distance dispersal to a new