FORCES OF EVOLUTIONPowerPoint PresentationSlide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18EVOLUTION IN SMALL POPULATIONSSlide 20Slide 21Slide 22Slide 23Slide 24Slide 25Inbreeding increases homozygositySlide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35FORCES OF EVOLUTIONI. Evolution in Small PopulationsA. Genetic Drift1. RANDOM change in allele frequencies across generations-change is NOT directed with respect to phenotype-small populations are more subject to chance events -certain alleles become more or less common by CHANCE alone2. Sampling effects in small populations-when population size is reduced some genetic variation is lost by chance-not all individuals in a population will reproduce every generationNe = effective number of individualsNe < N (census number)-certain genotypes go extinct solely due to chance samplingProbability ofp = 0.60 is small when N = 10Initial frequency of p = 0.60Sampling biasB. How small is genetically small?1. Population has a history of being smallFOUNDER EVENTS-small number of individuals started the populationIsland populations or populations at the periphery of species’ rangesA1 B1 C1 A2 B2 C2A1 C2Initial founders cannot carry all of the source population’s genetic diversityPOPULATION BOTTLENECKS-population size is drastically reduced due to some type of disease or natural disaster-remaining survivors do not carry all the allelic diversity of the original populationC. GENETIC DRIFT leads to FIXATION1. Change in allele frequencies due to drift-impossible to predict within a single generation or population-because change is RANDOM2. What can be predicted?-effects of genetic drift among a group of DEMESWhat happens to genetic variation in a group of demes? All start at same p0All start at same NAll experience driftThen you can estimate.3. All demes eventually fix A1 or A2lose genetic variationp --> 0 or p --> 1A1A1A2A2A24. Frequency of heterozygotes declines with decreasing population size.5. Which allele will fix? Fraction of demes fixing A1 = p0 = initial freq. of A1tracking demesN = 40 t = 0 t = 20 t = 8075% 25% 01Initially, 100% of demes have p0 = 0.75After fixation, 75% of demes: p = 1 (fix A1 ) 25% of demes: p = 0 (fix A2 )100% 0.756. RATE Of fixation is proportional toSmaller demes fix FASTERLarger demes fix SLOWER1 2N1 2N = 1/81 2N = 1/801 2N = 1/800D. Overall effects of genetic drift1. Changes the DISTRIBUTION of genetic information2. Variation within demesBECOMESVariation among demesp0 = 0.75p = 0 or p = 13. Promotes population subdivision and can result in speciation-many island species may have speciated due to driftHawaiian DrosophilaEVOLUTION IN SMALL POPULATIONSII. Interaction of selection and driftA. Overall response1. Selection can fail in small populations-chance events can fix an unfavored alleleOr cause a favored allele to go extinct2. Outcome is often unpredictable-even when selection succeeds in increasing the frequency of favored allele, it may never reach fixation due to chance deviations caused by drift.B. When does selection fail?1. Population size is sufficiently smallStrong genetic drift2. Selection is sufficiently weakCannot overcome the action of driftC. Leads to EFFECTIVE NEUTRALITY-interaction between population size and magnitude of selection is such that. Selective alleles become effectively neutral when the population is small.1. Relationship between natural selection and drifts > 1/NeSelection Ruless < 1/Ne Drift Rules Example:If Ne = 100 thenEVOLUTION IN SMALL POPULATIONSIII. Interaction of drift with gene flowA. Drift-migration equilibrium1. Balance between loss of alleles due to genetic drift and introduction of alleles from migrants2. Corridors for dispersal critical in conservation management of small populations. ptA. Drift-migration equilibrium1. Gene flow prevents extinction of small populations2. Corridors for dispersal critical in conservation management of small populationsEVOLUTION IN SMALL POPULATIONSIV. Nonrandom MatingA. Inbreeding1. Process of consanguineous (1st cousin or closer) mating with a close relative -mating with a close relative (first cousin or closer)-most extreme form of inbreeding is selfingmate with yourself--common in some plant speciesB. Genetic Consequences of Inbreeding(changes genotype frequency) (technically not evolution)1. increased HOMOZYGOSITY-deviation from HWE--too few heterozygotes**-exposes deleterious recessive alleles in homozygotes. When you put 2 recessive alleles together, bad things happen.Inbreeding increases homozygosityA1* A2 A1 A2 A1* A1 A1* A2 A1* A1* SAME allele can bepassed from bothparentsIdentical in state if they have the same nucleotide sequenceIdentical by descent if they are copies of the same allele from a common ancestorI by DI in SA Genotype is:Homozygous if it has two alleles identical in stateAutozygous if it has two alleles identical by descentInbreeding increases homozygosity by adding more autozygous individuals to populationC. Measuring Inbreeding1. Fixation index or coefficient of inbreedingF = inbreeding coefficient = probability a random individual is autozygous = probability the two alleles are I by dF ranges from 0 to 1 F = 0 -- no inbreeding (HW)F = 1 -- extreme inbreeding (selfing plants)2. Genotype frequencies with inbreedingP = p2(1-F) + pFH = 2pq(1-F)Q = q2(1-F) +qFF = 0 --> H-W frequenciesF = 1 --> P = p, H = 0, Q = qInbreeding does not change p and q, only P, H, and Q3. Estimating F from an inbred populationHobs = 2 p q - 2 F p q = 2 p q (1 - F)Solve for F to getHobs2 p qF = 1 -D. Phenotypic consequences of inbreeding1. inbreeding depression: decline in average fitness (w) of a population due to exposure of deleterious recessive alleles in autozygous state2. populations can “breed through” inbreeding depression and purge all the deleterious recessives-inbreeding is not always bad-can be good if you live in a constant environment, i.e., selfing in many plant species3. When does inbreeding occur?1) Small populations-founder events--individuals in a population are all descended from a few founders-population bottlenecks--population size crashes due to disaster or diseaseIndividuals that survive must repeatedly mate with close relatives to rebuild population numbers2) Limited dispersal capabilities-individuals only disperse a short distance from where they were born-close relatives remain