AGENTS OF EVOLUTIONPowerPoint PresentationSlide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15AGENTS OF EVOLUTIONI. Migration = Gene FlowA. Movement of genes from one population to another1. Causes deviations from HWE due to exchange of migrant alleles between populations- Change in allele frequencies in each population as a result of genetic exchangeB. Nature of gene flow1. Movement of individuals or their gametes between populations-zygotic gene flow: whole organism movesDiploid adults, juveniles, or zygotes can disperse-gametic gene flow: eggs or sperm moveHaploid gametes disperse. Pollen in plants or group spawning events in marine invertebrates.2. Integration of migrant alleles-results in a genetic mixing of genotypes between populations exchanging migrants3. Can introduce genetic variation at a much HIGHER rate than mutation-especially true if populations exchanging migrants are GENTICALLY SUBDIVIDED to begin with.C. Genetic consequences of migration1. Makes populations that exchange migrants more genetically SIMILAR-has a HOMOZYGOUS effect on genetic variation between populations -SLOWS DOWN the rate at which populations become genetically differentiated from one another -inhibits population subdivision-prevents speciationA2A2/B2B2A1A1/B1B1POP 1 POP 2A2A2/B1B1A1A1/B2B2POP 1 POP 2AGENTS OF EVOLUTIONII. Models of MigrationA. One-island model = unidirectional gene flowmm = migration rateB. Calculating allele frequency due to gene flow (one-way)1. Can calculate p´I = freq. of A1 in island population at generation tp´I = (1-m)(pI) + (m)(pC)Where pC = freq. of A1 in mainland (migrant) population pI = freq. of A1 in the island population in the previous generation2. Change in frequency of pI = ∆pI∆pI = m(pC - pI)Where:pI = frequency of A1 in existing population (island)pC = frequency of A1 in population where immigrants are coming from (mainland)∆p = change in frequency of pI after one generation of migrationm = coefficient of migration; proportion of migrant genes entering the population each generation3. Example:pI = 0.40 pC = 0.60Each generation the population receives 10% immigrants from the mainland and the island population is 100 individuals (10/100).So m =0.10; proportion of island population (Nl) that are migrants from main land. p´I = (1 - 0.10)(0.40) + (0.10)(0.60)pI = 0.40 p´I = 0.42∆p = 0.10(0.60 - 0.40) = 0.02Thus frequency of A1 increases in the island population 2% each generation until equilibrium is reached.AGENTS OF EVOLUTIONI. Interaction of Selection and MigrationA. Lake Erie Water Snakes (Nerodia sipedon)MORPH TYPESA1A1A1A2A2A2DC,BADC, DB, CA, BAAMainland--marsh Islands--rocky, open shoresMorph frequencies and DistributionbandedplainB. Are selection and migration balanced in this system?On islands, unbanded snakes (A2A2) are more fitw11 = 0.84 w12 = 0.84 w22 = 1.0more fitA2=q=0.73 A1 on mainland = 0.96Model in AlleleA1 with migration rate of no migration, then 1% then