Biology 212 General Genetics Spring 2007Lecture 28: Population Genetics IIIReading: Chapter 14 pp. 513-520Lecture outline:1. Natural selection2. Heterozygote superiority3. Mutation 4. MigrationLectureIn the real world, Hardy Weinberg conditions are seldom met. Collectively, the forces of selection, mutation and migration lead to changes in allele frequencies and contribute to biodiversity. From today’s lecture, you should be able to predict the effects of various evolutionary forces on allele frequencies.1. Natural selectionDarwin’s theory of evolution by natural selection 1859- More offspring are produced than can survive - Individuals differ in their traits- Those individuals whose traits are best adapted survive and reproduces=selection coefficient; a measure of natural selection acting on individual genotypesw= fitness = 1-s; measure of ability to survive and reproduceExample: peppered moth in UKLight gray gg vs. black Gg Countryside polluted city- Light gray blends with the birch tree bark in the country- Black blends in with sooty environment2. Heterozygote superiority (balanced polymorphism)natural selection acting on each of the dominant and recessive genotypess = selection against homozygous dominantt = selection against homozygous recessive1example: sickle cell anemiaDefect in human protein resulting in sickle shaped red blood cellsHbAHbA HbAHbS HbSHbSnormal Sickle cell trait Sickle cell anemiaSusceptible to malaria Resistant to malaria Resistant to malariaEnvironment where HbS frequency is high: tropical climates, AfricaThere, heterozygotes have greater fitness than either of the homozygous phenotypesBoth alleles are maintained in the population = “balanced polymorphism”3. Mutation- Variation in gene structure is due to mutation- Causes of mutations: chemicals, radiation, such as X-rays or radioactive materialsMutation rates are generally low (10-5 or 10-6/gamete/generation)Therefore mutation barely alters gene frequencies μA --> a <-- νμ = rate of mutation from A to aν = rate of back mutation (a to A) also known as reversion4. MigrationWhen individuals move from one population to another and interbreed with individuals in the second population, the gene frequencies may be altered.First population Second populationAA AA m aa aa aaAa Aa aa AaAA Aa aa Aam= proportion of migrantsIn this example, the allele frequency for A will increase and for a will decrease as the rateof migration increases25. Genetic driftChanges in gene frequencies due to chance occurances. See Fig. 14.27 for an example. Less likely to exert effect on large population than on small population. A statistical phenomenon.6. Interplay of evolutionary forcesa. fixation of one allele, extinction of the other allelecommon for selection, migrationb. equilibrium is established, so both A and a are represented in the populationexamples: - forward mutation balanced by back mutation- selection balanced by mutation- balanced polymorphism ^ ^equilibrium allele frequencies are symbolized by p and