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Evolution of Populations Marie ern Slide 2 Microevolution Genetic drift in small population Gene flow by migration Mutation Nonrandom mating Natural selection Slide 3 Genetic Drift Slide 4 Genetic Drift - small populations Slide 5 Genetic Drift - large populations Slide 6 Genetic Drift - Bottle neck effect Slide 7 northern elephant seals of the coast of Mexico During the 1890s, their population was reduced due to hunting to about 20 individuals. (It is also likely that one male would have fathered the offspring of the entire group.) Slide 8 Genetic Drift - Founder principle Drosophila subobscura (fruit fly) European American Slide 9 Genetic Drift - Founder principle Slide 10 Gene flow - migration Slide 11 Mutation Slide 12 Nonrandom mating inbreeding assortative mating Slide 13 Natural selection the differential ability to survive and produce viable, fertile offspring is likely to adapt a population to its environment (accumulates and maintains favorable genotypes) Slide 14 Genetic variation: the substrate for natural selection within populations (polymorphism) between populations (geographical variation) Slide 15 Polymorphism Slide 16 Geographic variation Slide 17 Geographic variation - Cline Slide 18 Sources of Genetic variation Mutation (affection of function) Sexual recombination Slide 19 Crossing over in meiosis Slide 20 Independent assortment Slide 21 Preservation of Genetic variation Diploidy (heterozygote protection) Balanced polymorphism Slide 22 Heterozygote advantage Slide 23 Frequency-dependent selection Slide 24 Modes of natural selection Slide 25 Directional selection is most common - during periods of environmental change - when members of a population migrate to some new habitat with different environmental conditions Slide 26 Directional selection peccaries Desert cactus population Slide 27 Diversifying selection Slide 28 a species of finch lives in Cameroon, West Africa small-billed birds (soft seeds) large-billed birds (hard seeds) Slide 29 Stabilizing selection This mode of selection reduces variation and maintains the status quo for a particular phenotypic character. It keeps the majority of human birth weights In the 3 4 kg range. For babies much smaller or larger than this, infant mortality is greater. Slide 30 Stabilizing selection Desert cactus population Slide 31 Literature Biology, eighth edition, Campbell, Reece Unit four: Mechanisms of Evolution Chapter 23: The Evolution of Populations Pages 468 486