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Chapter 23 ~ Evolution of Populations Population genetics • Population: group of individuals belonging to the same species in same area • Species: organisms that can interbreed and have fertile offspring • Gene pool: the total number of genes in a population at any one time • Population genetics: the study of genetic changes in populations • “Individuals are selected, but populations evolve.” Hardy-Weinberg Theorem Serves as a model for the genetic structure of a nonevolving population (equilibrium) 5 conditions: 1. 2. 3. 4. 5. Very large population size No migration No mutations Random mating No natural selection Hardy-Weinberg Equation • p =frequency of one allele = A • q =frequency of the other allele = 1 p+q=1.0 • P2=frequency of AA genotype • 2pq=frequency of Aa • q2=frequency of aa genotype p2 + 2pq + q2 = 1.0 Show BIO-FLIX Chapter 23 Bozeman: HW • https://www.youtube.com/watch?v=xPkOA nK20kw Microevolution: I • A change in the gene pool of a population over generations • Genetic Drift: changes in the gene pool of a small population due to chance (usually reduces genetic variability) LE 23-7 CWCW CRCR CRCR CRCW Only 5 of 10 plants leave offspring CRCR CWCW CRCW CWCW CRCR CRCW CRCW CRCR CRCR CRCR CRCW CRCW Generation 1 p (frequency of CR) = 0.7 q (frequency of CW) = 0.3 CWCW CRCR Only 2 of 10 plants leave offspring CRCR CRCR CRCR CRCR CRCR CRCR CRCR CRCR CRCW CRCW Generation 2 p = 0.5 q = 0.5 CRCR CRCR Generation 3 p = 1.0 q = 0.0 Microevolution: II • The Bottleneck Effect: – Type of genetic drift resulting from a reduction in population (natural disaster) where the surviving population does have the same genetic make up of the original population LE 23-8 Original population Bottlenecking event Surviving population Microevolution: III • Founder Effect: a cause of genetic drift due to colonization by a limited number of individuals from a parent population Microevolution: IV • Gene Flow: genetic exchange due to the migration of fertile individuals or gametes between populations (reduces differences between populations) – Note: “opposite of inbreeding” Selective Breeding – “Inbreeding” Out-breeding – genes “merge” Microevolution: V • Mutations: a change in an organism’s DNA / original source of genetic variation (raw material for natural selection) Microevolution: VI • Nonrandom mating: inbreeding and assortive mating (both shift frequencies of different genotypes) Microevolution: VII • Natural Selection: –differential success in reproduction –only form of evolution that adapts a population to its environment Population variation • Polymorphism: coexistence of 2 or more distinct forms of individuals (morphs) within the same population • Geographical variation: differences in genetic structure between populations (cline) Variation preservation • Prevention of natural selection’s reduction of variation • Diploidy 2nd set of chromosomes hides variation in the heterozygote • Balanced polymorphism 1- heterozygote advantage (hybrid vigor; i.e., malaria/sicklecell anemia); 2- frequency dependent selection (survival & reproduction of any 1 morph declines if it becomes too common; i.e., parasite/host) Natural selection • Fitness: contribution an individual makes to the gene pool of • • • • the next generation 3 types: A. Directional B. Diversifying C. Stabilizing Sexual selection • Sexual dimorphism: secondary sex characteristic distinction • Sexual selection: selection towards secondary sex characteristics that leads to sexual dimorphism