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Microevolution What is the smallest unit that can evolve? Do you remember how evolution a) Individual b) can Speciesbe c) Genus measured? d) Population Measure the Finalfrequency Answer? allele in the gene pool d! Calculating Allele Frequencies Phenotypes for flower color (R = red, r = white) 320 RR 160 Rr 20 rr 160 “R” and 160 “r” Number of alleles: 640 “R” Allele frequencies: FR = 800/1000 = .8 40 “r” Fr = 200/1000 = .2 Total alleles in gene pool: 1000 To measure evolution, calculate the allele frequency before and after a given amount of time. If the frequency is the same or constant no evolution has taken place A non-evolving population is said to be in “Hardy-Weinberg Equilibrium” Hardy-Weinberg (HW) Equations Let’s revisit the previous example involving flower color…. Let p = allele frequency of dominant allele (R) Let q = allele frequency of recessive allele (r) Then… p + q = 1 √ .8 + .2 = 1 HW Equation # 1 Hardy-Weinberg (HW) Equations If p = allele frequency of dominant allele (R), then p2 = frequency of homozygous genotype (RR) If q = allele frequency of recessive allele (r), then q2 = frequency of homozygous genotype (rr) If you complete the square, then the frequency of heterozygous genotype Rr must be 2pq So… p2 + 2pq + q2 = 1 √ .64 + .32 + .04 = 1 HW Equation # 2 5 Assumptions for HW equilibrium 1) 2) 3) 4) 5) Large Population Size No Migration No Mutation Random Mating No Natural Selection If all 5 conditions are met, no evolution will occur Any deviation results in microevolution of the population What is microevolution? A generation to generation change in a population’s allele frequency What causes microevolution? 5 opposing conditions to HW equilibrium 1) 2) 3) 4) 5) Genetic Drift Gene Flow Mutation Non-random mating Natural Selection Genetic Drift In a small population, some individuals may, just by chance, leave behind a few more descendents (and genes, of course!) than other individuals. Genetic Drift always reduces diversity 2 Examples of Genetic Drift 1. Population Bottleneck – A natural disaster or event causes a population’s size to be greatly reduced Initial Diverse Population Event that causes the population to be reduced in size Final population (not diverse) Over-poaching has bottlenecked the cheetah population, which is now susceptible to random changes in gene frequencies Reduction in diversity and inbreeding puts this species at risk of extinction 2 Examples of Genetic Drift 2. Founder Effect – Changes in gene frequencies that usually accompany starting a new population from a small number of individuals. Return to Causes of Microevolution Gene Flow • • Individuals from one group move into another group. Makes certain genes more frequent in the population. Return to Causes of Microevolution Mutation • A change in a DNA sequence – usually occurring because of errors in replication or repair. • Mutation is the ultimate source of genetic variation. Return to Causes of Microevolution Non-Random Mating • Individuals choose mates based on preferences for particular traits • 2 examples – 1) Sexual Selection – mates are chosen based upon behavior/appearance – 2) Inbreeding – the choice to mate with related individuals Female peacocks choose mates based upon the male’s plumage display Return to Causes of Microevolution Natural Selection • Increases or decreases in allele Strength of Selection frequencies due to environmental impact • Natural Selection can act upon a population in a variety of ways… Diversifying Selection = Directional Selection = Stabilizing Selection = environmental favors traits that are at conditions are varied selection that acts against one extreme of a range of such that individuals at extreme and both extremesphenotypes are traits favors favored intermediates