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MECHANISMS FOR EVOLUTION CHAPTER 23 Objectives – State the Hardy-Weinburg theorem – Write the Hardy-Weinburg equation and be able to use it to calculate allele and genotype frequencies – List the conditions that must be met to maintain Hardy Weinburg equilibrium VOCABULARY POPULATION SPECIES GENE POOL GENE FLOW BOTTLENECK EFFECT FOUNDER EFFECT HETEROZYGOTE ADVANTAGE HYBRID VIGOR STABILIZING SELECTION DIRECTIONAL SELECTION DIVERSIFYING SELECTION SEXUAL DIMORPHISM POPULATION – Localized group belonging to the same species SPECIES – Naturally breeding group of organisms that produce fertile offspring GENE POOL – Total aggregate of genes in a population at any one time Most species are not evenly distributed over a geographic range. Individuals are more likely to breed with others from their population center HARDY – WEINBURG THEOREM Describes a NON-EVOLVING population In the absence of other factors the segregation and recombination of alleles during meiosis and fertilization will not alter the overall genetic make-up of a population Imagine an isolated wildflower population with the following characteristics – Diploid with both pink and white flowers – Pink is dominant A and white is recessive a – There are 480 pink flowers and 20 white » 320 are AA » 160 are Aa (p + q)2 = 1 p+q=1 p2 + 2pq +q2 = 1 p2 = frequency of AA 2pq = frequency of Aa q2 = frequency of aa Calculate q2 first There are 1000 alleles – AA -- 320 x 2/plant = 640 – Aa --160 x 1/plant = 160 800 – aa -- 20 x 2/plant = 40 – Aa – 160 x 1/plant =160 – 200 – Frequency of A = 80% and a = 20% Condition for Hardy-Weinburg Large population No net mutation Isolated population Random mating No natural selection MICROEVOLUTION LEADS TO MACROEVOLUTION CAUSES OF MICROEVOLUTION GENETIC DRIFT – BOTTLENECK EFFECT – FOUNDER EFFECT GENE FLOW – genetic exchange between populations due to migration Mutation – a new mutation that is transmitted in a gamete can immediately change the gene pool NONRANDON MATING – Breed with other members of the “neighborhood” promotes inbreeding – Assortative mating – mate with others like themselves NATURAL SELECTION Variation within Populations Most heritable variation is measured by – Quantitative characters (vary along a continuum ie. Height) are polygenetic – Discrete characters (pink or white) are located on a single gene Polymorphism – two or more forms of a discrete character are represented in a population GEOGRAPHICAL VARIATION A cline is a graded change in some trait along a geographical axis. MODES OF SELECTION