LS 1 Notes Ch 1, 24, 25, 26, 27

LS 1 Notes Ch 1, 24, 25, 26, 27 - Chapter 25: Evolutionary...

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Chapter 25: Evolutionary Processes Key Concepts The Hardy-Weinberg principle acts as a null hypothesis when researchers want to test whether evolution or nonrandom mating is occurring at a particular gene. Each of the four evolutionary mechanisms has different consequences. Only natural selection produces adaptation. Genetic drift causes random fluctuations in allele frequencies. Gene flow equalizes allele frequencies between populations. Mutation introduces new alleles. Inbreeding changes genotype frequencies but does not change allele frequencies. Sexual selection leads to the evolution of traits that help individuals attract mates. It is usually stronger on males than on females. Section 25.1 Outline: Analyzing Change in Allele Frequencies: The Hardy-Weinberg Principle For a gene with two alleles A 1 and A 2 , three genotypes are possible: A 1 A 1 , A 1 A 2 , and A 2 A 2 . The frequency of A 1 is represented by p and the frequency of A 2 is represented by q . The frequency of the A 1 A 1 genotype in the new generation will be p 2 . The frequency of the A 2 A 2 genotype will be q 2 . The frequency of A 1 A 2 genotype will be 2 pq (Figure 25.1). Because all individuals in the new generation must have one of the three genotypes, the sum of the three genotype frequencies must equal 1 (100% of the population): p 2 + 2 pq + q 2 = 1. When allele frequencies are calculated for this new generation, the frequency of A 1 is still p and the frequency of A 2 is still q . The Hardy–Weinberg principle makes two fundamental claims: 1. if the frequencies of alleles A 1 and A 2 in a population are given by p and q , then the frequencies of genotypes A 1 A 1 , A 1 A 2 , and A 2 A 2 will be given by p 2 , 2 pq , and q 2 for generation after generation 2. when alleles are transmitted according to the rules of Mendelian inheritance, their frequencies do not change over time. For evolution to occur, some other factor or factors must come into play.
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The Hardy–Weinberg Model Makes Important Assumptions o The Hardy–Weinberg model assumes that none of the four evolutionary mechanisms can be acting on the population. o In addition, the model assumes that mating is random with respect to the gene in question. o The Hardy–Weinberg principle holds when the following five conditions are met with respect to the gene in question: 1. no natural selection 2. no genetic drift or random allele frequency changes 3. no gene flow 4. no mutation 5. random mating How Does the Hardy–Weinberg Principle Serve as a Null Hypothesis? o The Hardy–Weinberg principle serves as a null hypothesis for determining whether evolution is acting on a particular gene in a population. o
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LS 1 Notes Ch 1, 24, 25, 26, 27 - Chapter 25: Evolutionary...

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