Biol-113 Principles of Biology Chapter 23 PDF

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Biol-113 Principles of Biology Chapter 23 Evolutionary Processes Introduction ● Evolution—change in allele frequencies—driven by four processes: 1. Natural selection increases frequency of alleles that contribute to reproductive success in particular environment 2. Genetic drift causes allele frequencies to change randomly 3. Gene flow occurs when individuals leave one population, join another, and breed 4. Mutation modifies allele frequencies by continually introducing new alleles ● Two fundamental messages in this chapter: 1. Natural selection is not the only process responsible for evolution 2. Each of the four evolutionary processes has different consequences for genetic variation and fitness ● Modern Synthesis: - Era in early 1900s where evolutionary biologists, mathematicians, and geneticists collaborated to quantify evolution - Population genetics:▪One fruit of modern synthesis ➔ Study of processes that change allele and genotype frequencies in populations Null Hypothesis: The Hardy–Weinberg Principle ● 1908, G.H. Hardy and Wilhelm Weinberg: - Wanted to know what happened in an entire population when all individuals— and thus all possible genotypes—bred - Analyzed frequencies of alleles when individual in population mate and produce offspring The Gene Pool Concept ● To analyze consequences of matings: - Hardy and Weinberg imagined that alleles from all gametes in each generation go into single group called gene pool and then combine randomly - Calculated what would happen if pairs of gametes picked randomly, many times, and each pair combined to produce offspring - Calculations predict genotypes of offspring that population would produce, as well as frequency of each genotype ● Hardy-Weinberg principle: - Serves as mathematical null hypothesis for study of evolutionary processes ● Simplest situation: - Gene with two alleles, A1 and A2

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Frequency of A1 represented by p–Frequency of A2 represented by q in gene pool - Because there are only two alleles, p + q = 1 - These are the frequencies of two alleles, which sum to 1 (or 100% of the population) In this situation, three genotypes are possible: - A1A1, A1A2, and A2A2 - Model predicts the frequencies of the three genotypes: ➔ The frequency of the A1A1 genotype is ➔ The frequency of the A2A2 genotype is ➔ The frequency of the A1A2 genotype is 2pq Sum of three genotype frequencies must equal 1 (100% of population): - This is the Hardy–Weinberg equation When alleles are transmitted via meiosis and random combination of gametes: - Allele frequencies do not change - Therefore when allele frequencies are calculated for next generation: ● Frequency of A1 is still p and frequency of A2 is still q Hardy–Weinberg principle predicts the genotype frequencies and allele frequencies in the next generation If frequencies conform to predictions, then frequencies are in Hardy–Weinberg equilibrium

The Hardy–Weinberg Principle Makes Important Assumptions ● Hardy–Weinberg model is based on five assumptions about behavior of populations and alleles: 1. Random mating—no mate choice; gametes combine randomly 2. No natural selection—all individuals contribute equally to gene pool 3. No genetic drift (random allele frequency changes)—alleles not picked by chance because assumes population is large 4. No gene flow—no new alleles added or lost from gene pool 5. No mutation—new alleles introduced into gene pool Are MN Blood-Type Alleles in Humans in Hardy–Weinberg Equilibrium? ● Most human populations have two alleles for MN blood group: - Alleles for this gene are M and N, which code for protein on surface of red blood cell: ➔ M and N are codominant: - Heterozygotes have both M and N versions of protein on red blood cells - Genotype of person can be determined from blood samples: ➔ Genotype may be MM, MN, or NN ●

Analysis to determine if population is in Hardy–Weinberg equilibrium based on four steps: 1. Estimate genotype frequencies:

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➔ Divide total number of individuals with each genotype by total number of individuals in sample 2. Calculate observed allele frequencies from observed genotype frequencies: ➔ Example: Frequency of M allele = frequency of MM homozygotes + half the frequency of heterozygotes. 3. Use observed allele frequencies to calculate genotypes expected according to Hardy–Weinberg principle 4. Statistically compare observed and expected values Observed and expected genotype frequencies were almost identical: - For every population examined, genotypes at MN locus are in Hardy–Weinberg equilibrium - MN blood groups were not being affected by any of the four evolutionary processes - Mating was random with respect to this gene - Null hypothesis is accepted...