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Population Genetics Terminology Population Genetics • Population Genetics = The study of all the genetic traits in a population Genetic Equilibrium Genetic Drift • Gene Pool = combined genetic makeup of all the members of a population Genetic Equilibrium The Hardy-Weinberg Principle • A constant state of allele frequency • Population not evolving • Allele frequencies remain stable • This situation is also known as the: States that • Under certain conditions, allelic frequencies remain constant from generation to generation • If any one condition is not met, genetic equilibrium will be disrupted and the population may evolve HARDY-WEINBERG PRINCIPLE 1 Conditions required to maintain genetic equilibrium: • No natural selection – all alleles must be equally favorable • Random mating • No migration – no new alleles brought into population • No significant mutations – mutations would introduce new alleles into population NOTE: Evolution is a result of disruption in genetic equilibrium 4 Ways to Disrupt Genetic Equilibrium • Mutation: introducing new alleles into a population • Natural Selection: acting on variations – Directional selection – Stabilizing selection – Disruptive selection • Genetic Drift • Gene Flow Genetic Drift Example of Genetic Drift • is an accidental change in gene frequency • Sometimes, genetic equilibrium can be • The Amish population originated from 30 upset by random events in the environment (example: a fire) • can greatly affect small populations that originated from a small number of ancestors, since the genes of the ancestors represent a small fraction of the gene pool of the entire species. individuals, and has been isolated ever since due to religious practices. • One of these original settlers carried the recessive allele for short arms and legs, and extra fingers and toes • Today, 1 in 14 Amish have this trait, compared to 1 in 1000 in the rest of the US 2 Gene Flow Review Questions • The movement of individuals into or out of 1. List the conditions necessary for a population to maintain genetic equilibrium. a population changes the frequency of alleles in that populations gene pool. • When an individual leaves(emmigrates), its genes are lost • When an individual enters (immigrates), their genes are added to the population • Can greatly effect small populations, may not have any effect on large populations. 2. Why would migration and mutation have a greater effect on the gene frequencies of a very small population than a large one? Explain your reasoning. Answers to Review Q’s 1. No natural selection, random mating, no 2. migration, no significant mutations. Rapid evolutionary change due to migration and mutation is more likely to occur in a small population because of genetic drift and the limited number of mates. 3