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Population Genetics & Evolution Population Genetics & Evolution • Individuals do not evolve but populations do Genetic Equilibrium • Occurs when the frequency of alleles remains the same over generations within a population QuickTime™ and a decompressor are needed to see this picture. Genetic Equilibrium • Gene pool - All of the alleles of a population’s genes QuickTime™ and a decompressor are needed to see this picture. QuickTime™ and a decompressor are needed to see this picture. Genetic Equilibrium • Allele Frequency - percentage of a specific allele in the gene pool QuickTime™ and a decompressor are needed to see this picture. Genetic Equilibrium • A population that is in genetic equilibrium is NOT evolving Genetic Equilibrium • Anything that affects the genes in a gene pool can change allelic frequencies, which will disrupt a population’s genetic equilibrium and result in the process of evolution Mechanisms that disrupt a population’s genetic equilibrium • Mutations: introduce new alleles into the gene pool & change allelic frequencies QuickTime™ and a decompressor are needed to see this picture. Mechanisms that disrupt a population’s genetic equilibrium • Genetic drift - alteration of allelic frequencies by chance events • Occurs mainly in small populations that interbreed with each other Example of Genetic Drift • Allele for extra fingers & toes is 1 out of 14 in Amish people but 1 out of 1000 for the rest of the U.S. population QuickTime™ and a decompressor are needed to see this picture. QuickTime™ and a decompressor are needed to see this picture. QuickTime™ and a decompressor are needed to see this picture. Mechanisms that disrupt a population’s genetic equilibrium • Gene flow - transport of genes by migrating individuals Genes are lost from the gene pool when an individual leaves a population; genes are added when an individual enters a population QuickTime™ and a decompressor are needed to see this picture. Mechanisms that disrupt a population’s genetic equilibrium *The 3 mechanisms previously described primarily affect evolution of small, isolated gene pools (like those on islands) but their effect is often insignificant in larger, less isolated gene pools* Mechanisms that disrupt a population’s genetic equilibrium • Natural selection - the most significant factor that causes changes in established gene pools, whether they are large or small • There are 3 types of natural selection: stabilizing, directional, & disruptive Stabilizing Natural Selection • Favors the average individuals in a population • Reduces variation in a population Directional Natural Selection • Favors individuals with one of the extreme variations of a trait • Can result in rapid evolutionary change within a population Disruptive Natural Selection • Favors individuals with either extreme of a traits variation • Can lead to evolution of 2 distinct species The End