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Genes and Evolution EVOLUTION AS GENETIC CHANGE Natural Selection and Genes Evolution occurs when there is shift in the relative frequency of an allele in a gene pool. This occurs because natural selection results in individuals with certain traits and certain alleles surviving more easily and passing their alleles on to more offspring. Single-Gene Traits Over time one phenotype of the single-gene traits might be more advantageous. As time passes, more individuals will show that phenotype, and the alleles needed for it will have higher frequency. With single gene traits it is nearly impossible to select away an individual allele. Polygenic Traits Directional Selection This occurs when one of the extremes becomes most advantageous. Stabilizing Selection This happens when the middle of the curve is most fit. Disruptive Selection This occurs when both extremes are more advantageous than the middle. Small Populations Sometimes chance can change the frequency of an allele, rather than natural selection. This is called genetic drift. Typically, the founder effect, or when a small population gets separated from the larger population, creates this. Hardy-Weinberg Principle Allele frequencies in a population will stay constant unless some factor(s) force the population to change When the allele frequency stays constant, this is known as genetic equilibrium. Requirements for Genetic Equilibrium 1. Random Mating 2. Large Population 3. No movement in or out of the population 4. No mutations 5. No Natural Selection Answer These 1. What kind of trait can undergo disruptive selection? 1. Polygenic Trait 2. Single-gene trait 2. Describe an event that could cause stabilizing selection? 1. Something that makes being 6’3” advantageous 2. Something that makes being 5’7 advantageous 3. What is it called when the gene frequencies in gene pool remain constant? 1. Genetic Stabilization 2. Genetic Equilibrium 4. What are the five requirements to maintain genetic equilibrium? Random Mating, Large population, no movement in/out, no mutations and no natural selection