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Evolution of Populations

Evolution of Populations. Evolution as Genetic Change 16-2. Natural Selection on Single Gene Traits. Natural selection never acts on genes, always on the whole organism

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Evolution of Populations

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  1. Evolution of Populations Evolution as Genetic Change 16-2

  2. Natural Selection on Single Gene Traits • Natural selection never acts on genes, always on the whole organism • But, if that organism produce a gene change that affects survival, natural selection will act differently on that organism because of that gene change. • Ex: Pg 397-Lizard color • Brown vs. Black • Change the survival, change the gene pool

  3. Distribution of Phenotypes in Polygenic Traits • When many genes affect one trait, you get many phenotypes • The graph to the right shows the number of individuals (area under the curve) for the different phenotypes

  4. Natural Selection on Polygenic Traits • Directional Selection: when individuals at one end of the curve have higher fitness than individuals in the middle or at the other end-Thus the curve will shift left or right Ex:Beak Shape

  5. Natural Selection on Polygenic Traits • Stabilizing Selection:when individuals near the center of the curve have higher fitness than the others-Thus the curve will get narrow and tall -Survival favors the average Ex: Birth weight in humans

  6. Natural Selection on Polygenic Traits • Disruptive Selection:when individuals at opposite ends of the curve have higher fitness-Thus causes the curve to dip in the middle -Average struggle to survive Ex: Bird Beaks

  7. Genetic Drift • In small populations, individuals that carry a particular allele may leave more descendants than others, just by chance • Over time, this can cause a change in phenotype from the original population • Ex: Founders Effect- small subgroup of individuals populate an area, thus the descendants have their alleles only

  8. Genetic Equilibrium=No Evolution/Change • Hardy-Weinberg Principle: allele frequencies remain the same if… • Random Mating Occurs • Large Population • No immigration or emigration • No mutations • No natural selection

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