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Evolution: Microevolution

Evolution: Microevolution. Chapter 16. Microevolution evolutionary changes below species level refers to change within a species changes in gene frequency due to natural selection driving force of evol . Population Genetics study of microevolutionary change

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Evolution: Microevolution

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  1. Evolution:Microevolution Chapter 16

  2. Microevolution • evolutionary changes below species level • refers to change within a species • changes in gene frequency due to natural selection • driving force of evol. • Population Genetics • study of microevolutionary change • evol. takes place as changes occur in a gene pool • all of the genes present within a pop. • gene frequencies change as organisms adapt to a changing env. • alleles for more adaptive phenotypes increase in freq. • alleles for less adaptive phenotypes decrease in freq. • if gene frequencies change, then evolution occurs

  3. Hardy-Weinberg (HW) Principle • math. model used to predict behavior of allele freq. in an ideal pop. • allele freq. remain constant from generation to generation, unless outside forces change them • HW equilibrium • changes occur through natural selection and other means • p2 + 2pq + q2 = 1 • if a population is not in HW equilibrium, then evol. is occurring • gene freq. are changing

  4. Outcomes of Natural Selection • most traits within a pop. form a normal distribution • stabilizing selection • intermediate (avg.) phenotype is most adaptive and selected for • alleles for average individuals increase in number Stabilizing selection

  5. Fig. 16.9 Human birth weight, an example of stabilizing selection

  6. directional selection • one extreme phenotype is most adaptive and selected for • a new “average” is established Directional selection

  7. Fig. 16.10 An example of directional selection. Guppies become more colorful (blue and yellow lines on graph) in the absence of predation, and less colorful (red graph line) when in the presence of predation.

  8. disruptive selection • both extreme phenotypes are adaptive and selected for • two “averages” are formed Disruptive selection

  9. Fig. 16.8 Three different kinds of selection: (a) stabilizing, (b) directional, and (c) disruptive

  10. Another real example of the three different kinds of selection

  11. Other Ways Evolution Can Occur in Addition to Natural Selection • genetic drift • change in allele freq. due simply to random chance • more pronounced effect in small populations • can lead to the complete loss or fixation of certain alleles in a pop. An example of genetic drift

  12. Fig. 16.6 Genetic drift

  13. population bottleneck • pop. is greatly reduced in a small amount of time • usually due to a natural disaster or major disease of some kind • gene freq. change according to those possessed by the survivors Schematic view of a population bottleneck

  14. founder effect • small group of individuals leave main pop. and establish a new colony • gene freq. change according to those possessed by these “founders” • gene flow • exchange of genes between pops. of the same species Schematic view of founder effect

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