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Chapter 16: Evolution of Populations and Speciation

Chapter 16: Evolution of Populations and Speciation. 16-1 Variation of Traits in a Population. 16-2 Disruption of Genetic Equilibrium. 16-3 Formation of Species. 16-1 Genetic Equilibrium. I. Variation of Traits in a Population.

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Chapter 16: Evolution of Populations and Speciation

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  1. Chapter 16: Evolution of Populations and Speciation 16-1 Variation of Traits in a Population 16-2 Disruption of Genetic Equilibrium 16-3 Formation of Species

  2. 16-1 Genetic Equilibrium I. Variation of Traits in a Population • Variation ALLOWS for natural selection to act upon a population.

  3. (1) Population Genetics (population is SMALLEST unit capable of evolving) • Study of evolution from a GENETIC perspective (i.e., gene pool changes).

  4. (2) Bell Curve (within a LARGE sample population) • Measurable TRAITS display a BELL-CURVE pattern (i.e., MEDIAN form is FAVORED).

  5. (A) Causes of Genotypic Variation in a Population • Genetics CAN increase VARIATION in a population in THREE ways:

  6. (1) MUTATION from mutant copies of individual GENES.

  7. (2) RECOMBINATION AND CROSSING-OVER during meiosis add variation to sex cells. NOTE: On average, between 2-3 crossovers occur on each pair of chromosomes during meiosis.

  8. (3) RANDOM fusion of GAMETES ensure offspring are NOT clones of parents.

  9. II. Allele Frequencies and the Gene Pool • Some alleles are MORE NUMEROUS than other alleles in gene pools.

  10. (1) Gene Pool • TOTAL genes AVAILABLE in a population of a species (a dynamic NOT static pool of genes). • NOTE: SUM of ALL changes in a GENE POOL over a LONG period of TIME is known as “EVOLUTION.”

  11. (2) Allele Frequency (a calculation) • Frequency of an ALLELE among ALL alleles in a POPULATION. (e.g., PREDICT the likelihood of a certain PHENOTYPE being expressed).

  12. (A) Predicting Phenotype • Certain PHENOTYPES are EXPRESSED more often than OTHER TYPES.

  13. (1) Phenotypic Frequency • Frequency of a SPECIFIED phenotype within an entire POPULATION.

  14. Critical Thinking (1) Do you think it is easier to analyze genotype in organisms with complete dominance or in organisms with incomplete dominance?

  15. III. Hardy-Weinberg Genetic Equilibrium (i.e., a theoretical state) • A model showing HOW allele frequencies in a population CAN remain CONSTANT. (i.e., evolution does NOT occur). (1) NO mutations occur; (NO allelic frequency change). (2) Individuals neither ENTER nor LEAVE the population. (3) Population is LARGE (ideally, infinitely large) (4) Individuals mate RANDOMLY. (5) Natural selection does NOT occur. NOTE: This model ALLOWS us to consider what forces DISRUPT genetic equilibrium in a population (i.e., allow a POPULATION to EVOLVE).

  16. Sample Problem: Using the Hardy-Weinberg equation of… p2 + 2pq + q2 = 1 (p = dominant allele) (q = recessive allele) Calculate the expected number of heterozygotes (Aa) and homozygous dominant (AA) individuals in a population of 100 tigers, 10 of which are albino white (homozygous recessive, aa). q2 = (10/100) = 0. 1 Therefore q = 0.32 p = 1-q or 0.68 *Therefore, the frequency of homozygous dominant individuals is p2 or 0.46, or approximately 46% *And the frequency of heterozygous individuals is 2pq or 0.44, or approximately 44%

  17. 16-2 Disruption of Genetic Equilibrium • A VIOLATION of Hardy-Weinberg equilibrium can result in EVOLUTION (i.e., FIVE evolutionary forces).

  18. I. Mutation (introduces VARIATION, disrupts equilibrium) • Mutations produce NEW alleles INTO a population.

  19. II. Migration (disrupts equilibrium) • REMOVES or ADDS new members to a gene pool (i.e., new alleles/genes).

  20. (1) Immigration (WIDENS gene pool) • MOVEMENT of NEW genes INTO a population.

  21. (2) Emigration (SHRINKS gene pool) • Movement of genes OUT of a population.

  22. (3) Gene Flow (FLUIDITY of the waves in the gene pool) • NET movement of genes BETWEEN populations of SAME species.

  23. III. Genetic Drift (MORE likely experienced in SMALL populations) • CHANGE in allelic frequencies as a result of RANDOM events (chance).

  24. IV. Nonrandom Mating (includes Assortative Mating) • Results from GEOGRAPHIC PROXIMITY and a SUITABLE MATE (negative drawback, inbreeding).

  25. (1) Assortative Mating (e.g., NON-random mating) • Selection of a MATE with SIMILAR physical traits (to ITSELF).

  26. V. Natural Selection (selective forces of NATURE; for OR against) • An ONGOING process that DISRUPTS genetic equilibrium. • (NOTE: 4 types of natural selection include…) (A) Stabilizing Selection (B) Directional Selection (C) Disruptive Selection (D) Sexual Selection

  27. (A) STABILIZING Selection (selects AGAINST the extremes) • Individuals with AVERAGE form of a trait have the HIGHEST fitness (i.e., Selected FOR by nature)

  28. (B) DIRECTIONAL Selection (selection is SHIFTED one side) • Individuals with a MORE EXTREME form of a trait (one extreme OR the other, NOT BOTH) have GREATEST fitness.

  29. (C) DISRUPTIVE Selection (mean is selected AGAINST) • Individuals with EITHER extreme variation have GREATEST fitness.

  30. Critical Thinking (2) Human newborns with either a very high or very low birth weight are more likely to die in infancy. What type of selection does this seem to be?

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