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Evolution of Populations Chapter 23.3-23.4

Evolution of Populations Chapter 23.3-23.4. AP Biology. Recap. 23.1-23.2. Two Types of Evolution. Macroevolution. Microevolution. Change that occurs within a species Change in gene frequencies Focus of Darwin’s theory of evolution via natural selection.

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Evolution of Populations Chapter 23.3-23.4

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  1. Evolution of PopulationsChapter 23.3-23.4 AP Biology

  2. Recap 23.1-23.2

  3. Two Types of Evolution Macroevolution Microevolution Change that occurs within a species Change in gene frequencies Focus of Darwin’s theory of evolution via natural selection • Evolution that takes place over LONG periods of time • Change on a grand scale • Mass extinctions

  4. How can we study microevolution? • Study population genetics to understand changes that occur within a species • Hardy-Weinberg

  5. There are 3 mechanisms for evolution • Natural Selection • Leads to an accumulation of favorable adaptations in a population • Genetic Drift • Chance events that alter allele frequencies • Gene Flow • The transfer of alleles between populations

  6. Natural selection, genetic drift, and gene flow can alter allele frequencies in a population 23.3

  7. Review: Natural Selection • This is a change in gene frequencies due to differential reproductive success. • Example: antibiotic resistance • Variation already present • Introduction of antibiotic selects for those with higher resistance • Frequencies for the resistance gene increase over time

  8. Genetic Drift • Changes in the gene pool of a small population due to chance (random) • Founder Effect or Bottleneck Effect

  9. Genetic Drift: Founder Effect • A few (random) individuals from a population start a new population with a different allele frequency than the original population

  10. Genetic Drift: Bottleneck Effect • The population experiences a huge decrease in size • Result: severe reduction in diversity of the original gene pool because a small percentage of chance survivors remain • Endangered species can experience this

  11. BB BB BB BB BB bb Gene Flow • Migration of fertile individuals, or the transfer of gametes, between populations • Populations may gain or lose alleles • Reduces differences between neighboring populations

  12. Natural selection is the only mechanism that consistently causes adaptive radiation 23.4

  13. Natural Selection • Natural selection results in development of features which increase an organism’s likelihood for survival and reproduction—adaptations • Therefore, Darwin’s explanation of evolution goes like this: • Adaptation + natural selection  change within species • Leads to adaptive evolution

  14. Modes of Natural Selection • Directional Selection – favors variants of one extreme • Disruptive Selection – Opposite phenotypic extremes favored over intermediate • Stabilizing Selection – favors intermediate variants by selecting against extreme phenotypes

  15. Maintaining Variation • Natural selection is the main mechanism of evolution • For this to work, variation must be maintained • Variation is good (why?)

  16. Conserving Variation: Polymorphism • The presence of two or more distinct phenotypes in a population • Example: sexual dimorphism • Leads to the enigma of sexual reproduction • Evolutionary advantage outweighs cost of sex • Costs: energy, resources, competition • Advantage: pass on genes, variation of genes (2 parents vs. 1 parent, meiosis)

  17. Conserving Variation: Diploidy • The presence of homologous chromosomes (one from mom, one from dad) maintains variety in the population • A dominant allele can cover up a harmful recessive one • Example: cystic fibrosis

  18. Conserving Variation: Heterozygote Advantage • Heterozygotes more likely to survive than homozygotes • Example: sickle-cell anemia and malaria resistance

  19. Natural Selection isn’t perfect • Nature can only select for what traits are present. • Ancestry is a legacy that is modified; nothing is built from scratch. • Adaptations are often compromises. • Chance, natural selection and the environment interact.

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