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  1. Q.P.S. Update • Goal: • Summarize the effects of the different types of natural selections on gene pools.

  2. 15.2 Section Objectives – page 404 Section Objectives • Summarize the effects of the different types of natural selections on gene pools.

  3. Section 15.2 Summary– pages 404-413 Natural selection acts on variations • Variations increase or decrease an organism’s chance of survival in an environment. • There are three different types of natural selection that act on variation: stabilizing, directional, and disruptive.

  4. Section 15.2 Summary– pages 404-413 Natural selection acts on variations • Stabilizing selection is a natural selection that favors average individuals in a population. Normal variation Selection for average size spiders

  5. Section 15.2 Summary– pages 404-413 Natural selection acts on variations • Directional selection occurs when natural selection favors one of the extreme variations of a trait. Selection for longer beaks Normal variation

  6. Section 15.2 Summary– pages 404-413 Natural selection acts on variations • In disruptive selection, individuals with either extreme of a trait’s variation are selected for. Selection for light limpets Normal variation Selection for dark limpets

  7. Section 15.2 Summary– pages 404-413 Natural selection acts on variations • Natural selection can significantly alter the genetic equilibrium of a population’s gene pool over time. • Significant changes in the gene pool could lead to the evolution of a new species over time. (speciation)

  8. Section 15.2 Summary– pages 404-413 Physical barriers can prevent interbreeding • In nature, physical barriers can break large populations into smaller ones. • Geographic isolation occurs whenever a physical barrier divides a population. • A new species can evolve when a population has been geographically isolated.

  9. Section 15.2 Summary– pages 404-413 The Evolution of Species • When geographic isolation divides a population of tree frogs, the individuals no longer mate across populations.

  10. Section 15.2 Summary– pages 404-413 The Evolution of Species • The formation of a river may divide the frogs into two populations.

  11. Section 15.2 Summary– pages 404-413 The Evolution of Species • Over time, the divided populations may become two species that may no longer interbreed, even if reunited.

  12. Section 15.2 Summary– pages 404-413 A change in chromosome numbers and speciation • Mistakes during mitosis or meiosis can result in polyploid individuals. New polyploid species Abnormal gametes (2n) Fertilization Zygote (4n) Nondisjunction Sterile plant Fertilization Parent plant (2n) Meiosis begins Zygote (3n) Normal meiosis Normal gametes (n)

  13. Section 15.2 Summary– pages 404-413 Speciation rates • Gradualism is the idea that species originate through a gradual change of adaptations.

  14. Section 15.2 Summary– pages 404-413 Speciation rates • In 1972, Niles Eldredge and Stephen J. Gould proposed a different hypothesis known as punctuated equilibrium. • This hypothesis argues that speciation occurs relatively quickly, in rapid bursts, with long periods of genetic equilibrium in between.

  15. Speciation rates Section 15.2 Summary– pages 404-413 Loxodonta africana Elephas maximus 0 1 2 Millions of Years Ago Mammuthus primigenius Elephas 3 Loxodonta 4 Mammuthus 5 Primelephas 6 Ancestral species about 55 million years ago

  16. Section 15.2 Summary– pages 404-413 Speciation rates • Biologists generally agree that both gradualism and punctuated equilibrium can result in speciation, depending on the circumstances.

  17. Section 15.2 Summary– pages 404-413 Patterns of Evolution • Biologists have observed different patterns of evolution that occur throughout the world in different natural environments. • These patterns support the idea that natural selection is an important agent for evolution.

  18. Section 15.2 Summary– pages 404-413 Diversity in new environments • When an ancestral species evolves into an array of species to fit a number of diverse habitats, the result is called adaptive radiation.

  19. Section 15.2 Summary– pages 404-413 Diversity in new environments • Adaptive radiation in both plants and animals has occurred and continues to occur throughout the world and is common on islands. • Adaptive radiation is a type of divergent evolution, the pattern of evolution in which species that were once similar to an ancestral species diverge, or become increasingly distinct.

  20. Section 15.2 Summary– pages 404-413 Diversity in new environments Amakihi Extinct mamo Crested honeycreeper Possible Ancestral Lasan finch Kauai Niihau Molokai Oahu Maui Lanai Akialoa Kahoolawe Hawaii Akepa Apapane Maui parrotbill Akiapolaau Liwi Grosbeak finch Akikiki Palila Ou

  21. Section 15.2 Summary– pages 404-413 Diversity in new environments • Divergent evolution occurs when populations change as they adapt to different environmental conditions, eventually resulting in new species.

  22. Section 15.2 Summary– pages 404-413 Different species can look alike • A pattern of evolution in which distantly related organisms evolve similar traits is called convergent evolution. • Convergent evolution occurs when unrelated species occupy similar environments in different parts of the world.

  23. Section 2 Check Question 1 The fur of an Arctic fox turns white in the winter. Is this an example of natural selection? Why or why not? IN: 1.32

  24. Section 2 Check The answer is no. An individual cannot evolve a new phenotype (in this case, changing the color of its fur) within its lifetime in response to its environment. IN: 1.32

  25. Section 2 Check Question 2 Which type of natural selection does NOT favor the evolution of new species? A. divergent B. disruptive C. stabilizing D. directional IN: 1.3

  26. Section 2 Check The answer is C. Stabilizing selection reduces variation in a population. IN: 1.3

  27. Section 2 Check Question 3 Which of the following rarely affects a population’s genetic equilibrium? A. genetic drift B. lethal mutations C. gene flow D. disruptive selection

  28. Section 2 Check The answer is B. Organisms with lethal mutations do not survive. Therefore, organisms with lethal mutations cannot produce enough offspring to affect a population’s genetic equilibrium.

  29. Section 2 Check Question 4 Why are the Galapagos Islands rich in unique species of organisms? A. The islands are an area exhibiting an abnormal number of mutations. B. The islands are geographically isolated. C. The island species have been subjected to increased gene flow. D. The island species have been subjected to stabilizing selection. IN: 1.36

  30. Section 2 Check The answer is B. Geographic isolation has helped to keep the islands’ species unique. IN: 1.36