1. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 1 Populations may change through two major genetic forces:  Natural Selection (includes artificial seltn)  Genetic Drift Two other forces that change a population:  Mutations  Gene flow  Not much change, mutation is too rare & gene flow tends to equalize gene frequencies Mechanisms of Population Change

2. 2 Natural Selection Occurs because: Organisms with favorable traits… chance at survival The longer an organism lives, more offspring it may have More of the next generation will have the favorable trait than the previous generation

3. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 3 Natural Selection Individuals of one phenotype are favored Peppered moths

4. 4 Genetic Drift Differences in reproduction or survival between organisms due to random chance  Environmental disturbances can cause genetic drift as long as they act at random: Hurricanes Volcanic eruptions (Mt. St. Helens)  Can be from random events  seeds from white flowers are blown to gravel, some seeds from red flowers blown to good soil.

5. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 5 Genetic Drift

6. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 6 How Populations Change Mutation creates genetic variation (genes) Gene flow, genetic drift and natural selection act on this genetic variation to change allele frequencies in populations  Gene flow tends to equalize allele frequencies between 2 pops.  Genetic drift and natural selection tend to diverge allele frequencies between 2 pops. (populations tend to diverge)

7. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 7 Adaptations Adaptation –a characteristic that improves the survival of a population or individual

8. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 8 Adaptation An improvement in a population over time, changing a population to better fit its environment Only the result of natural selection acting on genetic variation, no other force adapts a population Environments change and new genetic variation arises, so adaptation never reaches perfection

9. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 9 What are Species? Members of a species generally look alike  Bald eagles in Alaska (top) & Colorado (bottom)  But not always Species –are reproductively isolated from other species, meaning that one can only reproduce within one’s own species

10. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 10 Walking Sticks Adapting to match their environment (two mating pairs) Striped form is well-hidden among the needle-like leaves w/ stripes, solid form among solid leaves

11. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 11 The striped form prefers needle-like leaves of the chemise (sheh-mēz) bush. The solid-green form prefers the solid green leaves of the wild lilac. The two forms are more likely to survive on their preferred plant.  Predators (birds) more likely to catch them if they are not on their preferred plant. Walking Sticks

12. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 12 Walking Sticks Is a complex adaptation, more than coloration. Lilac and chemise bush leaves have different pulp, fiber, chemical contents, and plant toxins. The solid-green walking sticks have enzymes to better digest the leaves of the wild lilac. The striped form has enzymes adapted to better digest the leaves of the leaves of the chemise bush.

13. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 13 Walking Sticks “Hybrid offspring” of a cross between solid and striped parents are less fit than offspring of a single form mating.  Do not blend as well on either Lilac or chemise bush leaves; predators more likely to eat them.  Do not digest either plant as well as single forms.

14. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 14 Walking Sticks Because “Hybrid offspring” (a solid & striped cross) are less fit than “single form” offspring, the population is adapting in one other way:  Individuals prefer to mate with others of the same color pattern.

15. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 15 Walking Sticks Population is diverging with respect to alleles for:  coloration (once were all green),  digestion (once were generalists),  dealing with plant toxins,  mating behavior. Natural selection is better adapting each form to their preferred plant.

16. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 16 W/ time, natural selection will further diverge the two forms & further decrease their interbreeding. When the two forms loose the ability to interbreed, they will become two different species.

17. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 17 Speciation When one species splits to form two species Results from reproductive isolation Often, it is a secondary consequence of changing populations Occasionally, is a direct consequence of a diverging population  The hybrid is less fit, walking sticks

18. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 18 Biological Evolution Is a change in the genetic characteristics of a population over time If any allele frequency in a population changes w/ time, the population is evolving Most people think of evolution as Darwin’s speciation, but this is not correct

19. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 19 Only populations can evolve; individuals cannot  (on molecular level, genes can evolve) Small scale change = microevolution Large scale change = macroevolution Biological Evolution

20. © 2006 W.W. Norton & Company, Inc. DISCOVER BIOLOGY 3/e 20 Biological Evolution Macroevolution  can lead to speciation, but it does not have to  has occurred in dogs

21. 21 Review Understanding evolution has impacted our use of Antibiotics and our understanding the development of drug resistance Doonesbury cartoon? Doonesbury cartoon Understanding evolution has impacted our understanding of the world (shown at right) Review of agents of change videoagents of change