1. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings PowerPoint ® Lecture Presentations for Biology Eighth Edition Neil Campbell and Jane Reece Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp Chapter 23 The Evolution of Populations

2. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Review Questions 1.Define the following: – Evolutionary Fitness – Adaptation 2.What is the advantage of a diverse gene pool for species in a changing environment. Why is low genetic diversity a threat to the survival of a species? 3.What are the 5 conditions of Hardy Weinberg equilibrium for a species that is not evolving 4.Use the 5 observations and 3 inferences of natural selection to describe how a preying mantis could evolve to resemble a leaf?

3. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Overview How Natural Selection affects populations – Directional selection – Disruptive selection – Stabilizing selection Geographic Variations in Species Sexual Selection Genetic Drift – Founder effect – Bottleneck events Gene flow

4. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings How does Natural selection affect Populations Only natural selection consistently results in adaptive evolution.

5. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Selection results in changes in allele frequency in a population Stabilizing Selection Disruptive Selection Directional Selection

6. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Directional, Disruptive, and Stabilizing Selection Three modes of natural selection: – Directional selection favors individuals at one end of the phenotypic range. – Disruptive selection favors individuals at both extremes of the phenotypic range. – Stabilizing selection favors intermediate variants and acts against extreme phenotypes.

7. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Natural Selection Original population (c ) Stabilizing selection (b) Disruptive selection (a ) Directional selection Phenotypes (fur color) Frequency of individuals Original population Evolved population

8. Geographic Clines show gradual differences in allele frequency across different environments 1.0 0.8 0.6 0.4 0.2 0 46 444240 3836 34 3230 Georgia Warm (21°C) Latitude (°N) Maine Cold (6°C) Ldh-B b allele frequency

9. Allele frequency of Sickle cell Allele vs. presence of Malaria caused by P. falciparum 0–2.5% Distribution of malaria caused by Plasmodium falciparum (a parasitic unicellular eukaryote) Frequencies of the sickle-cell allele 2.5–5.0% 7.5–10.0% 5.0–7.5% >12.5% 10.0–12.5%

10. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Sexual Selection also affects frequency of alleles in a population Many organisms do ritual displays to attract a mate. Sexual selection often results in sexual dimorphism: physical differences between the sexes

11. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Sexual Selection

12. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Random Events can also change allele frequencies in a gene pool Genetic Drift is a change in allele frequency in a gene pool due to random chance The smaller the population, the greater the influence genetic drift has on the frequency of alleles in a population. How does this connect to the conditions of Hardy Weinberg equilibrium

13. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Founder affect is an example of Genetic Drift Founder effect: occurs when a few individuals become isolated from a larger population. Allele frequencies in the small founder population can be different from those in the larger parent population.

14. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Bottleneck Effect is an example of Genetic Drift A bottleneck is a sudden reduction in population size due to a sudden, extreme change in the environment The resulting gene pool may no longer be reflective of the original population’s gene pool. Original population Bottleneck event Surviving population Gene Pool

15. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Effects of Genetic Drift: A Summary 1.Genetic drift is significant in small populations. 2.Genetic drift causes allele frequencies to change at random. 3.Genetic drift can lead to a loss of genetic variation within populations. 4.Genetic drift can cause harmful alleles to become fixed.

16. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Gene Flow can change allele frequency in a population Gene flow: the movement of alleles among populations. Can be caused by movement of fertile individuals (Immigration, Emigration) or movement of gametes (for example, pollen). Tends to reduce differences between populations over time.

17. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Gene Flow Can change allele frequency in a population Hybrid Zones are areas where two populations overlap and can interbreed How might the presence of geographic barriers promote speciation

18. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

19. Natural Selection - Adaptive Evolution (a) Color-changing ability in cuttlefish (b) Movable jaw bones in snakes Movable bones

20. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Because environments change, adaptive evolution is a continuous process. Genetic drift and gene flow are random and so do not consistently lead to adaptive evolution as they can increase or decrease the match between an organism and its environment.

21. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Natural Selection Cannot Fashion Perfect Organisms 1.Selection can act only on existing variations. 2.Evolution is limited by historical constraints. 3.Adaptations are often compromises. 4.Chance, natural selection, and the environment interact.

22. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings You should now be able to: 1.Explain the source of genetic variation. 2.Explain how sexual recombination generates genetic variability. 3.Define the terms population, species, gene pool, relative fitness, and neutral variation. 4.List the five conditions of Hardy-Weinberg equilibrium.

23. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings 5.Apply the Hardy-Weinberg equation to a population genetics problem. 6.Explain why natural selection is the only mechanism that consistently produces adaptive change. 7.Explain the role of population size in genetic drift.

24. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings 8.Distinguish among the following sets of terms: directional, disruptive, and stabilizing selection, sexual selection. 9.List four reasons why natural selection cannot produce perfect organisms.