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

19. 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.

20. 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.

21. 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.

22. 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.

23. 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.

24. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Speciation- CH 24 Brief Lecture What is a species? How do populations become separate species? Reproductive Barriers – Prezygotic – Habitat, Temporal, Behavioral, Mechanical and Gametic Isolation – Postzygotic – Reduced Hybrid Viability, Fertility, and Hybrid Breakdown – Polyploidy- a common mechanism in plants

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26. Reproductive Barriers Between Species Prezygotic Barriers Habitat Isolation Temporal Isolation Behavioral Isolation Mating attempt Mechanical Isolation Gametic Isolation Fertilization Reduced Hybrid Viability Reduced Hybrid Fertility Postzygotic Barriers Hybrid Breakdown Viable, fertile offspring (b) (d) (c) (g)(h) (i) (j) (l) (k)

27. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Concept 24.2: Speciation can take place with or without geographic separation Speciation can occur in two ways: – Allopatric speciation: geographic barrier separates populations. – Sympatric speciation: no geographic barrier

28. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Speciation (a) Allopatric speciation (b) Sympatric speciation

29. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Allopatric (“Other Country”) Speciation In allopatric speciation, gene flow is interrupted or macroevolution is the cumulative effect of many speciation and extinction events. reduced when a population is divided into geographically isolated subpopulations … A geographic barrier separates the original population. Separate populations may evolve independently through mutation, natural selection, and genetic drift. Reproductive isolation between populations generally increases as the distance between them increases. Barriers to reproduction are intrinsic; separation itself is not a biological barrier.

30. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Allopatric Speciation A. harrisi A. leucurus

31. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Continental Drift Caused Allopatric Speciation Mantellinae (Madagascar only): 100 species Rhacophorinae (India/Southeast Asia): 310 species Other Indian/ Southeast Asian frogs Millions of years ago (mya) 1 23 1 2 3 100 80 60 40 20 0 88 mya65 mya 56 mya India Madagascar

32. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Sympatric Speciation: Habitat Differentiation and Sexual Selection Sympatric speciation can also result from the appearance of new ecological niches. For example, the North American maggot fly can live on native hawthorn trees as well as more recently introduced apple trees. Sexual selection can drive sympatric speciation. Sexual selection for mates of different colors has likely contributed to the speciation in cichlid fish in Lake Victoria.

33. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Allopatric and Sympatric Speciation May Cause Reproductive Isolation: A Review In allopatric speciation, geographic separation restricts gene flow between populations. Reproductive isolation may then arise by natural selection, genetic drift, or sexual selection in the isolated populations. Even if contact is restored between populations, interbreeding is prevented. In sympatric speciation, a reproductive barrier isolates a subset of a population without geographic separation from the parent species. Sympatric speciation can result from polyploidy, natural selection, or sexual selection.

34. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Concept 24.3: Hybrid zones provide opportunities to study factors that cause reproductive isolation A hybrid zone is a region in which members of different species mate and produce hybrids. A hybrid zone can occur in a single band where adjacent species meet. Hybrids often have reduced fitness compared with parent species. The distribution of hybrid zones can be more complex if parent species are found in multiple habitats within the same region.

35. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Hybrid Zones EUROPE Fire-bellied toad range Hybrid zone Yellow-bellied toad range Yellow-bellied toad, Bombina variegata Fire-bellied toad, Bombina bombina Allele frequency (log scale) Distance from hybrid zone center (km) 4030 20 10 0 0.01 0.1 0.5 0.9 0.99

36. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Hybrid Zones over Time When closely related species meet in a hybrid zone, there are three possible outcomes: – Reinforcement -- Strengthening of reproductive barriers reducing gene flow. – Fusion -- Weakening of reproductive barriers with eventual fusion into one species. – Stabilizing -- Continued formation of hybrid individuals.

37. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Hybrid Zones Over Time Gene flow Population (five individuals are shown) Barrier to gene flow Isolated population diverges Hybrid zone Hybrid Possible outcomes : Reinforcement of gene flow barrier. OR Fusion into one species. Stability : Continued formation of hybrid individuals.

38. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Time Course of Speciation Broad patterns in speciation can be studied using the fossil record, morphological data, or molecular data. The fossil record includes examples of species that appear suddenly, persist essentially unchanged for some time, and then apparently disappear Niles Eldredge and Stephen Jay Gould coined the term punctuated equilibrium to describe periods of apparent stasis (no change) punctuated by brief periods of rapid change. The punctuated equilibrium model contrasts with a Darwinian model of gradualism: slow continuous change over time in a species’ existence.

39. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Patterns in Speciation Punctuated Equilibrium pattern Gradualism pattern Change / Time

40. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Sympatric Speciation via Polyploidy is Common in Plants 2n = 64n = 12 Failure of cell division after chromosome duplication gives rise to tetraploid tissue. 2n2n Gametes produced are diploid.. 4n4n Offspring with tetraploid karyotypes may be viable and fertile.

41. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Speciation Ancestral species: Triticum monococcum (2n = 14) AABB Wild Triticum (2n = 14) Product: AA BB DD T. aestivum (bread wheat) (2n = 42) Wild T. tauschii (2n = 14) DD