1. Copyright © 2009 Pearson Education, Inc., publishing as Pearson Benjamin Cummings. BIOLOGY A GUIDE TO THE NATURAL WORLD FOURTH EDITION DAVID KROGH The Means of Evolution: Microevolution

2. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. 17.1 What Is It That Evolves?

3. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Microevolution The smallest unit that evolves is a population. A population is defined as all the members of a single species living in a defined geographical area at a given time.

4. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Microevolution Figure 17.1 original coloration lighter coloration population A expanse of barren terrain population B darker coloration (a)Original environment (b)Altered environment

5. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. 17.2 Evolution as a Change in the Frequency of Alleles

6. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Genetic Basis of Evolution Genes exist in variant forms called alleles. In most species, no individual will possess more than two alleles for a given gene. One allele comes from the individual’s father, the other allele comes from the individual’s mother.

7. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Genetic Basis of Evolution A population, however, is likely to possess many alleles for a given gene.

8. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Genetic Basis of Evolution The sum total of alleles in a population is referred to as that population’s gene pool. The basis of evolution is a change in the frequency of alleles in a gene pool.

9. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Genetic Basis of Evolution Figure 17.2 maternal chromosome 3 paternal chromosome 3 alleles dark colorationlight coloration alleles maternal chromosome 3 paternal chromosome 3

10. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Genetic Basis of Evolution To the extent that a given set of alleles increases in frequency from one generation to the next within a population, the phenotypes, or observable characteristics, produced by those alleles will be exhibited to a greater extent within the population.

11. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Genetic Basis of Evolution With such a change, a population can be said to have evolved. Evolution at this level is referred to as microevolution: a change of allele frequencies within a population over a relatively short period of time.

12. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Genetic Basis of Evolution Conversely, macroevolution, a product of microevolution, is evolution on a larger scale. Macroevolution is evolution that results in the formation of new species or other large groupings of living things.

13. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Evolution and Genetics PLAY Animation 17.1: Evolution and Genetics

14. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. 17.3 Five Agents of Microevolution

15. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Five Agents of Microevolution Five evolutionary forces can result in changes in allele frequencies within a population. These agents of microevolution are: –mutation –gene flow –genetic drift –sexual selection –natural selection

16. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Five Agents of Microevolution Table 17.1

17. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Mutations A mutation is any permanent alteration in an organism’s DNA, and some mutations are heritable, meaning they can be passed on from one generation to the next.

18. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Mutations Figure 17.3 Deletion incorrect nucleotide sequence correct nucleotide sequence incomplete chromosome 5 complete chromosome 5 (a)(b) Normal Point mutation

19. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Mutations Mutation happens fairly infrequently, and most mutations either have no effect or are harmful. Yet rare adaptive mutations are vital to evolution in that they are the only means by which entirely new genetic information comes into being.

20. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Gene Flow Gene flow is the movement of genes from one population to another. It takes place through migration, meaning the movement of individuals from one population into the territory of another.

21. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Gene Flow Figure 17.4 (a)(b)Hawaiian silverswordTarweeds in California North America Pacific Ocean Hawaiian Islands

22. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Genetic Drift Genetic drift, the chance alteration of allele frequencies in a population, has its greatest effects on small populations.

23. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Genetic Drift Figure 17.5 little change in allele frequency (no alleles lost) dramatic change in allele frequency (potential to lose one allele) 50% of population survives, including 450 allele carriers 50% of population survives, with no allele carrier among them allele frequency = 1,000 10,000 = 10% allele frequency == 9% 450 5,000 allele frequency == 10% 1 10 allele frequency == 0% 0 5 (a)(b)Large population = 10,000 (allele carriers in red) Small population = 10 (allele carriers in red)

24. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Genetic Drift Genetic drift can have large effects on small populations through two common scenarios: –the Bottleneck Effect –the Founder Effect

25. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Genetic Drift: Bottleneck Effect The first of these is the bottleneck effect, defined as a change in allele frequencies due to chance during a sharp reduction in a population’s size.

26. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Genetic Drift: Bottleneck Effect Figure 17.6 “bottleneck” only allows a few individuals through Original population, original allele frequency. Hunting of seals in late 1800s greatly reduced population size. Surviving population had different allele frequency and little genetic diversity. This different allele frequency is reflected in today's population.

27. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Genetic Drift: Founder Effect The second is the founder effect: the fact that when a small subpopulation migrates to a new area to start a new population, it is likely to bring with it only a portion of the original population’s gene pool.

28. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Sexual Selection Sexual selection is a form of natural selection that can affect the frequency of alleles in a gene pool. It occurs when differences in reproductive success arise because of differential success in mating.

29. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Sexual Selection A given male in a population may, for example, sire many more offspring than the average male in the population. If so, this male’s alleles will increase in frequency in the next generation of the population.

30. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Sexual Selection Figure 17.7

31. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Natural Selection In a population, some individuals will be more successful than others in surviving, and hence reproducing, owing to traits that better adapt them to their environment. This phenomenon is known as natural selection.

32. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Natural Selection Natural selection is the only agent of microevolution that consistently acts to adapt organisms to their environments. As such, it is generally regarded as the most powerful force underlying evolution.

33. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Natural Selection Figure 17.9 Shift of average beak depth following drought average beak depth, 1976 average beak depth, 1978 1978 beak depth 1976 60 40 20 0 5 6 789 Beak depth (mm) 80 1011121314 Number of individuals

34. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. 17.4 Natural Selection and Evolutionary Fitness

35. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Evolutionary Fitness The phrase “survival of the fittest” is misleading because it implies that evolution works to produce generally superior beings who would be successful competitors in any environment.

36. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Evolutionary Fitness Evolutionary fitness, however, has to do only with the relative reproductive success of individuals in a given environment at a given time.

37. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Evolutionary Fitness One individual is said to be more fit than another to the extent that it has more offspring than another.

38. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Evolutionary Fitness Individuals are not born with invariable levels of fitness; instead, fitness can change in accordance with changes in the surrounding environment.

39. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. 17.5 Three Modes of Natural Selection

40. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Stabilizing Selection Natural selection has three modes: –stabilizing selection –directional selection –disruptive selection

41. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Three Modes of Natural Selection Figure 17.10 Stabilizing selectionDirectional selectionDisruptive selection Range of a particular characteristic (in this instance, lightness or darkness of coloration) In stabilizing selection, individuals that possess extreme values of a characteristic—here, both the lightest and the darkest colors—are selected against and die or fail to reproduce. Over succeeding generations, an increasing proportion of the population becomes average in coloration. In directional selection, one of the extremes of a characteristic is better suited to the environment, meaning that individuals at the other extreme are selected against. Over succeeding generations, the coloration of the population moves in a direction—in this case toward darker coloration. In disruptive selection, individuals with average coloration are selected against and die. Over succeeding generations, part of the population becomes lighter, while part becomes darker meaning the range of color variation in the population has increased. Time (many generations)

42. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Three Modes of Natural Selection Stabilizing selection moves a given character in a population toward intermediate forms and hence tends to preserve the status quo.

43. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Stabilizing Selection Figure 17.11 infant births Infant mortality is lowest among infants of average birth weight. infant deaths 30 20 10 0 6 2 345 Birth weight in pounds 40 50 60 70 7891011 5 0 15 20 Percent of infant deaths Percent of births in population

44. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Directional Selection Directional selection moves a given character toward one of its extreme forms.

45. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Directional Selection Figure 17.12 A. afarensis A. africanus H. habilis H. ergaster H. erectus H. sapiens Cranial capacity (the volume of the skull) has increased in hominins over time 4321Present 1400 1200 1000 800 600 400 200 Earliest fossil record (millions of years ago) Cranial capacity (cubic centimeters)

46. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Disruptive Selection Disruptive selection moves a given character toward two extreme forms.

47. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Disruptive Selection Figure 17.13

48. Copyright © 2009 Pearson Education, Inc., publishing as Benjamin Cummings. Three Modes of Natural Selection PLAY Animation 17.2: Three Modes of Natural Selection