1. A Guide to the Natural World David Krogh © 2011 Pearson Education, Inc. Chapter 17 Lecture Outline The Means of Evolution: Microevolution Biology Fifth Edition

2. © 2011 Pearson Education, Inc. 17.1 What Is It That Evolves?

3. © 2011 Pearson Education, Inc. 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. © 2011 Pearson Education, Inc. original coloration (a) Original environment (b) Altered environment lighter coloration expanse of barren terrain darker coloration population A population B Figure 17.1

5. © 2011 Pearson Education, Inc. 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.

6. © 2011 Pearson Education, Inc. Genetic Basis of Evolution A population, however, is likely to possess many alleles for a given gene.

7. © 2011 Pearson Education, Inc. 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.

8. © 2011 Pearson Education, Inc. 17.2 Evolution as a Change in the Frequency of Alleles

9. © 2011 Pearson Education, Inc. Figure 17.2 maternal chromosome 3 paternal chromosome 3 maternal chromosome 3 paternal chromosome 3 alleles dark coloration light coloration a1a1 a2a2 a2a2 a4a4

10. © 2011 Pearson Education, Inc. 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. © 2011 Pearson Education, Inc. 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. © 2011 Pearson Education, Inc. 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. © 2011 Pearson Education, Inc. 17.3 Five Agents of Microevolution

14. © 2011 Pearson Education, Inc. 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

15. © 2011 Pearson Education, Inc. Agents of Change: Five Forces That Can Bring about Change in Allele Frequencies in a Population Agent Description Mutation Gene flow Genetic drift Sexual selection Natural selection Alteration in an organism’s DNA; generally has no effect or a harmful effect. But beneficial or “adaptive” mutations are indispensable to evolution. The movement of alleles from one population to another. Occurs when individuals move between populations or when one population of a species joins another, assuming the second population has different allele frequencies than the first. Chance alteration of gene frequencies in a population. Most strongly affects small populations. Can occur when populations are reduced to small numbers (the bottleneck effect) or when a few individuals from a population migrate to a new, isolated location and start a new population (the founder effect). Occurs when some members of a population mate more often than other members. Some individuals will be more successful than others in surviving and hence reproducing, owing to traits that give them a better “fit” with their environment. The alleles of those who reproduce more will increase in frequency in a population. Table 17.1

16. © 2011 Pearson Education, Inc. 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.

17. © 2011 Pearson Education, Inc. Mutations Figure 17.3 Normal Point mutation incorrect nucleotide sequence correct nucleotide sequence Normal Deletion complete chromosome 5 incomplete chromosome 5 (a) (b)

18. © 2011 Pearson Education, Inc. 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.

19. © 2011 Pearson Education, Inc. 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.

20. © 2011 Pearson Education, Inc. Figure 17.4 (a) Hawaiian silversword(b) Tarweeds in California North America Hawaiian Islands Pacific Ocean gene flow

21. © 2011 Pearson Education, Inc. Genetic Drift Genetic drift, the chance alteration of allele frequencies in a population, has its greatest effects on small populations.

22. © 2011 Pearson Education, Inc. Genetic Drift Figure 17.5

23. © 2011 Pearson Education, Inc. Genetic Drift Genetic drift can have large effects on small populations through two common scenarios: the Bottleneck Effect the Founder Effect

24. © 2011 Pearson Education, Inc. 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.

25. © 2011 Pearson Education, Inc. 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.

26. © 2011 Pearson Education, Inc. 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.

27. © 2011 Pearson Education, Inc. 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.

28. © 2011 Pearson Education, Inc. 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.

29. © 2011 Pearson Education, Inc. Sexual Selection Figure 17.7

30. © 2011 Pearson Education, Inc. 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.

31. © 2011 Pearson Education, Inc. 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.

32. © 2011 Pearson Education, Inc. Number of individuals 80 60 40 20 0 6 5 8710 9 11 1213 14 1978 beak depth 1976 average beak depth, 1976 average beak depth, 1978 Beak depth (mm) Shift of average beak depth following drought Figure 17.9

33. © 2011 Pearson Education, Inc. 17.4 Natural Selection and Evolutionary Fitness

34. © 2011 Pearson Education, Inc. 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.

35. © 2011 Pearson Education, Inc. Evolutionary Fitness Evolutionary fitness, however, has to do only with the relative reproductive success of individuals in a given environment at a given time.

36. © 2011 Pearson Education, Inc. Evolutionary Fitness One individual is said to be more fit than another to the extent that it has more offspring than another.

37. © 2011 Pearson Education, Inc. Evolutionary Fitness Individuals are not born with invariable levels of fitness; instead, fitness can change in accordance with changes in the surrounding environment.

38. © 2011 Pearson Education, Inc. 17.5 Three Modes of Natural Selection

39. © 2011 Pearson Education, Inc. Stabilizing Selection Natural selection has three modes: stabilizing selection directional selection disruptive selection

40. © 2011 Pearson Education, Inc. Figure 17.10 Stabilizing selection Directional selectionDisruptive selection Time (many generations) Range of a particular characteristic (in this instance, lightness or darkness of coloration) In stabilizing selection, individuals that possess extreme values of a characteristic 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.

41. © 2011 Pearson Education, Inc. 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.

42. © 2011 Pearson Education, Inc. Stabilizing Selection Figure 17.11 70 0 50 60 40 30 20 10 32456789 11 0 5 10 15 20 infant births infant deaths Birth weight in pounds Percent of infant deaths Percent of births in population Infant mortality is lowest among infants of average birth weight

43. © 2011 Pearson Education, Inc. Directional Selection Directional selection moves a given character toward one of its extreme forms.

44. © 2011 Pearson Education, Inc. Directional Selection Figure 17.12 Cranial capacity (the volume of the skull) has increased in hominins over time H. sapiens H. erectus H. ergaster H. habilis A. africanus A. afarensis Earliest fossil record (millions of years ago) Present 1400 1200 1000 800 600 400 200 Cranial capacity (cubic centimeters) 4 321

45. © 2011 Pearson Education, Inc. Disruptive Selection Disruptive selection moves a given character toward two extreme forms.

46. © 2011 Pearson Education, Inc. Disruptive Selection Figure 17.13