Chapter 17 Processes of Evolution (Sections 17.1 - 17.5)

1. Chapter 17Processes of Evolution (Sections 17.1 - 17.5)

2. 17.1 Rise of the Super Rats • Rats that carry pathogens and parasites associated with infectious diseases thrive wherever people do • Fighting rats with poisons such as warfarin usually doesn’t exterminate rat populations – instead, it selects for rats that are genetically resistant to the poisons

3. Rats as Pests • Rats infesting rice fields in the Philippine Islands ruin more than 20% of the crop

4. 17.2 Individuals Don’t Evolve, Populations Do • Evolution starts with mutations in individuals, which introduces new alleles into a population • Sexual reproduction can quickly spread a mutation through a population • population • A group of organisms of the same species who live in a specific location and breed with one another more often than they breed with members of other populations

5. Variation in Populations • Individuals of a population share morphological, physiological, and behavioral traits with a heritable basis • Variations within a population arise from different alleles of shared genes: A trait with only two forms is dimorphic; traits with more than two distinct forms are polymorphic • Traits that vary continuously often arise by interactions among alleles of several genes, and may be influenced by environmental factors

6. Phenotypic Variation in Humans

7. Sources of Variation in Traits Genetic Event Effect • Mutation >Source of new alleles • Crossing over >Introduces new combinations of alleles into chromosomes • Independent >Mixes maternal and paternal assortment chromosomes • Fertilization >Combines alleles from two parents • Changes in >Transposition, duplication, or chromosome loss of chromosomes number or structure

8. An Evolutionary View of Mutations • Mutations are the original source of new alleles; many are lethal or neutral mutations • lethal mutation • Mutation that drastically alters phenotype • Causes death • neutral mutation • A mutation that has no effect on survival or reproduction

9. Adaptive Mutations • Occasionally, a change in the environment favors a mutation that had previously been neutral or even somewhat harmful • Through natural selection, a beneficial mutation tends to increase in frequency in a population over generations • Mutations are the source of Earth’s staggering biodiversity

10. Allele Frequencies • All alleles in a population form a gene pool • Microevolution (changes in the allele frequencies of a population) occurs constantly by processes of mutation, natural selection, genetic drift, and gene flow

11. Key Terms • gene pool • All of the alleles of all of the genes in a population; a pool of genetic resources • microevolution • Change in allele frequencies in a population or species • allele frequency • Abundance of a particular allele among members of a population

12. Genetic Equilibrium • A theoretical reference point, genetic equilibrium, occurs when the allele frequencies of a population do not change • It requires five conditions that are never met in nature, so natural populations are never in genetic equilibrium • genetic equilibrium • Theoretical state in which a population is not evolving

13. Conditions of Genetic Equilibrium • Five theoretical conditions of genetic equilibrium: (1) Mutations never occur (2) Population is infinitely large (3) Population is isolated from all other populations of the species (no gene flow) (4) Mating is random (5) All individuals survive and produce the same number of offspring

14. Key Concepts • Microevolution • Individuals of a population inherit different alleles, and so they differ in phenotype • Over generations, any allele may increase or decrease in frequency in a population • Such change is called microevolution

15. ANIMATION: Antibiotic resistance To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

16. 17.3 A Closer Look at Genetic Equilibrium • Researchers know whether a population is evolving by tracking deviations from a baseline of genetic equilibrium • We use deviations from genetic equilibrium to study how a population is evolving

17. The Hardy–Weinberg Formula • Gene pools can remain stable only when the five theoretical conditions of genetic equilibrium are being met • Hardy and Weinberg developed a simple formula that can be used to track whether a population of any sexually reproducing species is in a state of genetic equilibrium • The following example illustrates how the Hardy-Weinberg formula is used

18. Allele Frequencies in Butterflies (1) • Consider a hypothetical gene that encodes a blue pigment in butterflies: • Two alleles of this gene, B and b, are codominant • A butterfly homozygous for the B allele (BB) has dark-blue wings • A butterfly homozygous for the b allele (bb) has white wings • A heterozygous butterfly (Bb) has medium-blue wings

19. Allele Frequencies in Butterflies (2) • At genetic equilibrium, the proportions of the wing-color genotypes are: p2(BB) + 2pq(Bb) + q2(bb) = 1.0 where p and q are the frequencies of alleles B and b • This is the Hardy–Weinberg equilibrium equation; it defines the frequency of a dominant allele (B) and a recessive allele (b) for a gene that controls a particular trait in a population

20. Allele Frequencies in Butterflies (3) • The frequencies of B and b must add up to 1.0 • Example: If B occupies 90% of the loci, then b must occupy the remaining 10 percent (0.9 + 0.1 = 1.0) • No matter what the proportions: p + q = 1.0

21. Allele Frequencies in Butterflies (4) • The Punnett square below shows the genotypes possible in the next generation (BB, Bb, and bb) • The frequencies of the three genotypes add up to 1.0: p2 + 2pq + q2 = 1.0

22. Allele Frequencies in Butterflies (4) p B q b B BB (p2 ) p Bb (pq) bb (q2) q b Bb (pq) p. 260

23. Allele Frequencies in Butterflies (5) • If 1,000 individuals each produces two gametes: • 490 BB individuals make 980 B gametes • 420 Bb individuals make 420 B and 420 b gametes • 90 bb individuals make 180 b gametes • The frequency of alleles B and b among 2,000 gametes is: B = (980 + 420)÷ 2,000 alleles = 1,400 ÷ 2,000 = 0.7 = p b = (180 + 420) ÷ 2,000 alleles = 600 ÷2,000 = 0.3 = q

24. Allele Frequencies in Butterflies (6) • At fertilization, gametes combine at random and start a new generation • If the population size stays constant at 1,000, there will be 490 BB, 420 Bb, and 90 bb individuals • Allele frequencies for dark-blue, medium-blue, and white wings are the same as they were in the original gametes – the population is not evolving

25. Frequencies of Wing-Color Alleles

26. Starting Population Frequencies of Wing-Color Alleles 490 BB butterflies dark-blue wings 420 Bb butterflies medium-blue wings 90 bb butterflies white wings 2nd Generation 490 BB butterflies dark-blue wings 420 Bb butterflies medium-blue wings 90 bb butterflies white wings 3rd Generation 490 BB butterflies dark-blue wings 420 Bb butterflies medium-blue wings 90 bb butterflies white wings Fig. 17.3, p. 260

27. ANIMATION: How to find out if a population is evolving To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

28. Applying the Rule • In the real world, researchers can use the Hardy–Weinberg formula to estimate the frequency of carriers of alleles that cause genetic traits and disorders • Example: Hereditary hemochromatosis (HH) in Ireland • If the frequency of the autosomal recessive allele that causes HH is q = 0.14, then p = 0.86 • The carrier frequency (2pq) is calculated to be about 0.24 • Such information is useful to doctors and to public health professionals

29. 17.4 Patterns of Natural Selection • Natural selection occurs in three different patterns, depending on the organisms involved and their environment • natural selection • Process in which environmental pressures result in differential survival and reproduction of individuals of a population who vary in details of shared, heritable traits

30. Three Patterns of Natural Selection • Directional selection shifts the range of variation in traits in one direction • Stabilizing selection favors intermediate forms of a trait • Disruptive selection favors forms at the extremes of a range of variation

31. population before selection Three Patterns of Natural Selection directional selection stabilizing selection disruptive selection Fig. 17.4, p. 261

32. 17.5 Directional Selection • Directional selection shifts an allele’s frequency in a consistent direction, so forms at one end of a range of phenotypic variation become more common over time • directional selection • Mode of natural selection in which phenotypes at one end of a range of variation are favored

33. Directional Selection • Bell-shaped curves indicate continuous variation in a butterfly wing-color trait • Red arrows show which forms are being selected against; green, forms that are being favored

34. Directional Selection Fig. 17.5a, p. 262

35. Directional Selection Number of individuals in population Time 1 Range of values for the trait Fig. 17.5a, p. 262

36. Directional Selection Fig. 17.5b, p. 262

37. Directional Selection Time 2 Fig. 17.5b, p. 262

38. Directional Selection Fig. 17.5c, p. 262

39. Directional Selection Time 3 Fig. 17.5c, p. 262

40. Number of individuals in population Time 1 Range of values for the trait Time 2 Time 3 Directional Selection Stepped Art Fig. 17.5, p. 262

41. ANIMATION: Directional selection To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

42. The Peppered Moth • The peppered moth’s coloration camouflages it from predatory birds • When the air was clean, trees were light-colored, and so were most peppered moths • When smoke from coal-burning factories changed the environment, predatory birds ate more white moths – selection pressure favored darker moths

43. Directional Selection: Peppered Moth

44. Directional Selection: Peppered Moth Fig. 17.6a, p. 262

45. Directional Selection: Peppered Moth Fig. 17.6a, p. 262

46. Directional Selection: Peppered Moth Fig. 17.6b, p. 262

47. Directional Selection: Peppered Moth Fig. 17.6b, p. 262

48. ANIMATION: Change in moth population To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

49. Rock Pocket Mice • Directional selection also affects the color of rock pocket mice in Arizona’s Sonoran Desert • Mice with light fur are more common in areas with light-colored granite; mice with dark fur are more common in areas with dark basalt • Mice with coat colors that do not match their surroundings are more easily seen by predators, so they are preferentially eliminated from the populations

50. Directional Selection: Rock Pocket Mice