13.1 Evolution: Getting from There to Here

2. The word “evolution” refers to how an entity changes through time Darwin initially used the phrase “descent with modification” to explain the concept of evolution The concept of evolution helps explain the great paradox of biology: In life there exists both unity and diversity 13.1 Evolution: Getting fromThere to Here

3. Natural selection, the process that leads to evolution, occurs in steps 13.1 Evolution: Getting fromThere to Here • 1. Gene variation exists among individuals in a population • 2. This variation is often passed to offspring • 3. All populations overproduce offspring • 4. Individuals with traits that aid survival and reproduction have a better chance of contributing to the next generation • 5. Over time, the population changes such that the traits of the more successful reproducers are more prevalent

4. Macroevolution Evolutionary change on a grand scale Encompasses the origins of new species and major episodes of extinction Microevolution Evolutionary change on a small scale Encompasses the genetic changes that occur within a species over time These changes are the result of changes in gene frequencies 13.1 Evolution: Getting fromThere to Here

5. Jean-Baptiste Lamarck proposed that evolution occurred by inheritance of acquired characteristics Individuals passed on to offspring body and behavior changes acquired during their lives In Darwin’s theory, by contrast, variation is not created by experience It already exists when selection acts on it 13.1 Evolution: Getting fromThere to Here

6. Fig. 13.1 How did long necks evolve in giraffe

7. The Rate of Evolution • Different kinds of organisms evolve at different rates • Bacteria evolve much faster than eukaryotes • The rate of evolution also differs within the same group of species • In punctuated equilibrium, evolution occurs in spurts • In gradualism, evolution occurs in a gradual, uniform way

8. Fig. 13.2 a) Punctuated equilibrium b) Gradualism

9. 13.2 The Evidence For Evolution • Evidence for evolution comes from the following • Fossil record • Molecular record • Anatomical record

10. Fossil Record • Provides the most direct evidence for macroevolution • Fossils are the preserved remains, tracks, or traces of once-living organisms • They form when organisms become buried in sediment and calcium in hard surfaces mineralizes • Arraying fossils according to age often provides evidence of successive evolutionary change

11. Fig. 13.3 Evolution in the titanotheres Large blunt horns Small bony protuberance Hoofed mammals

12. Fossils have been found linking all the major groups Fig. 13.4 Whale “missing links” The forms linking mammals to reptiles are particularly well known

13. Molecular Record • New alleles arise by mutations and they come to predominance through favorable selection • Thus, evolutionary changes involve a continual accumulation of genetic changes • Distantly-related organisms accumulate a greater number of evolutionary differences than closely-related ones • This divergence is seen among vertebrates in the 146-amino acid hemoglobin b chain

14. Fig. 13.5 Molecules reflect evolutionary divergence The greater the evolutionary distance The greater the number of amino acid differences

15. Fig. 13.6 • This same pattern of divergence is seen with DNA sequences, such as that of the cytochrome c gene • The changes appear to accumulate at a constant rate • This phenomenon is referred to as a molecular clock Note: Different proteins evolve at different rates

16. Fig. 13.7 Relict developmental forms Anatomical Record • All vertebrates share a basic set of developmental instructions

17. Fig. 13.8 Homology among vertebrate limbs Anatomical Record • Homologous structures • Have different structure and function but are all derived from the same part of a common ancestor The same basic bones are present in each forelimb

18. Anatomical Record • Analogous structures • Resemble each other as a result of parallel evolutionary adaptations to similar environments • They are the result of convergent evolution • Different animals often adapt in similar fashion when challenged by similar opportunities

19. Fig. 13.9 Convergent evolution: many paths to one goal

20. Fig. 13.9 Convergent evolution: many paths to one goal

21. Anatomical Record • Vestigial organs • Structures that are no longer in use • The human appendix • Apes have a much larger appendix that is involved in digestion

22. 13.3 Evolution’s Critics • Critics of evolution raise seven principal objections • 1. Evolution is not solidly demonstrated • 2. There are no fossil intermediates • 3. The intelligent design argument • 4. Evolution violates the 2nd law of thermodynamics • 5. Proteins are too improbable • 6. Natural selection does not imply evolution • 7. The irreducible complexity argument All of these objections are without scientific merit!

23. Population genetics is the study of the properties of genes in a population Genetic variation in populations puzzled scientists Dominant alleles were believed to drive recessive alleles out of populations In 1908, G. Hardy and W. Weinberg pointed out that in large populations with random mating, allelefrequencies remain constant Dominant alleles do not, in fact, replace recessive ones 13.4 Genetic Change Within Populations: The Hardy-Weinberg Rule

24. Total number of individuals being considered A population that is in Hardy-Weinberg equilibrium is not evolving • Hardy and Weinberg came to their conclusion by analyzing allele frequencies in successive generations Number of individuals falling within a category • Frequency = • If a population of 100 cats has 84 black and 16 white • Then the frequencies of black and white phenotypes are 0.84 and 0.16, respectively

25. B allele  Black color b allele  White color • By convention • The more common allele (B) is designated p • The less common allele (b) is designated q • p + q = 1 • The Hardy-Weinberg equilibrium can be written as an equation • (p + q)2 = p2 + 2pq + q2 Individuals homozygous for allele b Individuals homozygous for allele B Individuals heterozygous for alleles B and b

26. = 0.4 √ 0.16 = 1 – 0.4 = 0.6 = (0.6)2 = 0.36 = 2(0.6)(0.4) = 0.48 The equation allows calculation of allele frequencies • Frequency of white (bb) cats = 16/100 = 0.16 • => q2 = 0.16 • => q= • p + q =1 => p = 1 – q What about genotype frequencies? • Frequency of the homozygous dominant genotype is • p2 36 out of 100 cats are black (BB) • Frequency of the heterozygous genotype is • 2pq 48 out of 100 cats are black (Bb)

27. Fig. 13.10

28. Hardy-Weinberg Assumptions • The Hardy-Weinberg equation is true only if the following five assumptions are met • 1. Large population size • 2. Random mating • 3. No mutation • 4. No migration • 5. No natural selection

29. 13.5 Why Allele Frequencies Change • Five evolutionary forces can significantly alter the allele frequencies of a population • 1. Mutation • 2. Migration • 3. Genetic drift • 4. Nonrandom mating • 5. Selection

30. Table 13.1 Mutation • Errors in DNA replication • The ultimate source of new variation • Mutation rates are too low to significantly alter allele frequencies on their own

31. Migration Table 13.1 • Movement of individuals from one population to another • Immigration: movement into a population • Emigration: movement out of a population • A very potent agent of change

32. Genetic Drift Table 13.1 • Random loss of alleles • More likely to occur in smaller population • Founder effect • Small group of individuals establishes a population in a new location • Bottleneck effect • A sudden decrease in population size to natural forces

33. Nonrandom Mating Table 13.1 • Mating that occurs more or less frequently than expected by chance • Inbreeding • Mating with relatives • Increases homozygosity • Outbreeding • Mating with non-relatives • Increases heterozygosity

34. Selection Table 13.1 • Some individuals leave behind more offspring than others • Artificial selection • Breeder selects for desired characteristics • Natural selection • Environment selects for adapted characteristics

35. 13.6 Forms of Selection • Selection is a statistical concept • One cannot predict the fate of any single individual • But it is possible to predict which kind of individual will tend to become more common in a population • Three types of natural selection have been identified • Stabilizing selection • Acts to eliminate both extreme phenotypes • Disruptive selection • Acts to eliminate intermediate phenotypes • Directional selection • Acts to eliminate a single extreme phenotype

36. Fig. 13.12 Three kinds of natural selection

37. Fig. 13.13 Stabilizing Selection Increase in the frequency of the intermediate phenotype In humans, infants with intermediate weight at birth have the highest survival rate In chicken, eggs of intermediate weight have the highest hatching success

38. Fig. 13.14 Disruptive Selection In the African seed-cracker finch, large- and small-beaked birds predominate Can open tough shells of large seeds • Intermediate-beaked birds are at a disadvantage • Unable to open large seeds • Too clumsy to open small seeds More adept at handling small seeds

39. Fig. 13.15 Directional Selection Drosophila flies that flew toward light were eliminated from the population The remaining flies were mated and the experiment repeated for 20 generations Phototropic flies are far less frequent in the population

40. Fig. 13.16 Sickled RBCs 13.7 Sickle-Cell Anemia • Sickle-cell anemia is a hereditary disease affecting hemoglobin molecules in the blood • It was first detected on December 31st, 1904

41. Fig. 13.17a • This causes hemoglobin molecules to clump • The result is sickled red blood cells • The sickle-cell mutation changes the 6th amino acid in the b-hemoglobin chain from glutamic acid to valine • In normal RBCs, the hemoglobin chains do not clump

42. Sickle-cell homozygosity leads to a reduced life span • Heterozygosity produces enough hemoglobin to keep RBCs healthy • The disease originated in Central Africa • It affects 1 in 500 African Americans • But it is almost unknown in other racial groups • Why is the defective allele still around? • People who are heterozygous for the sickle-cell allele have less susceptibility to malaria • This is an example of heterozygote advantage

43. Fig. 13.18 • Stabilizing selection is thus acting on the sickle-cell allele • It occurs because malarial resistance counterbalances lethal anemia

44. 13.8 Selection on Color in Guppies • Poecilia reticulata (guppy) is a popular aquarium fish • In nature, it is found in small streams in NE South America and in mountainous streams in Trinidad • Due to dispersal barriers, guppies can be found in two different pool environments • Below waterfalls, where risk of predation is high • Above waterfalls, where risk of predation is low

45. Fig. 13.19 The evolution of protective coloration in guppies A voracious predator of guppies Rarely preys on guppies

46. In the absence of predators, larger more colorful fish may produce more offspring In the presence of predators, smaller and less colorful fish are likely favored by selection • In low-predation pools, males • Display gaudy colors and spots • Reproduce at a late age • Attain larger adult sizes • In high-predation pools, males • Exhibit drab coloration • Reproduce younger • Attain relatively small adult sizes

47. Same results were obtained in field experiments Fig. 13.20 Indistinguishable from low-predation controls • The evolution of these differences in guppies was experimentally tested in laboratory greenhouses Smaller and drab in color Thus, natural selection can lead to rapid evolutionary change

48. 13.9 The Biological Species Concept • Speciation is the species-forming process • It involves progressive change • 1. Local populations become increasingly specialized • 2. Natural selection acts to keep them different enough • Ernst Mayr coined the biological species concept • “Species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups” • Reproductively isolated populations • Populations whose members do not mate with each other or who cannot produce fertile offspring

49. 13.10 Isolating Mechanisms • Reproductive isolating mechanisms are the barriers that prevent genetic exchange between species • Prezygotic isolating mechanisms • Prevent the formation of zygotes • Postzygotic isolating mechanisms • Prevent the proper functioning of zygotes after they have formed

50. Prezygotic Isolating Mechanisms