Chapter 24

1. Chapter 24 The Origin of Species

2. Overview: That “Mystery of Mysteries” • In the Galápagos Islands Darwin discovered plants and animals found nowhere else on Earth Video: Galápagos Tortoise

3. Fig. 24-1

4. Speciation, the origin of new species, is at the focal point of evolutionary theory • Evolutionary theory must explain how new species originate and how populations evolve • Microevolution consists of adaptations that evolve within a population, confined to one gene pool • Macroevolution refers to evolutionary change above the species level Animation: Macroevolution

5. Concept 24.1: The biological species concept emphasizes reproductive isolation • Species is a Latin word meaning “kind” or “appearance” • Biologists compare morphology, physiology, biochemistry, and DNA sequences when grouping organisms

6. The Biological Species Concept • The biological species concept states that a species is a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring; they do not breed successfully with other populations • Gene flow between populations holds the phenotype of a population together

7. Fig. 24-2 (a) Similarity between different species (b) Diversity within a species

8. Fig. 24-2a (a) Similarity between different species

9. Fig. 24-2b (b) Diversity within a species

10. Fig. 24-3 EXPERIMENT Example of a gene tree for population pair A-B Allele Population Gene flow event 1 B Allele 1 is more closely related to alleles 2, 3, and 4 than to alleles 5, 6, and 7. Inference: Gene flow occurred. A 2 3 A 4 A 5 B Alleles 5, 6, and 7 are more closely related to one another than to alleles in population A. Inference: No gene flow occurred. B 6 B 7 RESULTS Pair of populations with detected gene flow Estimated minimum number of gene flow events to account for genetic patterns Distance between populations (km) A-B 5 340 3 K-L 720 A-C 1,390 2–3 1,190 B-C 2 F-G 760 2 G-I 2 1,110 1–2 1,310 C-E

11. Fig. 24-3a EXPERIMENT Example of a gene tree for population pair A-B Allele Population Gene flow event 1 B Allele 1 is more closely related to alleles 2, 3, and 4 than to alleles 5, 6, and 7. Inference: Gene flow occurred. 2 A A 3 4 A 5 B Alleles 5, 6, and 7 are more closely related to one another than to alleles in population A. Inference: No gene flow occurred. 6 B B 7

12. Fig. 24-3b RESULTS Pair of populations with detected gene flow Estimated minimum number of gene flow events to account for genetic patterns Distance between populations (km) A-B 5 340 K-L 3 720 A-C 2–3 1,390 2 B-C 1,190 F-G 2 760 2 1,110 G-I C-E 1–2 1,310

13. Fig. 24-3c Grey-crowned babblers

14. Reproductive Isolation • Reproductive isolation is the existence of biological factors (barriers) that impede two species from producing viable, fertile offspring • Hybrids are the offspring of crosses between different species • Reproductive isolation can be classified by whether factors act before or after fertilization

15. Prezygotic barriers block fertilization from occurring by: • Impeding different species from attempting to mate • Preventing the successful completion of mating • Hindering fertilization if mating is successful

16. Habitat isolation: Two species encounter each other rarely, or not at all, because they occupy different habitats, even though not isolated by physical barriers

17. Fig. 24-4 Prezygotic barriers Postzygotic barriers Habitat Isolation Behavioral Isolation Temporal Isolation Mechanical Isolation Gametic Isolation Reduced Hybrid Viability Reduced Hybrid Fertility Hybrid Breakdown Individuals of different species Viable, fertile offspring Mating attempt Fertilization (c) (e) (f) (a) (g) (h) (l) (i) (d) (j) (b) (k)

18. Fig. 24-4a Prezygotic barriers Mechanical Isolation Habitat Isolation Temporal Isolation Behavioral Isolation Individuals of different species Mating attempt (e) (f) (a) (c) (d) (b)

19. Fig. 24-4i Prezygotic barriers Postzygotic barriers Reduced Hybrid Viability Reduced Hybrid Fertility Hybrid Breakdown Gametic Isolation Viable, fertile offspring Fertilization (h) (g) (l) (i) (j) (k)

20. Fig. 24-4b Prezygotic barriers Temporal Isolation Mechanical Isolation Habitat Isolation Behavioral Isolation Individuals of different species Mating attempt

21. Fig. 24-4j Postzygotic barriers Prezygotic barriers Reduced Hybrid Fertility Reduced Hybrid Viability Hybrid Breakdown Gametic Isolation Viable, fertile offspring Fertilization

22. Fig. 24-4c (a) Water-dwelling Thamnophis

23. Fig. 24-4d (b) Terrestrial Thamnophis

24. Temporal isolation: Species that breed at different times of the day, different seasons, or different years cannot mix their gametes

25. Fig. 24-4e (c) Eastern spotted skunk (Spilogale putorius)

26. Fig. 24-4f (d) Western spotted skunk (Spilogale gracilis)

27. Behavioral isolation: Courtship rituals and other behaviors unique to a species are effective barriers Video: Albatross Courtship Ritual Video: Giraffe Courtship Ritual Video: Blue-footed Boobies Courtship Ritual

28. Fig. 24-4g (e) Courtship ritual of blue- footed boobies

29. Mechanical isolation: Morphological differences can prevent successful mating

30. Fig. 24-4h (f) Bradybaena with shells spiraling in opposite directions

31. Gametic isolation: Sperm of one species may not be able to fertilize eggs of another species

32. Fig. 24-4k (g) Sea urchins

33. Postzygotic barriers prevent the hybrid zygote from developing into a viable, fertile adult: • Reduced hybrid viability • Reduced hybrid fertility • Hybrid breakdown

34. Reduced hybrid viability: Genes of the different parent species may interact and impair the hybrid’s development

35. Fig. 24-4l (h) Ensatina hybrid

36. Reduced hybrid fertility: Even if hybrids are vigorous, they may be sterile

37. Fig. 24-4m (i) Donkey

38. Fig. 24-4n (j) Horse

39. Fig. 24-4o (k) Mule (sterile hybrid)

40. Hybrid breakdown: Some first-generation hybrids are fertile, but when they mate with another species or with either parent species, offspring of the next generation are feeble or sterile

41. Fig. 24-4p (l) Hybrid cultivated rice plants with stunted offspring (center)

42. Limitations of the Biological Species Concept • The biological species concept cannot be applied to fossils or asexual organisms (including all prokaryotes)

43. Other Definitions of Species • Other species concepts emphasize the unity within a species rather than the separateness of different species • The morphological species concept defines a species by structural features • It applies to sexual and asexual species but relies on subjective criteria

44. The ecological species concept views a species in terms of its ecological niche • It applies to sexual and asexual species and emphasizes the role of disruptive selection • The phylogenetic species concept: defines a species as the smallest group of individuals on a phylogenetic tree • It applies to sexual and asexual species, but it can be difficult to determine the degree of difference required for separate species

45. Concept 24.2: Speciation can take place with or without geographic separation • Speciation can occur in two ways: • Allopatric speciation • Sympatric speciation

46. Fig. 24-5 (a) Allopatric speciation (b) Sympatric speciation

47. Allopatric (“Other Country”) Speciation • In allopatric speciation, gene flow is interrupted or reduced when a population is divided into geographically isolated subpopulations

48. The Process of Allopatric Speciation • The definition of barrier depends on the ability of a population to disperse • Separate populations may evolve independently through mutation, natural selection, and genetic drift

49. Fig. 24-6 A. harrisi A. leucurus

50. Evidence of Allopatric Speciation • Regions with many geographic barriers typically have more species than do regions with fewer barriers