Speciation and Macroevolution

1. Speciation and Macroevolution Chapter 20

2. Learning Objective 1 • What is the biological species concept? • List two potential problems with the concept

3. Biological Species Concept • Species • one or more populations • members interbreed in nature • produce fertile offspring • do not interbreed with different species

4. Sterile Hybrid

5. Problems • Biological species concept applies only to sexually reproducing organisms • Individuals assigned to different species may occasionally successfully interbreed

6. KEY CONCEPTS • According to the biological species concept, a species consists of individuals that can successfully interbreed with one another but not with individuals from other species

7. Learning Objective 2 • What is the significance of reproductive isolating mechanisms? • Distinguish among different prezygotic and postzygotic barriers

8. Reproductive Isolating Mechanisms • Restrict gene flow between species • Prezygotic barriers • prevent fertilization from taking place • Postzygotic barriers • prevent gene flow after fertilization has taken place (reproductive isolating mechanisms)

9. Prezygotic Barriers (1) • Temporal isolation • two species reproduce at different times of day, season, or year • Habitat isolation • two closely related species live and breed in different habitats in same geographic area

10. Temporal Isolation

11. Fig. 20-2a, p. 430

12. Fig. 20-2b, p. 430

13. Wood frog Leopard frog Mating activity March 1 April 1 May 1 Fig. 20-2c, p. 430

14. Insert “Temporal isolation among cicadas” temporal_isolation.swf

15. Prezygotic Barriers (2) • Behavioral isolation • distinctive courtship behaviors prevent mating between species • Mechanical isolation • incompatible structural differences in reproductive organs of similar species

16. Behavioral Isolation

17. Mechanical Isolation

18. Prezygotic Barriers (3) • Gametic isolation • gametes from different species are incompatible because of molecular and chemical differences

19. Postzygotic Barriers (1) • Hybrid inviability • interspecific embryos die during development • Hybrid sterility • prevents interspecific hybrids that survive to adulthood from reproducing successfully

20. Hybrid Sterility

21. Postzygotic Barriers (2) • Hybrid breakdown • prevents offspring of hybrids that survive to adulthood and successfully reproduce from reproducing beyond one or a few generations

22. KEY CONCEPTS • The evolution of different species begins with reproductive isolation, in which two populations are no longer able to interbreed successfully

23. Insert “Reproductive isolating mechanisms” isolating_mechanisms_m.swf

24. Explore reproductive isolation by clicking on the figures in ThomsonNOW

25. Learning Objective 3 • What is allopatric speciation? • Give an example

26. Allopatric Speciation (1) • Evolution of a new species • from ancestral population • Population becomes geographically isolated from rest of species • subsequently diverges

27. Allopatric Speciation (2) • More likely to occur if original isolated population is small • makes genetic drift more significant • Examples: • Death Valley pupfishes • Kaibab squirrels • Porto Santo rabbits

28. Allopatric Speciation

29. KEY CONCEPTS • In allopatric speciation, populations diverge into different species due to geographic isolation, or physical separation

30. Insert “Allopatric speciation on an archipelago” archipelago.swf

31. Explore allopatric speciation by clicking on the figures in ThomsonNOW.

32. Learning Objective 4 • What is sympatric speciation? • Give plant and animal examples

33. Sympatric Speciation • Does not require geographic isolation • More common in plants than animals

34. Sympatric Speciation in Plants • Usually results from allopolyploidy • polyploid individual (>2 sets of chromosomes) is hybrid derived from two species • Examples: • kew primroses • hemp nettles

35. Allopolyploidy in Plants

36. Species A Species B 2 n = 6 2 n = 4 P generation n = 2 n = 3 Gametes Hybrid AB F1 generation Fig. 20-9a, p. 435

37. No doubling of chromosome number Doubling of chromosome number 2 n = 10 Chromosomes either cannot pair or go through erratic meiosis Pairing now possible during meiosis n = 5 No gametes or sterile gametes — no sexual reproduction possible Viable gametes — sexual reproduction possible (self-fertilization) Fig. 20-9b, p. 435

38. Species A Species B 2 n = 4 2 n = 6 P generation n = 2 n = 3 Gametes Hybrid AB F1 generation No doubling of chromosome number Doubling of chromosome number 2 n = 10 Chromosomes either cannot pair or go through erratic meiosis Pairing now possible during meiosis n = 5 No gametes or sterile gametes — no sexual reproduction possible Viable gametes — sexual reproduction possible (self-fertilization) Stepped Art Fig. 20-9b, p. 435

39. Sympatric Speciation

40. Sympatric Speciation in Animals • Fruit maggot flies

41. Sympatric Speciation in Animals • Cichlids

42. Fig. 20-12a, p. 437

43. Fig. 20-12b, p. 437

44. Fig. 20-12c, p. 437

45. KEY CONCEPTS • In sympatric speciation, populations become reproductively isolated from one another despite living in the same geographic area

46. Insert “Sympatric speciation in wheat” wheat_speciation.swf

47. Explore sympatric speciation by clicking on the figure in ThomsonNOW.

48. Learning Objective 5 • Debate the pace of evolution by representing the views of either punctuated equilibrium or gradualism

49. Evolution • Punctuated equilibrium model • evolution proceeds in spurts • short periods of active speciation interspersed with long periods of stasis • Gradualism model • populations slowly diverge from one another by accumulation of adaptive characteristics

50. Punctuated Equilibrium and Gradualism