Reproductive Isolation and Speciation: Mechanisms and Patterns

1. Chapter 19: Speciation and Macroevolution

2. Biological Species Concept • Species consist of 1+ populations whose members are capable of interbreeding in nature to produce fertile offspring and do not interbreed with members of different species • Sexual reproduction • Reproductive isolation

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

4. Reproductive Isolating Mechanisms • Prevent interbreeding between 2 species • Preserve genetic integrity • Gene flow is prevented

5. Reproductive barriers - prezygotic • Prezygotic – prevent fertilization • Interspecific zygote never made • Types: • Temporal • Habitat • Behavioral • Mechanical • gametic

6. Temporal • Different times • Day, season, year • Ex: • Fruit flies – afternoon vs. morning • Frogs – late March vs. mid-April

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

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

9. Habitat • Same geographical area, different habitat • Ex: • Flycatchers • Open woods/farms • Deciduous forest • Wet thickets • Coniferous forest • Brushy pastures/ willow thickets

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

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

12. Behavioral • Courtship (signals before mating) • Aka “sexual isolation” • Ex: • Nest decoration, dance, song, vocalizations

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

14. Mechanical • Incompatible structures of genital organs • Ex: • Flowers adapted for different insect pollinators

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

16. Gametic • Egg and sperm incompatible after mating • Ex: • Aquatic animals – release egg and sperm at once; egg and sperm protein bind to each other

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

18. Reproductive barriers - Postzygotic • Prevent gene flow when fertilization occurs • Hybrid inviability • Hybrid sterility • Hybrid breakdown

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

20. Hybrid inviability • Increased likelihood of reproductive failure after fertilization • Spontaneous abortion – genes do not interact properly

21. Hybrid sterility • Interspecific hybrid lives but can’t reproduce • Incompatible courtship w/ either parent species • Gametes of hybrid abnormal during meiosis • Different chromosome #’s • Female horse – 64 • Male donkey – 62 • Mule - 63

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

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

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

25. Hybrid breakdown • Inability of a hybrid to reproduce due to some defect • F2’s • Ex: • 2 sunflower species – 80% F2 can’t reproduce

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

27. Reproductive isolation is the Key to Speciation • Speciation = evolution of a new species • 2 patterns • 1) Anagenic • 2) Cladogenic

28. Anagenesis • (phyletic evolution) • Relatively small, progressive evolutionary changes in a single lineage over long periods • Enough time  conversion of 1 species to another • Sequence of species occurs over time without an increase in the number of species

29. Cladogenesis • (branching evolution) • 2+ populations of an ancestral species split and diverge, eventually forming 2+ new species • Clade = cluster of species derived from a single common ancestor • Over time  increase species richness

30. When has speciation occurred? • Population is sufficiently different from its ancestral species that no genetic exchange can occur between them • 2 ways: • Allopatric • Sympatric

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

32. Allopatric Speciation • Occurs when 1 population becomes geographically separated from the rest of the species and then evolves by natural selection and/or genetic drift • Most common • Geographic isolation by: • Changing of Rivers, glaciers, mountains, land bridges, lakes • Birds vs. rats • Small population migrates or is dispersed • Colonize new area • Isolated gene pool  microevolution  new species

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

34. Sympatric Speciation • New species evolves within the same geographical region as the parent species • 2 ways: • Change in • Ploidy • Ecology

35. Ploidy • Polyploidy - 2+ chromosome sets • Plants – rapid speciation • Autopolyploid – multiple sets chromosomes from a single species • Allopolyploidy – multiple sets of chromosomes from 2+ species • Allopolyploid – diff # chromosomes from parents = new species • 1) extinct • 2)coexist • 3)replace parent species

36. Fig. 24-10-3 2n 2n = 6 4n = 12 4n Failure of cell division after chromosome duplication gives rise to tetraploid tissue. Gametes produced are diploid.. Offspring with tetraploid karyotypes may be viable and fertile.

37. Fig. 24-11-4 Species B 2n = 4 Unreduced gamete with 4 chromosomes Unreduced gamete with 7 chromosomes Hybrid with 7 chromosomes Meiotic error Viable fertile hybrid (allopolyploid) 2n = 10 Normal gamete n = 3 Normal gamete n = 3 Species A 2n = 6

38. Allopatric and Sympatric Speciation: A Review • In allopatric speciation, geographic isolation restricts gene flow between populations • Reproductive isolation may then arise by natural selection, genetic drift, or sexual selection in the isolated populations • Even if contact is restored between populations, interbreeding is prevented

39. In sympatric speciation, a reproductive barrier isolates a subset of a population without geographic separation from the parent species • Sympatric speciation can result from polyploidy, natural selection, or sexual selection

40. Ecology • Parasitic insects • Ex: fruit maggot flies • Switched host from hawthorn tree fruits to domestic apples • Mutation  disruptive selection  different ecological opportunity

41. Evolutionary Change – rapid or gradual?2 models • Punctuated Equilibrium – fossil record accurately reflects evolution as it actually occurs • Long periods of stasis are punctuated by short periods of rapid speciation triggered by changes in the environment • Speciation in “spurts” • Short periods evolution, long periods stability • Accounts for abrupt appearance of new species with few intermediate forms

42. Fig. 24-17 (a) Punctuated pattern Time (b) Gradual pattern

43. Gradualism – traditional view of evolution • Evolution proceeds continuously over long periods • Rarely observed, fossil record incomplete • Populations slowly diverge from 1 another by the gradual accumulation of adaptive characteristics within each population

44. Macroevolution • Dramatic changes that occur over long time spans in evolution • Attempts to explain large phenotypic changes (novelties) • Important aspects • Evolutionary novelties • Adaptive radiation • Mass extinction

45. Macroevolution

46. Adaptive radiation • Evolutionary diversification of many related species from 1 or a few ancestors in a short period • Adaptive zones: new ecological opportunities that were not exploited by an ancestor • Islands – common – fewer species there • Ex: Darwin’s finches, honeycreeper birds, silversword plants

47. Extinction • End of lineage; last member of species dies • Permanent • Makes adaptive zones vacant • Background extinction • Continuous, low-level • Mass extinction • Numerous species die at once • Adaptive radiation follows

48. Extinction video • Causes of mass extinction • Climate change / Earth’s temp. • Catastrophes • Comet/asteroid  dust (block light)  food chain disrupted, drop in temp. • Competition • Humans  animal / plant habitats

49. Microevolution vs. Macroevolution Chance events “lucky” to survive Right place, right time • Genetics • Well suited survive