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 • Microevolution consists of adaptations that evolve within a population, confined to one gene pool • Changes over time in allele frequencies in a population • Macroevolution refers to evolutionary change above the species level

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

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

9. 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

10. 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

11. Habitat isolation: Two species encounter each other rarely, or not at all, because they occupy different habitats, even though not isolated by physical barriers • Two species of garter snake live in same area, but one is primarily terrestrial and the other lives mainly in the water.

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

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

14. Temporal isolation: Species that breed at different times of the day, different seasons, or different years cannot mix their gametes • Two skunk species live in the same area, but one mates in the late winter and one mates in the late summer.

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

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

17. Behavioral isolation: Courtship rituals and other behaviors unique to a species are effective barriers • Certain courtship rituals are specific to particular species.

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

19. Mechanical isolation: Morphological differences can prevent successful mating • Shells of two snail species spiral in a different direction, preventing alignment of genital openings.

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

21. Gametic isolation: Sperm of one species may not be able to fertilize eggs of another species • Gametes from similar species of sea urchins are unable to fuse to form zygotes because they lack the appropriate recognition proteins on the surfaces of eggs and sperm.

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

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

24. Reduced hybrid viability: Genes of the different parent species may interact and impair the hybrid’s development • Some salamander species occasionally hybridize, but the offspring often do not develop or are frail if they do complete development.

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

26. Reduced hybrid fertility: Even if hybrids are vigorous, they may be sterile • The mule, a hybrid of a horse and a donkey, is robust but sterile.

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

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

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

30. 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 • Hybrids between strains of rice are vigorous but plants in the next generation may be small and sterile because they carry too many mutant recessive alleles.

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

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

33. 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 • Body shape and other structural features • It applies to sexual and asexual species but relies on subjective criteria • Disagreement on which structures are most important in distinguishing species.

34. The ecological species concept views a species in terms of its ecological niche • Types of food eaten, tolerance to dry conditions • It applies to sexual and asexual species and emphasizes ecological niches

35. 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

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

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

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

39. 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

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

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

42. Reproductive isolation between populations generally increases as the distance between them increases

43. Barriers to reproduction are intrinsic; separation itself is not a biological barrier

44. Sympatric (“Same Country”) Speciation • In sympatric speciation, speciation takes place in geographically overlapping populations

45. Polyploidy • Polyploidy is the presence of extra sets of chromosomes due to accidents during cell division • An autopolyploid is an individual with more than two chromosome sets, derived from one species • Can mate successfully with another autopolyploid, but not with parental generation.

46. Fig. 24-10-1 2n = 6 4n = 12 Failure of cell division after chromosome duplication gives rise to tetraploid tissue.

47. Fig. 24-10-2 2n 2n = 6 4n = 12 Failure of cell division after chromosome duplication gives rise to tetraploid tissue. Gametes produced are diploid..

48. 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.

49. An allopolyploid is a species with multiple sets of chromosomes derived from different species • Fertile when mating with one another, but cannot mate with parental species

50. Fig. 24-11-1 Species B 2n = 4 Unreduced gamete with 4 chromosomes Meiotic error Normal gamete n = 3 Species A 2n = 6