The Origin of Species: Understanding Speciation and Evolution

1. Chapter 24 The Origin of Species

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

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

5. Two basic patterns of evolutionary change: • Anagenesis (phyletic evolution) transforms one species into another • Cladogenesis (branching evolution) is the splitting of a gene pool, giving rise to one or more new species Animation: Macroevolution

6. LE 24-2 Anagenesis Cladogenesis

7. Concept 24.1: The biological species concept emphasizes reproductive isolation • Species is a Latin word meaning “kind” or “appearance”

8. The Biological Species Concept • Members of a biological species are reproductively compatible, at least potentially; they cannot interbreed with other populations.

9. LE 24-3 Similarity between different species. Diversity within a species.

10. Reproductive Isolation • Reproductive isolation is the existence of biological factors (barriers) that impede two species from producing viable, fertile hybrids • Two types of barriers: prezygotic and postzygotic

11. Prezygotic barriers impede mating or hinder fertilization if mating does occur: • Habitat isolation • Temporal isolation • Behavioral isolation • Mechanical isolation • Gametic isolation

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

13. LE 24-4a Prezygotic barriers impede mating or hinder fertilization if mating does occur Habitat isolation Temporal isolation Behavioral isolation Mechanical isolation Gametic isolation Individuals of different species Mating attempt Fertilization TEMPORAL ISOLATION BEHAVIORAL ISOLATION MECHANICAL ISOLATION GAMETIC ISOLATION HABITAT ISOLATION Postzygotic barriers prevent a hybrid zygote from developing into a viable, fertile adult Reduced hybrid fertility Reduced hybrid viability Hybrid breakdown Viable, fertile offspring Fertilization REDUCED HYBRID VIABILITY REDUCED HYBRID FERTILITY HYBRID BREAKDOWN

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

17. Behavioral isolation: Courtship rituals and other behaviors unique to a species are effective barriers

19. Mechanical isolation: Morphological differences can prevent successful mating

21. Gametic isolation: Sperm of one species may not be able to fertilize eggs of another species Video: Albatross Courtship Ritual Video: Blue-footed Boobies Courtship Ritual Video: Giraffe Courtship Ritual

23. LE 24-4aa Prezygotic barriers impede mating or hinder fertilization if mating does occur Habitat isolation Temporal isolation Behavioral isolation Mechanical isolation Gametic isolation Individuals of different species Mating attempt Fertilization TEMPORAL ISOLATION BEHAVIORAL ISOLATION MECHANICAL ISOLATION GAMETIC ISOLATION HABITAT ISOLATION

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

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

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

29. 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

31. LE 24-4ab Postzygotic barriers prevent a hybrid zygote from developing into a viable, fertile adult Hybrid breakdown Reduced hybrid fertility Reduced hybrid viability Viable, fertile offspring Fertilization REDUCED HYBRID VIABILITY REDUCED HYBRID FERTILITY HYBRID BREAKDOWN

32. Limitations of the Biological Species Concept • The biological species concept does not apply to • Asexual organisms • Fossils • Organisms about which little is known regarding their reproduction

33. Other Definitions of Species • Morphological: defines a species by structural features • Paleontological: focuses on morphologically discrete species known only from the fossil record • Ecological: views a species in terms of its ecological niche • Phylogenetic: defines a species as a set of organisms with a unique genetic history

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

35. LE 24-5 Allopatric speciation Sympatric speciation

36. Allopatric (“Other Country”) Speciation • In allopatric speciation, gene flow is interrupted or reduced when a population is divided into geographically isolated subpopulations • One or both populations may undergo evolutionary change during the period of separation

37. LE 24-6 A. leucurus A. harrisi

38. To determine if allopatric speciation has occurred, reproductive isolation must have been established

39. LE 24-7a Initial population of fruit flies (Drosophila pseudoobscura) Some flies raised on starch medium Some flies raised on maltose medium Mating experiments after several generations

40. LE 24-7b Female Female Different populations Same population Starch Maltose Same population 18 15 22 9 Starch Male Male 8 15 20 12 Maltose Different populations Mating frequencies in experimental group Mating frequencies in control group

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

42. Polyploidy • Polyploidy is presence of extra sets of chromosomes due to accidents during cell division • It has caused the evolution of some plant species • An autopolyploid is an individual with more than two chromosome sets, derived from one species

43. LE 24-8 Offspring with tetraploid karyo- types may be viable and fertile—a new biological species. Failure of cell division in a cell of a growing diploid plant after chromosome duplication gives rise to a tetraploid branch or other tissue. Gametes produced by flowers on this tetraploid branch are diploid. 2n 2n = 6 4n = 12 4n

44. An allopolyploid is a species with multiple sets of chromosomes derived from different species

45. LE 24-9 Unreduced gamete with 4 chromosomes Unreduced gamete with 7 chromosomes Hybrid with 7 chromosomes Viable fertile hybrid (allopolyploid) Meiotic error; chromosome number not reduced from 2n to n Species A 2n = 4 2n = 10 Normal gamete n = 3 Normal gamete n = 3 Species B 2n = 6

46. Habitat Differentiation and Sexual Selection • Sympatric speciation can also result from the appearance of new ecological niches • In cichlid fish, sympatric speciation has resulted from nonrandom mating due to sexual selection

47. LE 24-10 P. pundamilia P. nyererei Monochromatic orange light Normal light

48. Allopatric and Sympatric Speciation: A Summary • In allopatric speciation, a new species forms while geographically isolated from its parent population • In sympatric speciation, a reproductive barrier isolates a subset of a population without geographic separation from the parent species

49. Adaptive Radiation • Adaptive radiation is the evolution of diversely adapted species from a common ancestor upon introduction to new environmental opportunities