Chapter 24 The Origin of Species

1. Chapter 24The Origin of Species

2. Question? • What is a species? • Evolution theory must also explain how species originate. • Darwin’s “Mystery of mysteries” • In the Galápagos Islands Darwin discovered plants and animals found nowhere else on Earth

3. Overview: That “Mystery of Mysteries” • 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

4. Fig. 24-1 How did this flightless bird come to live on the isolated Galápagos Islands?

5. Microevolutionconsists of adaptations that evolve within a population, confined to one gene pool • Macroevolution refers to evolutionary change above the species level • Example – the appearance of feathers during the evolution of birds from one group of dinosaurs

6. Two Basic Patterns of Evolutionary Change” • 1. Anagenesis • ana = new, genos = race • Accumulation of heritable changes that gradually transforms a species into a species with different characteristics

7. Two Basic Patterns of Evolutionary Change” • 2. Cladogenesis • klados = branch, genos = race • Branching evolution in which a new species arises from a population that buds from a parent species

8. 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

9. Fig. 24-2a Similarity between different species

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

11. Question • How many species of African Violets are here?

12. 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

13. Fig. 24-3c Does gene flow occur between widely separated populations? 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. Postzygotic barriers prevent the hybrid zygote from developing into a viable, fertile adult: • Reduced hybrid viability • Reduced hybrid fertility • Hybrid breakdown

17. EXAMPLES OF PrezygoticBarriers

18. Habitat isolation: Two species encounter each other rarely, or not at all, because they occupy different habitats, even though not isolated by physical barriers Terrestrial Thamnophis Water-dwelling Thamnophis

19. Habitat Isolation • Populations live in different habitats or ecological niches. • Ex – mountains vs lowlands.

20. Temporal isolation: Species that breed at different times of the day, different seasons, or different years cannot mix their gametes Eastern spotted skunk (Spilogaleputorius) Western spotted skunk (Spilogalegracilis)

21. Breeding seasons or time of day different. Ex – flowers open in morning or evening. Temporal Isolation

22. Behavioral isolation: Courtship rituals and other behaviors unique to a species are effective barriers Courtship ritual of blue- footed boobies

23. Mating or courtship behaviors different. Different sexual attractions operating. Ex – songs and dances in birds. Behavioral Isolation

24. Mechanical isolation: Morphological differences can prevent successful mating Bradybaena with shells spiraling in opposite directions

25. Mechanical Isolation • Structural differences that prevent gamete transfer. • Ex – anthers not positioned to put pollen on a bee, but will put pollen on a bird.

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

27. Gametic Isolation • Gametes fail to attract each other and fuse. • Ex – chemical markers on egg and sperm fail to match.

28. EXAMPLES OF PostzygoticBarriers

29. Reduced Hybrid Viability • Zygote fails to develop or mature. • Ex – when different species of frogs or salamanders hybridize.

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

31. Reduced hybrid fertility: Even if hybrids are vigorous, they may be sterile • Hybrids are viable, but can't reproduce sexually. • Chromosome count often “odd” so meiosis won’t work. • Ex – donkeys and horses produce mules

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

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

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

35. Hybrids ‘Heaven Scent’ an F1 hybrid between 2 species, but sterile.

36. Hybrid Breakdown • Offspring are fertile, but can't compete as well as the “pure breeds”. • Ex – many plant hybrids

37. 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 Hybrid cultivated rice plants with stunted offspring (center)

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

39. 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

40. Morphospecies • Organisms with very similar morphology or physical form.

41. Problem • Where does extensive phenotype variation fit?

42. 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

43. Question • How many species of African Violets are here?

44. African Violets • 1950-60’s –20 species described • 70,000 cultivars or human created hybrids

45. African Violets • 2007 – lumping occurred to 6 species and 10 subspecies • 2008 – AVSA now recognizes 9 species and 8 subspecies • point – species definitions can be fluid and change