Chapter 24 The Origin of Species

1. Chapter 24The Origin of Species

2. Question? • What is a species? • Comment - Evolution theory must also explain how species originate. • Darwin’s “Mystery of mysteries”

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

4. Two Concepts of Species 1. Morphospecies 2. Biological Species

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

6. Problem • Where does extensive phenotype variation fit?

7. Two Schools 1. Splitters - Break apart species into new ones on the basis of small phenotype changes. 2. Lumpers - Group many phenotype variants into one species.

8. Biological Species • A group of organisms that could interbreed in nature and produce fertile offspring.

9. Key Points • Could interbreed. • Fertile offspring. ‘Heaven Scent’ an F1 hybrid between two species, but sterile. The parents are probably separate species.

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

11. African Violets • 2007 – lumping occurred to 6 species and 10 subspecies • 2008 – AVSA now recognizes 9 species and 8 subspecies • 2012 – all plants lumped under a different genus with 6 species and 10 subspecies. • pt – species definitions can be fluid and change.

12. Speciation Requires: 1. Variation in the population. 2. Selection. 3. Isolation.

13. Reproductive Barriers • Serve to isolate a populations from other gene pools. • Create and maintain “species”.

14. Main Types of Barriers Prezygotic - Prevent mating or fertilization. Postzygotic - Prevent viable, fertile offspring.

15. Prezygotic - Types 1. Habitat Isolation 2. Temporal Isolation 3. Behavioral Isolation 4. Mechanical Isolation 5. Gametic Isolation

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

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

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

19. 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.

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

21. Postzygotic Types 1. Reduced Hybrid Viability 2. Reduced Hybrid Fertility 3. Hybrid Breakdown

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

23. Reduced Hybrid Fertility • Hybrids are viable, but can't reproduce sexually. • Chromosome count often “odd” so meiosis won’t work. • Ex – donkeys and horses produce mules

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

25. Question • Actively evolving species like Quercus and Saintpaulia. • Good isolation mechanisms or poor ones? • Isolation mechanisms may not have fully developed yet.

26. How do species occur? 1. Allopatric Speciation 2. Sympatric Speciation Both work through a block of gene flow between two populations.

28. Allopatric Speciation • Allopatric = other homeland • Ancestral population split by a geographical feature. • Comment – the size of the geographical feature may be very large or small.

29. Example • Pupfish populations in Death Valley. • Generally happens when a specie’s range shrinks for some reason.

31. Conditions Favoring Allopatric Speciation 1. Founder's Effect - with the peripheral isolate. 2. Genetic Drift – gives the isolate population variation as compared to the original population.

32. Conditions Favoring Allopatric Speciation 3. Selection pressure on the isolate differs from the parent population. (environment is different on the edges)

33. Result • Gene pool of isolate changes from the parent population and new species can form.

34. Sympatric Speciation • Sympatric = same homeland • New species arise within the range of parent populations. • Can occur In a single generation.

36. Plants • Polyploids may cause new species because the change in chromosome number creates postzygotic barriers.

37. Polyploid Types 1. Autopolyploid - when a species doubles its chromosome number from 2N to 4N. 2. Allopolyploid - formed as a polyploid hybrid between two species. • Ex: wheat

38. Autopolyploid

39. Allopolyploid

40. Adaptive Radiation • Rapid emergence of several species from a common ancestor (often Allopatric speciation) • Common in island and mountain top populations or other “empty” environments.

41. Mechanism • Resources are temporarily infinite. • Most offspring survive. • Result - little Natural Selection and the gene pool can become very diverse.

42. When the Environment Saturates • Natural Selection resumes. • New species form rapidly if isolation mechanisms work.

43. Examples • Galapagos – Finches • Usambaras Mountains – African violets

44. Question • What if isolation fails and populations come in contact with each other again? • Hybrid zones – a region where members of different species meet and mate and produce hybrid offspring.

46. Possibile Outcomes • 1. Reinforcement – barriers hold and hybrids disappear. • 2. Fusion – barriers give way and species fuse. • 3. Stability – barriers constant, continued production of hybrids

48. Speed of speciation? • How long does it take a new species to form? • How many genes are involved?

49. Gradualism Evolution • Darwinian style evolution. • Small gradual changes over long periods time.

50. Gradualism Predicts: • Long periods of time are needed for evolution. • Fossils should show continuous links.