Chapter 24: The Origin of Species

1. Chapter 24:The Origin of Species

2. Question? • What is a species? • Comment - Evolution theory must also explain how species originate.

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.

10. Morphospecies & Biological Species • Often overlap. • Serve different purposes.

11. African Violets • 15+ species • 5+ varieties (natural hybrids) • 50,000 cultivars

12. Example Problem • S. magungensis Cluster: • S. Magungensis • S. magungensis var. minima • S. magungensis var. occidentalis

13. Current Placement • One species and two varieties. • However : • Not all live in the same habitat. • Not all flower under the same temperatures.

14. Possible New Placement • Two species and one variety. • S. magungensis var. occidentalis may be a new species.

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

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

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

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

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

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

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

22. 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.

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

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

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

26. Reduced Hybrid Fertility • Hybrids are viable, but can't reproduce sexually. • Chromosome count often “odd” so meiosis won’t work. • Ex - mules

27. Hybrid Breakdown • Offspring are fertile, but can't compete successfully with the “pure breeds”. • Ex – many plant hybrids

28. Introgression • Transfer of alleles between two species, but only a partial gene flow. • Result – Some intermixing of genes between two species.

29. Importance • A possible mechanism for gene flow between similar species.

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

31. Other Concepts of Species 1. Recognition Species Concept 2. Cohesion Species Concept 3. Ecological Species Concept 4. Evolutionary Species Concept

32. Recognition Species Concept • Species are defined by the ability of the individuals in the population to recognize certain characteristics in each other. • Ex – mate recognition

33. Cohesion Species Concept • Emphasizes cohesion of phenotypes (complex of genes and set of adaptations).

34. Ecological Species Concept • Emphasizes a species role or function in the environment.

35. Evolutionary Species Concept • Emphasizes evolutionary lineages and ecological roles.

36. Comment • AP exam will focus on morphological and biological species concepts.

37. Modes of Speciation 1. Allopatric Speciation 2. Sympatric Speciation Both work through a block of gene flow between two populations.

39. 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.

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

41. Another Example

42. 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.

43. Conditions Favoring Allopatric Speciation 3. Selection pressure on the isolate differs from the parent population.

44. Result • Gene pool of isolate changes from the parent population. • New Species can form.

45. Comment • Populations separated by geographical barriers may not evolve much. • Ex - Pacific and Atlantic Ocean populations separated by the Panama Isthmus.

46. Examples • Fish - 72 identical kinds. • Crabs - 25 identical kinds. • Echinoderms - 25 identical kinds.

47. Adaptive Radiation • Rapid emergence of several species from a common ancestor. • Common in island and mountain top populations or other “empty” environments. • Ex – Galapagos Finches

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

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

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