Chapter 23 The Origin of Species

1. Chapter 23The Origin of Species

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

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

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

5. Problem • Where does extensive phenotype variation fit?

6. 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.

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

8. Key Points • Could interbreed. • Fertile offspring. Heaven Scent an F1 hybrid between 2 species, but sterile.

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

10. African Violets • Originally ~20 species • 70,000 cultivars

11. Problem • What is a species? • Some plants didn’t fit placement. • Plants freely interbreed. • Answer – coming up later

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. Behavioral Isolation 3. Temporal Isolation 4. Mechanical Isolation 5. Gametic Isolation

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

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

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

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 hybridize.

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

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

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

27. 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.

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

29. 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.

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

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

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

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

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

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

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

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

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

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

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

42. Problem • Gradualism doesn’t fit the fossil record very well. (too many “gaps”).

43. Punctuated Evolution • theory that deals with the “pacing” of evolution. • Elridge and Gould – 1972.

44. Punctuated Equilibrium • Evolution has two speeds of change: • Gradualism or slow change • Rapid bursts of speciation

45. Predictions • Speciation can occur over a very short period of time (1 to 1000 generations). • Fossil record will have gaps or missing links.

47. Predictions • New species will appear in the fossil record without connecting links or intermediate forms. • Established species will show gradual changes over long periods of time.

48. Possible Mechanism • Adaptive Radiation, especially after mass extinction events allow new species to originate. • Saturated environments favor gradual changes in the current species.

49. Comment • Punctuated Equilibrium is the newest ”Evolution Theory”. • Best explanation of fossil record evidence to date.

50. Origin of Evolutionary Novelty • How do macroevolution changes originate? • Several ideas discussed in textbook (read them) • Exaptation • Heterochrony • Homeosis