1. Chapter 24 The Origin of Species

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

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

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

5. Problem u Where does extensive phenotype variation fit?

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

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

8. Reproductive Barriers u Serve to isolate a population from other gene pools. u Create and maintain “species”.

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

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

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

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

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

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

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

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

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

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

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

20. How do species occur? Block gene flow between two populations. 1. Allopatric Speciation 2. Sympatric Speciation

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

23. Example u Pupfish populations in Death Valley. u Generally happens when a species’s range shrinks for some reason.

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

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

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

28. Sympatric Speciation u Sympatric = same homeland u New species arise within the range of parent populations. u Can occur in a single generation.

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

31. 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. u Ex: wheat

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

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

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

35. Examples u Galapagos – Finches

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

37. Gradualism Evolution u Darwinian style evolution. u Small gradual changes over long periods time.

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

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

40. Punctuated Evolution u Theory that deals with the “pacing” of evolution. u Elridge and Gould – 1972.

41. Punctuated Equilibrium u Evolution has two speeds of change: 1. Gradualism or slow change 2. Rapid bursts of speciation

43. Predictions u Speciation can occur over a very short period of time (1 to 1000 generations). u Too fast for fossils to record. u Fossil record will have gaps or missing links.

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

45. Possible Mechanism u Rapid: Adaptive Radiation, especially after mass extinction events u Gradual: Saturated environments favor gradual changes

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

47. How many genes? u Can be as little as 1 gene which prevents interbreeding. Ex – snails u Probably a larger number of genes in many cases.

48. 1 Gene = 2 species