Chapter 24: The Origin of Species

1. Chapter 24:The Origin of Species

2. Essential Knowledge • 1.c.1 – Speciation and extinction have occurred throughout the Earth’s history (24.3 & 24.4). • 1.c.2 – Speciation may occur when two populations become reproductively isolated from each other (24.1). • 1.c.3 – Populations of organisms continue to evolve (24.2). • 2.e.2 – Timing and coordination of physiological events are regulated by multiple mechanisms (24.1).

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

4. Morphospecies • Organisms with very similar morphology

5. 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

6. Biological Species • A group of organisms that could interbreed in nature and produce fertile offspring • Don’t identify species based upon similar likeness ALONE!

7. Key Points • Could interbreed • Fertile offspring • May or may not be similar in phenotype

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

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

10. Ecological Species Concept • Emphasizes a specie’s role or function in the environment. • Called an organism’s niche

11. (Added Slide ) Lions and tigers are ecologically isolated Live in forest Live in open grassland • Hybridization has been successful in captivity • But it does not occur in the wild Tiglon

12. Evolutionary Species Concept • Emphasizes evolutionary lineages and ecological roles • Used extensively in comparative biology • Popular among paleontologists

13. Speciation • Speciation = formation of a NEW species • Caused by disruptive selection (see Ch 23) • 3 Requirements for speciation: • Variation in the population • Selection • Isolation

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

15. Main Types of Reproductive Barriers • Prezygotic– • Prevent mating or fertilization • Postzygotic– • Prevent viable, fertile offspring

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

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

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

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

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

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

22. Postzygotic - Types 1. Reduced Hybrid Viability 2. Reduced Hybrid Fertility 3. Hybrid Breakdown

23. Reduced Hybrid Viability • Zygote fails to mature or develop • Ex: • When two different species of frogs hybridize • Egg is fertilized (zygote IS formed) • Zygote never develops properly (if at all)

24. Reduced Hybrid Fertility • Hybrids/offspring are living, viable • Offspring cannot reproduce sexually (nonfertile) • Chromosome count often “odd” • Meiosis doesn’t work out • Ex: mules, ligers

25. Hybrid Breakdown • Hybrid/offspring are living, viable AND fertile • Offspring cannot compete successfully with the “pure breeds” • Ex: • Most plant hybrids • Plants are “born” and can produce seeds • Blooms/plants may lack certain color or size of petal

26. Hybrids between them produced defective embryos in the lab • Their mating calls also differ substantially

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

29. 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

30. Example • Pupfish populations in Death Valley • Generally happens when a species range shrinks for some reason

31. Another Example

32. 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 3. Selection pressure on the isolate differs from the parent population

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

34. Comment • Populations separated by geographical barriers may not evolve much • Ex - Pacific and Atlantic Ocean populations separated by the Panama Isthmus • Fish - 72 identical kinds • Crabs - 25 identical kinds • Echinoderms - 25 identical kinds

35. 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

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

38. Plants • Polyploids may cause new species because the change in chromosome number creates postzygoticbarriers • Animals: Don't form polyploids and will use other mechanisms

39. 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

40. Autopolyploid

41. Allopolyploid

42. Rate of Evolution??

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

44. Gradualism Predicts: • Long periods of time are needed for evolution • Fossils should show continuous links • Problem: Gaps in fossil record

45. Punctuated Evolution • New theory on rate/pace of evolution • Elridge and Gould – 1972

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

47. Predictions • Speciation can occur over a very short period of time • Fossil record will have gaps or missing links • New species will appear in the fossil record • Established species will show

49. Comment • Punctuated Equilibrium is the newest, most widely accepted view • Best explanation of timing

50. Summary • Identify several concepts of "species". • Recognize reproductive barriers that keep two species from reproducing with each other. • Recognize various mechanisms of speciation and hybrid zones. • Identify the Gradual and Punctuated interpretations of speciation. • Identify genetic mechanisms of speciation.