Chapter 24 The Origin of Species Part C

1. Chapter 24The Origin of SpeciesPart C

2. Allopatric and Sympatric Speciation: A Review • In allopatric speciation, geographic isolation restricts gene flow between populations • Reproductive isolation may then arise by natural selection, genetic drift, or sexual selection in the isolated populations • Even if contact is restored between populations, interbreeding is prevented

3. Allopatric and Sympatric Speciation: A Review • In sympatric speciation, a reproductive barrier isolates a subset of a population without geographic separation from the parent species • Sympatric speciation can result from polyploidy, natural selection, or sexual selection

4. Animals • Don't form polyploids and will use other mechanisms.

5. Concept 24.3: Hybrid zones provide opportunities to study factors that cause reproductive isolation • A hybrid zone is a region in which members of different species mate and produce hybrids

6. Patterns Within Hybrid Zones • A hybrid zone can occur in a single band where adjacent species meet • Hybrids often have reduced fitness compared with parent species • The distribution of hybrid zones can be more complex if parent species are found in multiple habitats within the same region

7. A narrow hybrid zone for B. variegataand B. bombina in Europe EUROPE Fire-bellied toad range Hybrid zone Fire-bellied toad, Bombina bombina Yellow-bellied toad range Yellow-bellied toad, Bombina variegata 0.99 0.9 Allele frequency (log scale) 0.5 0.1 0.01 20 40 30 10 0 10 20 Distance from hybrid zone center (km)

8. Hybrid Zones over Time • When closely related species meet in a hybrid zone, there are three possible outcomes: • Strengthening of reproductive barriers • Weakening of reproductive barriers • Continued formation of hybrid individuals

9. Fig. 24-14-1 Formation of a hybrid zone and possible outcomes for hybrids over time Gene flow Barrier to gene flow Population (five individuals are shown)

10. Fig. 24-14-2 Formation of a hybrid zone and possible outcomes for hybrids over time Isolated population diverges Gene flow Barrier to gene flow Population (five individuals are shown)

11. Fig. 24-14-3 Formation of a hybrid zone and possible outcomes for hybrids over time Isolated population diverges Hybrid zone Gene flow Hybrid Barrier to gene flow Population (five individuals are shown)

12. Fig. 24-14-4 Formation of a hybrid zone and possible outcomes for hybrids over time Isolated population diverges Possible outcomes: Hybrid zone Reinforcement OR Fusion Gene flow Hybrid OR Barrier to gene flow Population (five individuals are shown) Stability

13. Reinforcement: Strengthening Reproductive Barriers • The reinforcement of barriers occurs when hybrids are less fit than the parent species • Over time, the rate of hybridization decreases • Where reinforcement occurs, reproductive barriers should be stronger for sympatric than allopatric species

14. Fig. 24-15 Reinforcement of barriers to reproduction in closely related species of European flycatchers Allopatric male pied flycatcher Sympatric male pied flycatcher 28 Pied flycatchers 24 Collared flycatchers 20 16 Number of females 12 8 4 (none) 0 Females mating with males from: Other species Own species Own species Other species Sympatric males Allopatric males

15. Fusion: Weakening Reproductive Barriers • If hybrids are as fit as parents, there can be substantial gene flow between species • If gene flow is great enough, the parent species can fuse into a single species

16. Fig. 24-16 The breakdown of reproductive barriers Pundamilia nyererei Pundamilia pundamilia Pundamilia “turbid water,” hybrid offspring from a location with turbid water

17. Stability: Continued Formation of Hybrid Individuals • Extensive gene flow from outside the hybrid zone can overwhelm selection for increased reproductive isolation inside the hybrid zone • In cases where hybrids have increased fitness, local extinctions of parent species within the hybrid zone can prevent the breakdown of reproductive barriers

18. Concept 24.4: Speciation can occur rapidly or slowly and can result from changes in few or many genes • Evolution has two speeds of change: • Gradualism or slow change • Rapid bursts of speciation

19. Many questions remain concerning how long it takes for new species to form, or how many genes need to differ between species

20. The Time Course of Speciation • Broad patterns in speciation can be studied using the fossil record, morphological data, or molecular data

21. Patterns in the Fossil Record • The fossil record includes examples of species that appear suddenly, persist essentially unchanged for some time, and then apparently disappear

22. Patterns in the Fossil Record • Niles Eldredge and Stephen Jay Gould coined the term punctuated equilibrium to describe periods of apparent stasis punctuated by sudden change • The punctuated equilibrium model contrasts with a model of gradual change in a species’ existence

23. Fig. 24-17 Two models for the tempo of speciation (a) Punctuated pattern Time (b) Gradual pattern

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

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

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

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

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

30. 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.

31. Speciation Rates • The punctuated pattern in the fossil record and evidence from lab studies suggests that speciation can be rapid • The interval between speciation events can range from 4,000 years (some cichlids) to 40,000,000 years (some beetles), with an average of 6,500,000 years

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

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

34. Rapid speciation in a sunflower hybrid zone (a) The wild sunflower Helianthus anomalus

35. Studying the Genetics of Speciation • The explosion of genomics is enabling researchers to identify specific genes involved in some cases of speciation • Depending on the species in question, speciation might require the change of only a single allele or many alleles

36. Fig. 24-19 Single-gene speciation

37. From Speciation to Macroevolution • Macroevolution is the cumulative effect of many speciation and extinction events

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

39. Another idea • Mutations in developmental or control genes (Chapter 21) • Looking very promising as a source of macroevolution

40. Exaptation • When a structure that was adapted for one context is co-opted for another function. • Ex. – feathers and flying

41. Heterochrony • Changes in the timing or rate of development. • Allometric Growth • Paedomorphsis

42. 1. Allometric Growth – changes in the relative rates of growth of various parts of the body. • Ex. – skull growth in primates

43. 2. Paedomorphosis – when an adult retains features that are present in the juvenile form. Ex. – gills in adult salamanders

45. Ex - Homeosis Changes in the basic body design or arrangement of body parts. Ex. – Hox gene clusters that gave rise to vertebrates from invertebrates.

47. Gene Duplications • Allow genes to be used for other functions such as in the previous slide. • Many other examples are known.

48. Future of Evolution ? • Look for new theories and ideas to be developed, especially from new fossil finds and from molecular (DNA) evidence.

49. Evolutionary Trends • Evolution is not goal oriented. It does not produce “perfect” species. • Remember – species survive because of their adaptations. They don’t adapt to survive.