Biology 212 Chapter 24

1. Biology 212 Chapter 24 The Origin of Species (Macroevolution)

2. You will be able to… • Differentiate between various isolating factors that lead to speciation • Compare Allopatric to Sympatric speciation • Contrast gradualism to punctuated equilibrium • Explain mechanisms leading towards macroevolution

3. 1700’s Linnaeus • Founder of taxonomy • Used structural differences • “Morphological species” morphos = shape

4. 1942: Ernst Mayr • “Married” Darwinian evolution with Mendel’s genetics • First proposed biological species concept • Reproductive isolation causes speciation

5. Reproductive isolating mechanisms • Prevent gene flow between species • Prezygotic barriers • Prevent mating or fertilization • Postzygotic barriers • Reproductive failure after fertilization

6. Temporal isolation • Mating at • different times of year • different times of day

7. Gametic isolation • Incompatible egg and sperm • Molecular recognition on surface of cells • is different (protein incompatibility)

8. Habitat isolation • Different habitats in same area

9. Habitat isolation During breeding season they stick to their own habitats: Least flycatcher = open woods & farms Acadian flycatcher = deciduous forests, beech trees, and swampy woods Alder flycatcher = wet thickets of Alder Yellow-bellied flycatcher = conifer woods Willow flycatcher = brushy pasture and willow thickets

10. Behavioral (sexual) isolation • Required courtship & mating behaviors Male satin bowerbird builds a bower (dark tunnel) of twigs lined with shiny or colorful objects to attract females Different species decorate with different colors

12. Mechanical isolation: • Incompatible shapes of genital organs • Includes reproductive organs of plants that use pollinators

13. Only small bees (pollen on back) can land on petal of black sage (Salvia mellifera) Only large bees (pollen on wings ) brush against stamens of white sage (S. apiana) Mechanical isolation: Flower stamen & stigma vary in design to either give & accept pollen from a bee’s wings or a bee’s back

14. Postzygotic barriers • Hybrid inviability • hybrid embryos die when genetic regulation fails during development • Sheep & goat hybrid embryo dies before birth: chromosome mismatch (sheep 54, goats 60).

15. Postzygotic barriers • Hybrid sterility • problems during meiosis cause abnormal gametes • Mule is a donkey-horse hybrid – sterile

16. Postzygotic barriers • Hybrid breakdown • Allele interactions in hybrid = deleterious result. • Frail offspring

17. Speciation • A population becomes reproductively isolated e.g. Genetic drift • Separated gene pools diverge Natural Selection acts differently for different environments • Genetic exchange stops (Gene Flow stops)

18. Comparison: • Genetic drift &Natural Selection increase chances for speciation • Gene Flow reduces chance for speciation • Gene Flow must stop or • greatly minimized for • speciation to occur

19. Allopatric (allo = other, patric = place)geographically isolated populations Parapatric (para = beside, patric = place) a continuously distributed population Sympatric(sym = same, patric = place) within the same range of ancestral population

20. Allopatric speciation • Geographically separatedpopulations = geographic isolation • Most common form of speciation • Genetic drift occurs faster in small populations

21. Antelope Squirrel species separated by Grand Canyon have diverged in fur color Ammospermophilus harrisii Ammospermophilus leucurus

22. Parapatric Speciation • No physical barrier to gene flow: • population is continuous • But! No random mating: • mating with geographic neighbors, • but not all over full range • So why is there speciation occurring?

23. Parapatric Speciation • Divergence happens because of • reducedGene Flow within population • varying selection pressures across popltn’s range. • e.g. Grasses starting to evolve different flowering times Anthoxanthum odoratum grass species

24. Sympatric speciation • Gene flow is reduced between parts of a population • Divergence of two populations in same geographic region: • exploiting a new niche

25. Sympatric speciation • Other reproductive isolating mechanisms at work • Polyploidy: extra chromosome # • Especially common in plants

26. Sympatric speciation in animals • Population occupies a new ecological niche • Nogene flow even though species live in same area

27. Two species of maggot flies are sympatric in the northern half of one fly’s range

28. apple maggot flies Hawthorn fruit Apples (not native) Males select mates on fruit species they grow up on Females lay eggs on fruit species they grow up on

29. Sympatric speciation: sexual selection

30. Polyploid organisms: Failure to reduce chromosome # results in polyploid organism. Only reproduce successfully w/ other polyploids. • Allopolyploid • Hybrid with multiple sets of chromosomes from two species • Autopolyploid • Hybrid with multiple sets of chromosomes from a single species

31. Sympatric speciation by allopolyploidy common in plants Wheat: diploid strain = ancestral tetraploid = Durum wheat hexaploid = Bread wheat.

32. Polyploid Animals: Flatworms, leeches, & brine shrimp. Triploids: sterile since 1/3 of chromosomes can’t pair Parthogenesis: female asexual reproduction Seedless watermelons are triploids Farmed oysters are also triploid

33. African clawed frog is an allotetraploid. Related species, e.g. Xenopus tropicalis, are diploid. Potato flowers. Potato is an autotetraploid. Polyploidy rare for mammals: XY sex chromosome Red vizcacha rat: Tympanoctomybarrerae, Argentina

34. Rate of evolutionary change • Fossil record often lacks transitional forms between two species • Is fossil record simply incomplete? • Or, does it accurately reflect evolution as it really occurs?

35. Gradualism • Continuous evolution over long periods • Traditional view (Darwinian) • Populations gradually accumulate adaptations • Different selective pressures in different environments (natural selection)

36. Gradualism: Parent population turns into extant population.

37. Punctuated equilibrium Niles Eldredge & Stephen Jay Gould in 1970’s. • Long periods of stasis (~2 My) • Punctuated by periods of rapid speciation (~ 1,000 to 100,000 yrs) • Triggered by D’s in environment • Accounts for abrupt appearance of new species in fossil record

38. Punctuated Equilibrium: Part of parent population changes. Parent population may still exist.

39. Could both models reflect evolutionary change under different situations?

40. Hybrid Zones Different but related species mix & mate

41. Dynamics in Hybrid Zones: Interplay btwn magnitude of dispersal and intensity of selection determines width of hybrid zone (and fate of hybrids).

42. Hybrid Zones may form when ancient barrier is reduced or eliminated allowing species to reconnect

43. Dynamics in Hybrid Zones: • If hybrids are: • less fit (relative to parent species) then natural selectionreinforcesreproductive isolating mechanisms & speciation will be completed

44. Dynamics in Hybrid Zones: • If hybrids are: • 2) equally or more fit everywhere than parental species, then species will merge into one species.

45. Dynamics in Hybrid Zones: • If hybrids are: • 3) less fit in parental ranges & equally fit in hybrid zone, then stable hybrid zone will form.

46. Dynamics in Hybrid Zones: • If hybrids are: • 4) less fit in parental ranges but more fit than parental types in hybrid zone - new, hybrid species may form.

47. Evidence for speciation: Prediction: populations of same species in differentgeographic locations would be genetically different. Are they?

48. Larus gulls: circumpolar "ring“ species British Herring Gull can hybridize w/ N. American Herring Gull, which can hybridize w/ Vega (E. Siberia) Gull.

49. Larus gulls: circumpolar "ring“ species These can hybridize w/ Birula's Gull, & they can hybridize with Heuglin's gull, which can hybridize with Siberian Lesser Black-backed Gull.

50. Larus gulls: circumpolar "ring“ species But, Lesser Black-backed Gull & Herring Gull too different to normally hybridize.