1. 22
The Origin of Species

2. Aim: How can we describe the phenomenon of speciation
Aim: How can we describe the phenomenon of speciation? Do Now: How can two organisms that were originally the same species, become two different species? Homework: Read chapter 22 and complete concept questions.

3. Speciation is the process by which one species splits into two or more species
Speciation explains the features shared between organisms due to inheritance from their recent common ancestor 3

4. Speciation forms a conceptual bridge between microevolution and macroevolution
Microevolution consists of changes in allele frequency in a population over time
Macroevolution refers to broad patterns of evolutionary change above the species level 4

5. Concept 22.1: The biological species concept emphasizes reproductive isolation
Species is a Latin word meaning “kind” or “appearance”
Biologists compare morphology, physiology, biochemistry, and DNA sequences when grouping organisms 5

6. The Biological Species Concept
The biological species concept states that a species is a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring; they do not breed successfully with other populations
Gene flow between populations holds the populations together genetically 6

7. (a) Similarity between different species
Figure 22.2
Figure 22.2 The biological species concept is based on the potential to interbreed rather than on physical similarity
(a) Similarity between different species
(b) Diversity within a species 7

8. Reproductive Isolation
Reproductive isolation is the existence of biological barriers that impede two species from producing viable, fertile offspring
Hybrids are the offspring of crosses between different species
Reproductive isolation can be classified by whether barriers act before or after fertilization 8

9. Prezygotic barriers block fertilization from occurring by:
Impeding different species from attempting to mate
Preventing the successful completion of mating
Hindering fertilization if mating is successful 9

10. Habitat isolation Temporal isolation Behavioral isolation
Figure 22.3a
Prezygotic barriers
Habitat
isolation
Temporal
isolation
Behavioral
isolation
MATING
ATTEMPT
(a)
(c)
(e)
Figure 22.3a Exploring reproductive barriers (part 1: prezygotic barriers)
(d)
(b) 10

11. Habitat isolation: Two species encounter each other rarely, or not at all, because they occupy different habitats, even though not isolated by physical barriers 11

12. Figure 22.3aa
(a)
Figure 22.3aa Exploring reproductive barriers (part 1a: habitat isolation, water) 12

13. Figure 22.3ab
(b)
Figure 22.3ab Exploring reproductive barriers (part 1b: habitat isolation, terrestrial) 13

14. Temporal isolation: Species that breed at different times of the day, different seasons, or different years cannot mix their gametes 14

15. Figure 22.3ac
(c)
Figure 22.2ac Exploring reproductive barriers (part 1c: temporal isolation, winter) 15

16. Figure 22.3ad
(d)
Figure 22.2ad Exploring reproductive barriers (part 1d: temporal isolation, summer) 16

17. Behavioral isolation: Courtship rituals and other behaviors unique to a species are effective barriers 17

18. Figure 22.3ae
(e)
Figure 22.ae Exploring reproductive barriers (part 1e: behavioral isolation) 18

19. Mechanical isolation Gametic isolation
Figure 22.3b
Prezygotic barriers
Mechanical
isolation
Gametic
isolation
MATING
ATTEMPT
FERTILIZATION
(f)
(g)
Figure 22.3b Exploring reproductive barriers (part 2: prezygotic barriers) 19

20. Mechanical isolation: Morphological differences prevent successful mating20

21. Figure 22.3bf
(f)
Figure 22.3bf Exploring reproductive barriers (part 2f: mechanical isolation) 21

22. Gametic isolation: Sperm of one species may not be able to fertilize eggs of another species22

23. Figure 22.3bg
(g)
Figure 22.3bg Exploring reproductive barriers (part 2g: gametic isolation) 23

24. Postzygotic barriers prevent the hybrid zygote from developing into a viable, fertile adult by:
Reduced hybrid viability
Reduced hybrid fertility
Hybrid breakdown 24

25. Reduced hybrid viability Reduced hybrid fertility Hybrid breakdown
Figure 22.3c
Postzygotic barriers
Reduced
hybrid
viability
Reduced
hybrid
fertility
Hybrid
breakdown
VIABLE,
FERTILE
OFFSPRING
FERTILIZATION
(h)
(i)
(l)
(j)
Figure 22.3c Exploring reproductive barriers (part 3: postzygotic barriers)
(k) 25

26. Reduced hybrid viability: Genes of the different parent species may interact and impair the hybrid’s development or survival 26

27. Figure 22.3ch
(h)
Figure 22.3ch Exploring reproductive barriers (part 3h: reduced hybrid viability) 27

28. Reduced hybrid fertility: Even if hybrids are vigorous, they may be sterile28

29. Figure 22.3ci
(i)
Figure 22.3ci Exploring reproductive barriers (part 3i: reduced hybrid fertility) 29

30. Figure 22.3cj
(j)
Figure 22.3cj Exploring reproductive barriers (part 3j: reduced hybrid fertility) 30

31. Figure 22.3ck
(k)
Figure 22.3ck Exploring reproductive barriers (part 3k: reduced hybrid fertility) 31

32. Hybrid breakdown: Some first-generation hybrids are fertile, but when they mate with another species or with either parent species, offspring of the next generation are feeble or sterile 32

33. Figure 22.3cl
(l)
Figure 22.3cl Exploring reproductive barriers (part 3l: hybrid breakdown) 33

34. Aim: How can we continue to describe the phenomenon of speciation
Aim: How can we continue to describe the phenomenon of speciation? Do Now: Other that molecular similarities, what other information can be used to define a species?

35. Prezygotic barriers Habitat isolation Temporal isolation Behavioral
Figure 22.3d
Prezygotic
barriers
Habitat
isolation
Temporal
isolation
Behavioral
isolation
MATING
ATTEMPT
Mechanical
isolation
Gametic
isolation
MATING
ATTEMPT
FERTILIZATION
Figure 22.3d Exploring reproductive barriers (part 4: art summary)
Postzygotic
barriers
Reduced
hybrid
viability
Reduced
hybrid
fertility
Hybrid
breakdown
VIABLE,
FERTILE
OFFSPRING
FERTILIZATION 35

36. Limitations of the Biological Species Concept
The biological species concept cannot be applied to fossils or asexual organisms (including all prokaryotes)
The biological species concept emphasizes absence of gene flow
However, gene flow can occur between distinct species
For example, grizzly bears and polar bears can mate to produce “grolar bears” 36

37. Grizzly bear (U. arctos) Polar bear (U. maritimus)
Figure 22.4
Grizzly bear (U. arctos)
Figure 22.4 Hybridization between two species of bears in the genus Ursus
Hybrid “grolar bear”
Polar bear (U. maritimus) 37

38. Other Definitions of Species
Other species concepts emphasize the unity within a species rather than the separateness of different species
The morphological species concept defines a species by structural features
It applies to sexual and asexual species but relies on subjective criteria 38

39. The ecological species concept views a species in terms of its ecological niche
It applies to sexual and asexual species and emphasizes the role of disruptive selection
The phylogenetic species concept defines a species as the smallest group of individuals on a phylogenetic tree
It applies to sexual and asexual species, but it can be difficult to determine the degree of difference required for separate species 39

40. Concept 22.2: Speciation can take place with or without geographic separation
Speciation can occur in two ways:
Allopatric speciation
Sympatric speciation 40

41. Allopatric (“Other Country”) Speciation
In allopatric speciation, gene flow is interrupted when a population is divided into geographically isolated subpopulations.
For example, the flightless cormorant of the Galápagos likely originated from a flying species on the mainland. 41

42. The Process of Allopatric Speciation
The definition of a geographic barrier depends on the ability of a population to disperse.
For example, a canyon may create a barrier for small rodents, but not birds, coyotes, or pollen. 42

43. Reproductive isolation may arise as a result of genetic divergence.
Separate populations may evolve independently through mutation, natural selection, and genetic drift.
Reproductive isolation may arise as a result of genetic divergence.
For example, mosquito fish in the Bahamas comprise several isolated populations in different ponds. 43

44. body shape that enables rapid bursts of speed (a) Under low predation:
Figure 22.6
Under high predation:
body shape that enables
rapid bursts of speed
(a)
Under low predation:
body shape that favors
long, steady swimming
(b)
Figure 22.6 Reproductive isolation as a by-product of selection 44

45. Evidence of Allopatric Speciation
Fifteen pairs of sister species of snapping shrimp (Alpheus) are separated by the Isthmus of Panama
These species originated from 9 million to 3 million years ago, when the Isthmus of Panama formed and separated the Atlantic and Pacific waters 45

46. A. formosus A. nuttingi ATLANTIC OCEAN Isthmus of Panama PACIFIC OCEAN
Figure 22.7
A. formosus
A. nuttingi
ATLANTIC
OCEAN
Isthmus of Panama
PACIFIC
OCEAN
Figure 22.7 Allopatric speciation in snapping shrimp (Alpheus)
A. panamensis
A. millsae 46

47. Regions with many geographic barriers typically have more species than do regions with fewer barriers
Reproductive isolation between populations generally increases as the geographic distance between them increases 47

48. Barriers to reproduction are intrinsic; separation itself is not a biological barrier
Intrinsic reproductive barriers can develop in experimentally isolated populations 48

49. Initial population of fruit flies (Drosophila pseudoobscura)
Figure 22.8
Experiment
Initial population
of fruit flies
(Drosophila
pseudoobscura)
Some flies raised
on starch medium
Some flies raised
on maltose medium
Mating experiments
after 40 generations
Results
Female
Female
Starch
population 1
Starch
population 2
Starch
Maltose
Figure 22.8 Inquiry: Can divergence of allopatric populations lead to reproductive isolation?
Starch
population 1
Starch 22 9 18 15
Male
Male
population 2
Starch
Maltose 8 20 12 15
Number of matings
in experimental group
Number of matings
in control group 49

50. Sympatric (“Same Country”) Speciation
In sympatric speciation, speciation takes place in populations that live in the same geographic area
Sympatric speciation occurs when gene flow is reduced between groups that remain in contact through factors including
Polyploidy
Habitat differentiation
Sexual selection 50

51. Polyploidy
Polyploidy is the presence of extra sets of chromosomes due to accidents during cell division
Polyploidy is much more common in plants than in animals
An autopolyploid is an individual with more than two chromosome sets, derived from one species
The offspring of matings between autopolyploids and diploids have reduced fertility 51

52. Tetraploid cell 4n  12 New species (4n)
Figure 22.UN01
Cell
division
error
2n  6
Tetraploid cell
4n  12
Figure 22.UN01 In-text figure, autopolyploid, p. 425 2n
New species
(4n)
Gametes produced
by tetraploids 52

53. Allopolyploids cannot interbreed with either parent species
An allopolyploid is a species with multiple sets of chromosomes derived from different species
Allopolyploids cannot interbreed with either parent species 53

54. Meiotic error; chromosome number not reduced from 2n to n
Figure
Species A
2n  6
Species B
2n  4
Meiotic error; chromosome
number not reduced from 2n to n
Normal
gamete
n  3
Unreduced gamete
with 4 chromosomes
Figure One mechanism for allopolyploid speciation in plants (step 1) 54

55. Meiotic error; chromosome number not reduced from 2n to n
Figure
Species A
2n  6
Species B
2n  4
Meiotic error; chromosome
number not reduced from 2n to n
Normal
gamete
n  3
Unreduced gamete
with 4 chromosomes
Hybrid with
7 chromosomes
Figure One mechanism for allopolyploid speciation in plants (step 2) 55

56. Meiotic error; chromosome number not reduced from 2n to n
Figure
Species A
2n  6
Species B
2n  4
Meiotic error; chromosome
number not reduced from 2n to n
Normal
gamete
n  3
Unreduced gamete
with 4 chromosomes
Hybrid with
7 chromosomes
Normal
gamete
n  3
Figure One mechanism for allopolyploid speciation in plants (step 3)
Unreduced gamete
with 7 chromosomes 56

57. Meiotic error; chromosome number not reduced from 2n to n
Figure
Species A
2n  6
Species B
2n  4
Meiotic error; chromosome
number not reduced from 2n to n
Normal
gamete
n  3
Unreduced gamete
with 4 chromosomes
Hybrid with
7 chromosomes
Normal
gamete
n  3
Figure One mechanism for allopolyploid speciation in plants (step 4)
Unreduced gamete
with 7 chromosomes
New species:
viable fertile hybrid
(allopolyploid)
2n  10 57

58. Allopatric speciation: forms a new species while
Figure 22.5
Figure 22.5 Two main modes of speciation
Allopatric speciation: forms
a new species while
geographically isolated.
(a)
(b)
Sympatric speciation: a subset
forms a new species without
geographic separation. 58

59. Aim: How can we continue to describe the phenomenon of speciation
Aim: How can we continue to describe the phenomenon of speciation? Do Now: Describe the difference between allopatric and sympatric speciation.

60. Habitat Differentiation
Sympatric speciation can also result from the appearance of new ecological niches
For example, the North American maggot fly can live on native hawthorn trees as well as more recently introduced apple trees 60

61. Sexual Selection Sexual selection can drive sympatric speciation
Sexual selection for mates of different colors has likely contributed to speciation in cichlid fish in Lake Victoria 61

62. Monochromatic orange light Normal light P. pundamilia
Figure 22.10
Experiment
Monochromatic
orange light
Normal light
P. pundamilia
Figure Inquiry: Does sexual selection in cichlids result in reproductive isolation?
P. nyererei 62

63. Concept 22.3: Hybrid zones reveal factors that cause reproductive isolation
A hybrid zone is a region in which members of different species mate and produce hybrids
Hybrids are the result of mating between species with incomplete reproductive barriers 63

64. Patterns Within Hybrid Zones
A hybrid zone can occur in a single band where adjacent species meet
For example, two species of toad in the genus Bombina interbreed in a long and narrow hybrid zone 64

65. B. variegata-specific allele Distance from hybrid zone center (km)
Figure 22.11
Fire-bellied
toad range
Fire-bellied toad, Bombina
bombina
Hybrid zone
Yellow-bellied
toad range
0.99
Hybrid
zone
0.9
Figure A narrow hybrid zone for Bombina toads in Europe
B. variegata-specific allele
Frequency of
0.5
Yellow-bellied
toad range
Fire-bellied
toad range
Yellow-bellied toad, Bombina
variegata
0.1
0.01 40 30 20 10 10 20
Distance from hybrid zone center (km) 65

66. Hybrids often have reduced fitness compared with parent species
The distribution of hybrid zones can be more complex if parent species are found in patches within the same region 66

67. Hybrid Zones over Time
When closely related species meet in a hybrid zone, there are three possible outcomes:
Reinforcement
Fusion
Stability 67

68. Gene flow Population Barrier to gene flow Figure 22.12-1
Figure Formation of a hybrid zone and possible outcomes for hybrids over time (step 1)
Population
Barrier to
gene flow 68

69. Isolated population diverges. Gene flow Population Barrier to
Figure
Isolated
population
diverges.
Gene flow
Figure Formation of a hybrid zone and possible outcomes for hybrids over time (step 2)
Population
Barrier to
gene flow 69

70. Isolated population diverges. Hybrid zone Gene flow Population
Figure
Isolated
population
diverges.
Hybrid
zone
Gene flow
Figure Formation of a hybrid zone and possible outcomes for hybrids over time (step 3)
Population
Barrier to
gene flow
Hybrid
individual 70

71. Isolated population diverges. Possible outcomes: Hybrid zone
Figure
Isolated
population
diverges.
Possible
outcomes:
Hybrid
zone
Reinforcement
Fusion
Gene flow
Figure Formation of a hybrid zone and possible outcomes for hybrids over time (step 4)
Population
Barrier to
gene flow
Hybrid
individual
Stability 71

72. Reinforcement occurs when hybrids are less fit than the parent species
Natural selection strengthens (reinforces) reproductive barriers, and, over time, the rate of hybridization decreases
Where reinforcement occurs, reproductive barriers should be stronger for sympatric than for allopatric species 72

73. This might be causing the fusion of many species
Fusion of the parent species into a single species may occur if hybrids are as fit as parents, allowing substantial gene flow between species
For example, researchers think that pollution in Lake Victoria has reduced the ability of female cichlids to distinguish males of different species
This might be causing the fusion of many species 73

74. Pundamilia pundamilia
Figure 22.13
Pundamilia nyererei
Pundamilia pundamilia
Figure Fusion: the breakdown of reproductive barriers
Pundamilia “turbid water,”
hybrid offspring from a
location with turbid water 74

75. In a stable hybrid zone, hybrids continue to be produced over time
Stability of the hybrid zone may be achieved if extensive gene flow from outside the hybrid zone can overwhelm selection for increased reproductive isolation inside the hybrid zone
In a stable hybrid zone, hybrids continue to be produced over time 75

76. Concept 22.4: Speciation can occur rapidly or slowly and can result from changes in few or many genes
Many questions remain concerning how long it takes for new species to form, or how many genes need to differ between species 76

77. The Time Course of Speciation
Broad patterns in speciation can be studied using the fossil record, morphological data, or molecular data 77

78. 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
These periods of apparent stasis punctuated by sudden change are called punctuated equilibria
The punctuated equilibrium model contrasts with a model of gradual change in a species’ existence 78

79. (a) Punctuated model Time (b) Gradual model Figure 22.14
Figure Two models for the tempo of speciation, based on patterns observed in the fossil record
(b) Gradual model 79

80. Speciation Rates
The punctuated pattern in the fossil record and evidence from lab studies suggest that speciation can be rapid
For example, the sunflower Helianthus anomalus originated from the hybridization of two other sunflower species and quickly diverged into a new species 80

81. The interval between speciation events can range from 4,000 years (some cichlids) to 40 million years (some beetles), with an average of 6.5 million years 81

82. Studying the Genetics of Speciation
A fundamental question of evolutionary biology persists: How many genes change when a new species forms?
Depending on the species in question, speciation might require the change of only a single allele or many alleles
For example, in Japanese Euhadra snails, the direction of shell spiral affects mating and is controlled by a single gene 82

83. In monkey flowers (Mimulus), two loci affect flower color, which influences pollinator preference
Pollination that is dominated by either hummingbirds or bees can lead to reproductive isolation of the flowers
In other species, speciation can be influenced by larger numbers of genes and gene interactions 83

84. From Speciation to Macroevolution
Macroevolution is the cumulative effect of many speciation and extinction events 84