1. Concept 24.1: 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
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2. 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 phenotype of a population together
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3. Figure 24.2
The biological species concept is based on the potential to interbreed rather than on physical similarity
(a) Similarity between different species
Figure 24.2 The biological species concept is based on the potential to interbreed rather than on physical similarity.
(b) Diversity within a species

4. Reproductive Isolation
Reproductive isolation is the existence of biological factors (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 factors act before or after fertilization
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5. Exploring: Reproductive Barriers
Figure 24.3_a
Exploring: Reproductive Barriers
Prezygotic barriers
Postzygotic barriers
Habitat
Isolation
Temporal
Isolation
Behavioral
Isolation
Mechanical
Isolation
Gametic
Isolation
Reduced Hybrid
Viability
Reduced Hybrid
Fertility
Hybrid
Breakdown
Individuals of
different
species
MATING
ATTEMPT
VIABLE,
FERTILE
OFFSPRING
FERTILIZATION
(a)
(c)
(e)
(f)
(g)
(h)
(i)
(l)
(d)
(j)
(b)
Figure 24.3 Exploring: Reproductive Barriers
(k)

6. Exploring: Reproductive Barriers
Figure 24.3_b
Exploring: Reproductive Barriers
Prezygotic barriers
Habitat
Isolation
Temporal
Isolation
Behavioral
Isolation
Mechanical
Isolation
Gametic
Isolation
Individuals of
different
species
MATING
ATTEMPT
FERTILIZATION
(a)
(c)
(e)
(f)
(g)
(d)
(b)
Figure 24.3 Exploring: Reproductive Barriers

7. Postzygotic barriers Reduced Hybrid Viability Reduced Hybrid Fertility
Figure 24.3_c
Postzygotic barriers
Reduced Hybrid
Viability
Reduced Hybrid
Fertility
Hybrid
Breakdown
VIABLE,
FERTILE
OFFSPRING
FERTILIZATION
(h)
(i)
(l)
(j)
Figure 24.3 Exploring: Reproductive Barriers
(k)

8. 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
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9. Habitat isolation: Two species encounter each other rarely, or not at all, because they occupy different habitats, even though not isolated by physical barriers
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10. Temporal isolation: Species that breed at different times of the day, different seasons, or different years cannot mix their gametes
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11. Behavioral isolation: Courtship rituals and other behaviors unique to a species are effective barriers
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12. Mechanical isolation: Morphological differences can prevent successful mating
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13. Gametic Isolation: Sperm of one species may not be able to fertilize eggs of another species
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14. Postzygotic barriers prevent the hybrid zygote from developing into a viable, fertile adult:
Reduced hybrid viability
Reduced hybrid fertility
Hybrid breakdown
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15. Reduced hybrid viability: Genes of the different parent species may interact and impair the hybrid’s development
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16. Figure 24.3i
(i)
Figure 24.3 Exploring: Reproductive Barriers

17. Figure 24.3j
(j)
Figure 24.3 Exploring: Reproductive Barriers

18. Figure 24.3k
(k)
Figure 24.3 Exploring: Reproductive Barriers

19. Reduced hybrid fertility: Even if hybrids are vigorous, they may be sterile
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20. 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
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21. 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”
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22. Grizzly bear (U. arctos)
Figure 24.4
Grizzly bear (U. arctos)
Polar bear (U. maritimus)
Figure 24.4 Hybridization between two species of bears in the genus Ursus.
Hybrid “grolar bear”

23. 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
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24. 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
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25. Concept 24.2: Speciation can take place with or without geographic separation
Speciation can occur in two ways:
Allopatric speciation
Sympatric speciation
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26. Allopatric speciation. A population forms a new species while
Figure 24.5
Figure 24.5 Two main modes of speciation.
(a)
Allopatric speciation.
A population forms a
new species while
geographically isolated
from its parent population.
(b)
Sympatric speciation.
A subset of a population
forms a new species
without geographic
separation.

27. Allopatric (“Other Country”) Speciation
In allopatric speciation, gene flow is interrupted or reduced 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
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28. The Process of Allopatric Speciation
The definition of 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
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29. A. harrisii A. leucurus Figure 24.6
Figure 24.6 Allopatric speciation of antelope squirrels on opposite rims of the Grand Canyon.

30. 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, mosquitofish in the Bahamas comprise several isolated populations in different ponds
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31. (a) Under high predation (b) Under low predation
Figure 24.7
(a) Under high predation
(b) Under low predation
Figure 24.7 Reproductive isolation as a by-product of selection.

32. Evidence of Allopatric Speciation
15 pairs of sibling species of snapping shrimp (Alpheus) are separated by the Isthmus of Panama
These species originated 9 to 13 million years ago, when the Isthmus of Panama formed and separated the Atlantic and Pacific waters
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33. A. formosus A. nuttingi Atlantic Ocean Isthmus of Panama Pacific Ocean
Figure 24.8
A. formosus
A. nuttingi
Atlantic Ocean
Isthmus of Panama
Pacific Ocean
Figure 24.8 Allopatric speciation in snapping shrimp (Alpheus).
A. panamensis
A. millsae

34. Atlantic Ocean Isthmus of Panama Pacific Ocean Figure 24.8b
Figure 24.8 Allopatric speciation in snapping shrimp (Alpheus).

35. Figure 24.8c
Figure 24.8 Allopatric speciation in snapping shrimp (Alpheus).
A. formosus

36. Figure 24.8d
Figure 24.8 Allopatric speciation in snapping shrimp (Alpheus).
A. panamensis

37. Figure 24.8e
Figure 24.8 Allopatric speciation in snapping shrimp (Alpheus).
A. nuttingi

38. Figure 24.8f
Figure 24.8 Allopatric speciation in snapping shrimp (Alpheus).
A. millsae

39. Regions with many geographic barriers typically have more species than do regions with fewer barriers
Reproductive isolation between populations generally increases as the distance between them increases
For example, reproductive isolation increases between dusky salamanders that live further apart
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40. Degree of reproductive isolation
Figure 24.9
2.0
1.5
Degree of reproductive isolation
1.0
0.5
Figure 24.9 Reproductive isolation increases with distance in populations of dusky salamanders.
Geographic distance (km)

41. Barriers to reproduction are intrinsic; separation itself is not a biological barrier
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42. EXPERIMENT Initial population of fruit flies (Drosophila
Figure 24.10
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 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

43. EXPERIMENT Initial population of fruit flies (Drosophila
Figure 24.10a
EXPERIMENT
Initial population
of fruit flies
(Drosophila
pseudoobscura)
Some flies raised
on starch medium
Some flies raised on
maltose medium
Figure Inquiry: Can divergence of allopatric populations lead to reproductive isolation?
Mating experiments
after 40 generations

44. RESULTS Female Female Starch Maltose 22 9 18 15 Male 8 20 12 15
Figure 24.10b
RESULTS
Female
Female
Starch
population 1
Starch
population 2
Starch
Maltose
Starch
population 1
Starch 22 9 18 15
Male
Male
Maltose
Figure Inquiry: Can divergence of allopatric populations lead to reproductive isolation? 8 20
population 2
Starch 12 15
Number of matings
in experimental group
Number of matings
in control group

45. Sympatric (“Same Country”) Speciation
In sympatric speciation, speciation takes place in geographically overlapping populations
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46. 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
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47. An allopolyploid is a species with multiple sets of chromosomes derived from different species
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48. Species A 2n = 6 Species B 2n = 4 Meiotic error; chromosome number not
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.
Normal
gamete
n = 3
Unreduced gamete
with 7 chromosomes
New species:
viable fertile hybrid
(allopolyploid) 2n = 10

49. Many important crops (oats, cotton, potatoes, tobacco, and wheat) are polyploids
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50. 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
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51. 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
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52. Monochromatic orange light
Figure 24.12
EXPERIMENT
Monochromatic
orange light
Normal light
P. pundamilia
Figure Inquiry: Does sexual selection in cichlids result in reproductive isolation?
P. nyererei

53. 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
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54. 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
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55. Concept 24.3: 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
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56. 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
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57. B. variegata-specific allele
Figure 24.13
EUROPE
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)

58. 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
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59. Hybrid Zones over Time
When closely related species meet in a hybrid zone, there are three possible outcomes:
Reinforcement
Fusion
Stability
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60. Possible outcomes: Isolated population Hybrid diverges zone
Figure
Possible
outcomes:
Isolated
population
diverges
Hybrid
zone
Reinforcement OR
Fusion OR
Figure Formation of a hybrid zone and possible outcomes for hybrids over time.
Gene flow
Population
Hybrid
individual
Barrier to
gene flow
Stability

61. 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
For example, in populations of flycatchers, males are more similar in allopatric populations than sympatric populations
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62. Females choosing between these males: Females choosing between
Figure 24.15
Females choosing between
these males:
Females choosing between
these males: 28
Sympatric pied male
Allopatric pied male 24
Sympatric collared male
Allopatric collared male 20 16
Number of females 12 8
Figure Reinforcement of barriers to reproduction in closely related species of European flycatchers. 4
(none)
Own
species
Other
species
Own
species
Other
species
Female mate choice
Female mate choice

63. 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
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
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64. Pundamilia pundamilia
Figure 24.16
Pundamilia nyererei
Pundamilia pundamilia
Figure Fusion: The breakdown of reproductive barriers.
Pundamilia “turbid water,”
hybrid offspring from a location
with turbid water

65. 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
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66. Concept 24.4: 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
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67. The Time Course of Speciation
Broad patterns in speciation can be studied using the fossil record, morphological data, or molecular data
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68. 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
Niles Eldredge and Stephen Jay Gould coined the term punctuated equilibria 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
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69. (a) Punctuated pattern Time (b) Gradual pattern Figure 24.17
Figure Two models for the tempo of speciation.

70. 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
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71. chromosomes are shown)
Figure 24.19
EXPERIMENT
H. annuus
gamete
H. petiolarus
gamete
F1 experimental hybrid
(4 of the 2n = 34
chromosomes are shown)
RESULTS
H. anomalus
Figure Inquiry: How does hybridization lead to speciation in sunflowers?
Chromosome 1
Experimental hybrid
H. anomalus
Chromosome 2
Experimental hybrid

72. chromosomes are shown)
Figure 24.19a
EXPERIMENT
H. annuus
gamete
H. petiolarus
gamete
F1 experimental hybrid
(4 of the 2n = 34
chromosomes are shown)
Figure Inquiry: How does hybridization lead to speciation in sunflowers?

73. H. anomalus Experimental hybrid RESULTS Chromosome 1 H. anomalus
Figure 24.19b
RESULTS
H. anomalus
Chromosome 1
Experimental hybrid
H. anomalus
Figure Inquiry: How does hybridization lead to speciation in sunflowers?
Chromosome 2
Experimental hybrid

74. 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
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75. 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
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76. 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
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77. M. cardinalis flower-color allele
Figure 24.20
(a)
Typical
Mimulus
lewisii
(b)
M. lewisii with an
M. cardinalis flower-color
allele
Figure A locus that influences pollinator choice.
(c)
Typical
Mimulus
cardinalis
(d)
M. cardinalis with an
M. lewisii flower-color
allele

78. From Speciation to Macroevolution
Macroevolution is the cumulative effect of many speciation and extinction events
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79. Tetraploid cell 4n = 12 New species (4n)
Figure 24.UN01
Cell
division
error
2n = 6
Tetraploid cell
4n = 12 2n
Figure 24.UN01 In-text figure, p. 495 2n
New species
(4n)
Gametes produced
by tetraploids

80. Allopatric speciation Sympatric speciation
Figure 24.UN02
Original population
Figure 24.UN02 Summary figure, Concept 24.2
Allopatric speciation
Sympatric speciation

81. Ancestral species: Triticum Wild Wild monococcum Triticum T. tauschii
Figure 24.UN03
Ancestral species:
Triticum
monococcum
(2n = 14)
Wild
Triticum
(2n = 14)
Wild
T. tauschii
(2n = 14)
Product:
Figure 24.UN03 Test Your Understanding, question 10
T. aestivum
(bread wheat)
(2n = 42)

82. Figure 24.UN04
Figure 24.UN04 Appendix A: answer to Test Your Understanding, question 10