1. Chapter 24
The Origin of Species

2. How did this flightless bird come to live on the isolated Galápagos Islands?
Figure 24.1 How did this flightless bird come to live on the isolated Galápagos Islands?

3. Animation: Macroevolution Right-click slide / select “Play”
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4. Process of species change
Speciation, the origin of new species, is at the focal point of evolutionary theory
Scale of evolution:
Microevolution consists of changes in allele frequency in a population over time
Macroevolution refers to broad patterns of evolutionary change above the species level
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5. Concept 24.1: The biological species concept emphasizes reproductive isolation
Biological Species Concept (one way to divide species):
A species is a group of populations whose members can breed and produce viable, fertile offspring
Ability to mate = formation of a species
Gene flow between populations holds together the phenotype of a population (ongoing exchange of alleles)
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6. It’s all about ability to breed
(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

7. Reproductive Isolation is the driving force behind Speciation
A new species will form when reproductive isolation occurs
Absence of gene flow
Reproductive Isolation:
The existence of biological factors (barriers) that prevent two individuals of a species from mating and producing viable and fertile offspring
Hybrids:
Offspring of crosses between two different species
Barriers may contribute to reproductive isolation before or after fertilization (pre- or postzygotic barriers)
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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
5 types of prezygotic barriers
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9. Prezygotic barriers (a) (c) (e) (f) (g) (d) (b) Figure 24.3_b 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

10. Reduced hybrid viability Reduced hybrid fertility Hybrid breakdown
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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11. 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)

12. 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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13. 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”

14. Other Definitions of Species
1. morphological species concept:
A species is characterized by its body shape
It applies to sexual and asexual species but relies on subjective criteria
2. ecological species concept:
A species is characterized by its ecological niche
It applies to sexual and asexual species and emphasizes the role of disruptive selection
3. phylogenetic species concept:
A species is the smallest group of individuals that share a common ancestor
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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15. Concept 24.2: Speciation can take place with or without geographic separation
Speciation can occur in two ways:
Allopatric speciation
Sympatric speciation
What do the route words mean?
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16. 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.

17. 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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18. 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
A. harrisii
A. leucurus
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19. Reproductive isolation may arise as a result of genetic divergence
Once isolated:
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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20. (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.

21. Evidence of Allopatric Speciation: a case study
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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22. 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

23. What types of regions have the greatest isolation?
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24. What types of regions have the greatest isolation?
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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25. Degree of reproductive isolation
Dusky salamanders
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)

26. 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

27. Sympatric (“Same Country”) Speciation
In sympatric speciation, speciation takes place in geographically overlapping populations
How can this occur if parent species is in same area? Provide an example.
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28. Sympatric (“Same Country”) Speciation
In sympatric speciation, speciation takes place in geographically overlapping populations
In a sympatric speciation, various factors can limit gene flow:
Polyploidy
Habitat differentiation
Sexual selection
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29. 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. Why do you think?
An autopolyploid is an individual with more than two chromosome sets, derived from one species
When would this originate?
An allopolyploid is a species with multiple sets of chromosomes derived from different species (see diagram on proceeding slides)
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30. 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
Figure One mechanism for allopolyploid speciation in plants.

31. 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.

32. 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

33. 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

34. Many important crops (oats, cotton, potatoes, tobacco, and wheat) are polyploids
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35. 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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36. Sexual Selection: Sexual selection for mates of different colors has likely contributed to speciation in cichlid fish in Lake Victoria
Monochromatic
orange light
EXPERIMENT
Normal light
P. pundamilia
Figure Inquiry: Does sexual selection in cichlids result in reproductive isolation?
P. nyererei

37. 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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38. 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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39. 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
Can you think of an example of where and how this would occur?
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40. 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 (see next slide)
Would you expect hybrids to have more, less and the same fitness as the parents?
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41. 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)

42. B. variegata-specific allele
Figure 24.13b
0.99
Hybrid
zone
0.9
B. variegata-specific allele
Frequency of
Yellow-bellied
toad range
Fire-bellied
toad range
0.5
0.1
Figure A narrow hybrid zone for Bombina toads in Europe.
0.01 40 30 20 10 10 20
Distance from hybrid zone center (km)

43. 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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44. Gene flow Population Barrier to gene flow Figure 24.14-1
Figure Formation of a hybrid zone and possible outcomes for hybrids over time.
Gene flow
Population
Barrier to
gene flow

45. Isolated population diverges Gene flow Population Barrier to gene flow
Formation of a hybrid zone and possible outcomes for hybrids over time.
Isolated
population
diverges
Figure Formation of a hybrid zone and possible outcomes for hybrids over time.
Gene flow
Population
Barrier to
gene flow

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

47. 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

48. 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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49. 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

50. Fusion: Weakening Reproductive Barriers
If hybrids are as fit as parents, there can be substantial gene flow between species
How would increasing gene flow affect separate species?
For example, researchers think that pollution in Lake Victoria has reduced the ability of female cichlids to distinguish males of different species
How would this affect different cichlid species?
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51. 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

52. Concept 24.4: Speciation can occur rapidly or slowly and can result from changes in few or many genes
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53. The Time Course of Speciation
By what tools can we determine rate of speciation?
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54. The Time Course of Speciation
By what tools can we determine rate of speciation?
fossil record,
morphological data, or
molecular data
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55. 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
Compare punctuated equlibrium to uniformitarianism.
Which model is more correct?
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56. Represent these two models in graphic form
Figure 24.17
(a)
Punctuated
pattern
Time
(b)
Gradual
pattern
Figure Two models for the tempo of speciation.
Represent these two models in graphic form

57. 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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58. Figure 24.18
Figure A hybrid sunflower species and its dry sand dune habitat.

59. How does hybridization lead to speciation in sunflowers??
EXPERIMENT
H. annuus
gamete
H. petiolarus
gamete
How does hybridization lead to speciation in sunflowers??
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

60. Average rates of speciation
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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61. Studying the Genetics of Speciation
A fundamental question of evolutionary biology persists: How many genes change when a new species forms?
For example:
In Japanese Euhadra snails, the direction of shell spiral affects mating and is controlled by a single gene
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
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62. 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

63. VOCAB REVIEW
The cumulative effect of many speciation and extinction events is called ________________ .
The ____________species concept defines a species by structural features.
The sudden change in a species after a long period of no change is called ____________.

64. The new species would be called a ___________.
Figure 24.UN01
Cell
division
error
The new species would be called a ___________.
2n = 6
4n = 12 2n
Figure 24.UN01 In-text figure, p. 495 2n
New species
(4n)
Gametes produced

65. Fill in the blanks Original population _______ speciation
Figure 24.UN02
Fill in the blanks
Original population
Figure 24.UN02 Summary figure, Concept 24.2
_______ speciation
________ speciation

66. Answers to VOCAB REVIEW
Macroevolution
Morphological
Punctuated equlibrium
Polyploid, or tetraploid more specifically
Allopatric
Sympatric

67. TRY…Test Your Understanding, question 10

68. Test Your Understanding, question 10
Figure 24.UN03
Test Your Understanding, question 10
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)

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