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Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
PowerPoint® Lecture Presentations for
Biology Eighth Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp
Chapter 24Chapter 24
The Origin of Species
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Overview: That “Mystery of Mysteries”
• In the Galápagos Islands Darwin discovered plants and animals found nowhere else on Earth
Video: Galápagos TortoiseVideo: Galápagos Tortoise
Fig. 24-1
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• Speciation, the origin of new species, is at the focal point of evolutionary theory
• Evolutionary theory must explain how new species originate and how populations evolve
• Microevolution consists of adaptations that evolve within a population, confined to one gene pool
• Macroevolution refers to evolutionary change above the species level
Animation: MacroevolutionAnimation: Macroevolution
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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
Fig. 24-2
(a) Similarity between different species
(b) Diversity within a species
Fig. 24-2a
(a) Similarity between different species
Fig. 24-2b
(b) Diversity within a species
Fig. 24-3
EXPERIMENT
RESULTS
Example of a gene tree for population pair A-B
Allele PopulationGene flow event 1 B
B
B
B
5
6
7
2
3
4
A
A
A
Allele 1 is more closely related toalleles 2, 3, and 4 than toalleles 5, 6, and 7.Inference: Gene flow occurred.
Alleles 5, 6, and 7 are more closelyrelated to one another than toalleles in population A.Inference: No gene flow occurred.
Pair ofpopulationswith detectedgene flow
Estimated minimumnumber of gene flowevents to account forgenetic patterns
Distance betweenpopulations (km)
A-B
K-L
A-C
B-C
F-G
G-I
C-E
5
3
2–3
2
2
2
1–2
340
720
1,390
1,190
1,110
760
1,310
Fig. 24-3a
EXPERIMENT
Example of a gene tree for population pair A-B
Allele PopulationGene flow event 1 B
B
B
B
5
6
7
2
3
4
A
A
A
Allele 1 is more closely related toalleles 2, 3, and 4 than toalleles 5, 6, and 7.Inference: Gene flow occurred.
Alleles 5, 6, and 7 are more closelyrelated to one another than toalleles in population A.Inference: No gene flow occurred.
Fig. 24-3b
RESULTS
Pair ofpopulationswith detectedgene flow
Estimated minimumnumber of gene flowevents to account forgenetic patterns
Distance betweenpopulations (km)
A-B
K-L
A-C
B-C
F-G
G-I
C-E
5
3
2–3
2
2
2
1–2
340
720
1,390
1,190
760
1,110
1,310
Fig. 24-3c
Grey-crowned babblers
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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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• 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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Habitat isolation: Two species encounter each other rarely, or not at all, because they occupy different habitats, even though not isolated by physical barriers
Fig. 24-4
Prezygotic barriers
Habitat Isolation
Individualsof
differentspecies
Temporal Isolation Behavioral Isolation
Matingattempt
Mechanical Isolation Gametic Isolation
Fertilization
Reduced Hybrid Viability Reduced Hybrid Fertility
Postzygotic barriers
Hybrid Breakdown
Viable,fertile
offspring
(a)
(b)
(d)
(c) (e) (f) (g) (h) (i)
(j)
(l)
(k)
Fig. 24-4a
Habitat Isolation Temporal Isolation
Prezygotic barriers
Behavioral Isolation
Matingattempt
Mechanical Isolation
(f)(e)(c)(a)
(b)
(d)
Individualsof
differentspecies
Fig. 24-4i
Prezygotic barriers
Gametic Isolation
Fertilization
Reduced Hybrid Viability
Postzygotic barriers
Reduced Hybrid Fertility Hybrid Breakdown
Viable,fertile
offspring
(g) (h) (i)
(j)
(l)
(k)
Fig. 24-4b
Prezygotic barriers
Habitat Isolation Temporal Isolation Behavioral Isolation Mechanical Isolation
Individualsof
differentspecies
Matingattempt
Prezygotic barriers
Fig. 24-4j
Gametic Isolation
Fertilization
Reduced Hybrid Viability Reduced Hybrid Fertility
Postzygotic barriers
Hybrid Breakdown
Viable,fertile
offspring
Fig. 24-4c
(a)
Water-dwelling Thamnophis
Fig. 24-4d
(b)
Terrestrial Thamnophis
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• Temporal isolation: Species that breed at different times of the day, different seasons, or different years cannot mix their gametes
Fig. 24-4e
(c)
Eastern spotted skunk(Spilogale putorius)
Fig. 24-4f
(d)
Western spotted skunk(Spilogale gracilis)
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• Behavioral isolation: Courtship rituals and other behaviors unique to a species are effective barriers
Video: Blue-footed Boobies Courtship RitualVideo: Blue-footed Boobies Courtship Ritual
Video: Giraffe Courtship RitualVideo: Giraffe Courtship Ritual
Video: Albatross Courtship RitualVideo: Albatross Courtship Ritual
Fig. 24-4g
(e)
Courtship ritual of blue-footed boobies
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• Mechanical isolation: Morphological differences can prevent successful mating
Fig. 24-4h
(f)
Bradybaena with shellsspiraling in oppositedirections
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Gametic isolation: Sperm of one species may not be able to fertilize eggs of another species
Fig. 24-4k
(g)
Sea urchins
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• 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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• Reduced hybrid viability: Genes of the different parent species may interact and impair the hybrid’s development
Fig. 24-4l
(h)
Ensatina hybrid
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Reduced hybrid fertility: Even if hybrids are vigorous, they may be sterile
Fig. 24-4m
(i)
Donkey
Fig. 24-4n
( j)
Horse
Fig. 24-4o
(k)
Mule (sterile hybrid)
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• 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
Fig. 24-4p
(l)
Hybrid cultivated rice plants withstunted offspring (center)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Limitations of the Biological Species Concept
• The biological species concept cannot be applied to fossils or asexual organisms (including all prokaryotes)
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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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• 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
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Concept 24.2: Speciation can take place with or without geographic separation
• Speciation can occur in two ways:
– Allopatric speciation
– Sympatric speciation
Fig. 24-5
(a) Allopatric speciation (b) Sympatric speciation
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Allopatric (“Other Country”) Speciation
• In allopatric speciation, gene flow is interrupted or reduced when a population is divided into geographically isolated subpopulations
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The Process of Allopatric Speciation
• The definition of barrier depends on the ability of a population to disperse
• Separate populations may evolve independently through mutation, natural selection, and genetic drift
Fig. 24-6
A. harrisi A. leucurus
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Evidence of Allopatric Speciation
• Regions with many geographic barriers typically have more species than do regions with fewer barriers
Fig. 24-7
Mantellinae(Madagascar only):100 species
Rhacophorinae(India/SoutheastAsia): 310 species
Other Indian/Southeast Asianfrogs
Millions of years ago (mya)1 2 3
1 2 3
100 80 60 40 20 0
88 mya 65 mya 56 mya
India
Madagascar
Fig. 24-7a
Mantellinae(Madagascar only):100 species
Rhacophorinae(India/SoutheastAsia): 310 species
Other Indian/Southeast Asianfrogs
Millions of years ago (mya)
100 80 60 40 20 01 2 3
Fig. 24-7b
India
88 mya 65 mya 56 mya
Madagascar
1 2 3
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• Reproductive isolation between populations generally increases as the distance between them increases
Fig. 24-8
Geographic distance (km)
Deg
ree
of
rep
rod
uct
ive
iso
lati
on
00
50 100 150 250200 300
0.5
1.0
1.5
2.0
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• Barriers to reproduction are intrinsic; separation itself is not a biological barrier
Fig. 24-9EXPERIMENT
RESULTS
Initial population
Some fliesraised on
starch medium Mating experimentsafter 40 generations
Some fliesraised on
maltose medium
FemaleFemale
StarchStarch Starch
Maltose population 1 population 2
Mal
e Sta
rch
Mal
tose
Mal
eS
tarc
hS
tarc
hp
op
ula
tio
n 1
po
pu
lati
on
2
22
8 20
9 18
12
15
15
Mating frequenciesin experimental group
Mating frequenciesin control group
Fig. 24-9a
EXPERIMENT
Initial population
Some fliesraised on
starch medium Mating experimentsafter 40 generations
Some fliesraised on
maltose medium
Fig. 24-9b
RESULTS
FemaleFemale
StarchStarch Starch
Maltose population 1 population 2
Mal
e Sta
rch
Mal
tose M
ale
Sta
rch
Sta
rch
po
pu
lati
on
1p
op
ula
tio
n 2
22
8 20
9 18
12
15
15
Mating frequenciesin experimental group
Mating frequenciesin control group
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Sympatric (“Same Country”) Speciation
• In sympatric speciation, speciation takes place in geographically overlapping populations
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Polyploidy
• Polyploidy is the presence of extra sets of chromosomes due to accidents during cell division
• An autopolyploid is an individual with more than two chromosome sets, derived from one species
Fig. 24-10-1
2n = 6 4n = 12
Failure of celldivision afterchromosomeduplication givesrise to tetraploidtissue.
Fig. 24-10-2
2n = 6 4n = 12
Failure of celldivision afterchromosomeduplication givesrise to tetraploidtissue.
2n
Gametesproducedare diploid..
Fig. 24-10-3
2n = 6 4n = 12
Failure of celldivision afterchromosomeduplication givesrise to tetraploidtissue.
2n
Gametesproducedare diploid..
4n
Offspring withtetraploidkaryotypes maybe viable andfertile.
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• An allopolyploid is a species with multiple sets of chromosomes derived from different species
Fig. 24-11-1
Species A2n = 6
Normalgameten = 3
Meioticerror
Species B2n = 4
Unreducedgametewith 4chromosomes
Fig. 24-11-2
Species A2n = 6
Normalgameten = 3
Meioticerror
Species B2n = 4
Unreducedgametewith 4chromosomes
Hybridwith 7chromosomes
Fig. 24-11-3
Species A2n = 6
Normalgameten = 3
Meioticerror
Species B2n = 4
Unreducedgametewith 4chromosomes
Hybridwith 7chromosomes
Unreducedgametewith 7chromosomes
Normalgameten = 3
Fig. 24-11-4
Species A2n = 6
Normalgameten = 3
Meioticerror
Species B2n = 4
Unreducedgametewith 4chromosomes
Hybridwith 7chromosomes
Unreducedgametewith 7chromosomes
Normalgameten = 3
Viable fertilehybrid(allopolyploid)2n = 10
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• Polyploidy is much more common in plants than in animals
• Many important crops (oats, cotton, potatoes, tobacco, and wheat) are polyploids
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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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Sexual Selection
• Sexual selection can drive sympatric speciation
• Sexual selection for mates of different colors has likely contributed to the speciation in cichlid fish in Lake Victoria
Fig. 24-12
EXPERIMENT
Normal lightMonochromatic
orange light
P.pundamilia
P. nyererei
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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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• 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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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
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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
Fig. 24-13
EUROPE
Fire-belliedtoad range
Hybrid zone
Yellow-belliedtoad rangeYellow-bellied toad,
Bombina variegata
Fire-bellied toad,Bombina bombina
Alle
le f
req
uen
cy (
log
sca
le)
Distance from hybrid zone center (km)
40 30 20 2010 100
0.01
0.1
0.5
0.9
0.99
Fig. 24-13a
Yellow-bellied toad,Bombina variegata
Fig. 24-13b
Fire-bellied toad, Bombina bombina
Fig. 24-13c
Fire-belliedtoad range
Yellow-belliedtoad range
Hybrid zone
All
ele
freq
uen
cy (
log
sca
le)
Distance from hybrid zone center (km)40 30 20 2010 100
0.01
0.1
0.5
0.9
0.99
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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
Fig. 24-14-1
Gene flow
Population(five individualsare shown)
Barrier togene flow
Fig. 24-14-2
Gene flow
Population(five individualsare shown)
Barrier togene flow
Isolated populationdiverges
Fig. 24-14-3
Gene flow
Population(five individualsare shown)
Barrier togene flow
Isolated populationdiverges
Hybridzone
Hybrid
Fig. 24-14-4
Gene flow
Population(five individualsare shown)
Barrier togene flow
Isolated populationdiverges
Hybridzone
Hybrid
Possibleoutcomes:
Reinforcement
OR
OR
Fusion
Stability
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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
Fig. 24-15
Sympatric malepied flycatcher
Allopatric malepied flycatcher
Pied flycatchers
Collared flycatchers
Nu
mb
er o
f fe
mal
es
(none)
Females matingwith males from:
Ownspecies
Otherspecies
Sympatric males
Ownspecies
Otherspecies
Allopatric males
0
4
8
12
16
20
24
28
Fig. 24-15a
Sympatric malepied flycatcher
Allopatric malepied flycatcher
Fig. 24-15b
Pied flycatchers
Collared flycatchers
28
24
20
16
12
8
4
0
(none)
Nu
mb
er o
f fe
mal
es
Females matingwith males from:
Ownspecies
Otherspecies
Sympatric males
Ownspecies
Otherspecies
Allopatric males
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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
Fig. 24-16
Pundamilia nyererei Pundamilia pundamilia
Pundamilia “turbid water,”hybrid offspring from a locationwith turbid water
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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
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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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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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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 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
Fig. 24-17
(a) Punctuated pattern
(b) Gradual pattern
Time
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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
Fig. 24-18
(a) The wild sunflower Helianthus anomalus
H. anomalus
H. anomalus
H. anomalus
(b) The genetic composition of three chromosomes in H. anomalus and in experimental hybrids
Chromosome 1
Chromosome 2
Chromosome 3
Experimental hybrid
Experimental hybrid
Experimental hybrid
Key
Region diagnostic forparent species H. petiolaris
Region diagnostic forparent species H. annuus
Region lacking information on parental origin
Fig. 24-18a
(a) The wild sunflower Helianthus anomalus
Fig. 24-18b
(b) The genetic composition of three chromosomes in H. anomalus and in experimental hybrids
Region lacking information on parental origin
Region diagnostic forparent species H. petiolaris
Region diagnostic forparent species H. annuus
Key
Experimental hybrid
Experimental hybrid
Experimental hybrid
Chromosome 3
Chromosome 2
Chromosome 1
H. anomalus
H. anomalus
H. anomalus
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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
Fig. 24-19
Fig. 24-20
(a) Typical Mimulus lewisii (b) M. lewisii with an M. cardinalis flower-color allele
(c) Typical Mimulus cardinalis (d) M. cardinalis with an M. lewisii flower-color allele
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From Speciation to Macroevolution
• Macroevolution is the cumulative effect of many speciation and extinction events
Fig. 24-UN1
Original population
Allopatric speciation Sympatric speciation
Fig. 24-UN2Ancestral species:
Triticummonococcum(2n = 14)
AA BB
WildTriticum(2n = 14)
Product:
AA BB DD
T. aestivum(bread wheat)(2n = 42)
WildT. tauschii(2n = 14)
DD
Fig. 24-UN3
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
You should now be able to:
1. Define and discuss the limitations of the four species concepts
2. Describe and provide examples of prezygotic and postzygotic reproductive barriers
3. Distinguish between and provide examples of allopatric and sympatric speciation
4. Explain how polyploidy can cause reproductive isolation
5. Define the term hybrid zone and describe three outcomes for hybrid zones over time
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