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The Origin of Species
Chapter 24
Chapter 24
The Origin of Species
Overview: That “Mystery of Mysteries”
• In the Galápagos Islands Darwin discovered
plants and animals found nowhere else on Earth
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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 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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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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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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Figure 24.2
(a) Similarity between different species
(b) Diversity within a species
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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Figure 24.3_a
Prezygotic barriers
Habitat
IsolationTemporal
Isolation
Behavioral
Isolation
Mechanical
Isolation
Gametic
Isolation
Reduced Hybrid
Viability
Reduced Hybrid
Fertility
Hybrid
Breakdown
Individuals of
differentspecies
MATINGATTEMPT FERTILIZATION
VIABLE,FERTILE
OFFSPRING
Postzygotic barriers
(a) (c) (e)
(d)
(b)
(g)
(k)
(h) (i)
(j)
(l)(f)
Figure 24.3_b
Prezygotic barriers
Habitat
Isolation
Temporal
Isolation
Behavioral
Isolation
Mechanical
Isolation
Gametic
Isolation
Individuals of
differentspecies
MATINGATTEMPT
FERTILIZATION
(a) (c) (e) (f)
(b)
(g)
(d)
Reduced Hybrid
Viability
Reduced Hybrid
Fertility
Hybrid
Breakdown
FERTILIZATION
VIABLE,FERTILE
OFFSPRING
Postzygotic barriers
(k)
(h) (i)
(j)
(l)
Figure 24.3_c
• 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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• 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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• Temporal isolation: Species that breed at
different times of the day, different seasons,
or different years cannot mix their gametes
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• Behavioral isolation: Courtship rituals and other
behaviors unique to a species are effective
barriers
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Albatross Courtship Ritual
Giraffe Courtship Ritual
Blue Footed Boobies Courtship Ritual
• Mechanical isolation: Morphological differences
can prevent successful mating
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• Gametic Isolation: Sperm of one species may not
be able to fertilize eggs of another species
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• 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
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• Reduced hybrid fertility: Even if hybrids are
vigorous, they may be sterile
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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
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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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Figure 24.4Grizzly bear (U. arctos)
Polar bear (U. maritimus)
Hybrid “grolar bear”
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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• 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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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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Figure 24.5
(a) (b)Allopatric speciation.
A population forms a
new species while
geographically isolated
from its parent population.
Sympatric speciation.
A subset of a population
forms a new species
without geographic
separation.
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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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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Figure 24.6
A. harrisii A. leucurus
• 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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Figure 24.7
(a) Under high predation (b) Under low predation
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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Figure 24.8
A. formosus
Atlantic Ocean
A. nuttingi
Isthmus of Panama
Pacific Ocean
A. panamensis A. millsae
• 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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Figure 24.9
Deg
ree
of
rep
rod
uc
tiv
e i
so
lati
on
Geographic distance (km)
0 50 100 150 200 250 300
2.0
1.5
1.0
0.5
0
• Barriers to reproduction are intrinsic;
separation itself is not a biological barrier
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Figure 24.10 EXPERIMENT
RESULTS
Initial population
of fruit flies
(Drosophila
pseudoobscura)
Some flies raised
on starch medium
Mating experiments
after 40 generations
Some flies raised on
maltose medium
Female
Starch Maltose
Ma
leM
alt
ose
Sta
rch
Number of matings
in experimental group
22 9
8 20
Female
Starch
population 1
Ma
leS
tarc
h
po
pu
lati
on
2
Number of matings
in control group
18 15
12 15
Starch
population 2
Sta
rch
po
pu
lati
on
1
Figure 24.10a
EXPERIMENT
Initial population
of fruit flies
(Drosophila
pseudoobscura)
Some flies raised
on starch medium
Mating experiments
after 40 generations
Some flies raised on
maltose medium
Figure 24.10b
RESULTS
Female
Starch Maltose
Male
Malt
ose
Sta
rch
Number of matings
in experimental group
22 9
8 20
Female
Starch
population 1
Male
Sta
rch
po
pu
lati
on
2Number of matings
in control group
18 15
12 15
Starch
population 2
Sta
rch
po
pu
lati
on
1
Sympatric (“Same Country”) Speciation
• In sympatric speciation, speciation takes place
in geographically overlapping populations
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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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• An allopolyploid is a species with multiple
sets of chromosomes derived from different
species
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Figure 24.11-1
Species A
2n = 6
Species B
2n = 4
Normal
gamete
n = 3
Meiotic error;
chromosome number not
reduced from 2n to n
Unreduced gamete
with 4 chromosomes
Figure 24.11-2
Species A
2n = 6
Species B
2n = 4
Normal
gamete
n = 3
Meiotic error;
chromosome number not
reduced from 2n to n
Unreduced gamete
with 4 chromosomes
Hybrid with
7 chromosomes
Figure 24.11-3
Species A
2n = 6
Species B
2n = 4
Normal
gamete
n = 3
Meiotic error;
chromosome number not
reduced from 2n to n
Unreduced gamete
with 4 chromosomes
Hybrid with
7 chromosomes
Unreduced gamete
with 7 chromosomes
Normal
gamete
n = 3
Figure 24.11-4
Species A
2n = 6
Species B
2n = 4
Normal
gamete
n = 3
Meiotic error;
chromosome number not
reduced from 2n to n
Unreduced gamete
with 4 chromosomes
Hybrid with
7 chromosomes
Unreduced gamete
with 7 chromosomes
Normal
gamete
n = 3
New species:
viable fertile hybrid
(allopolyploid) 2n = 10
• 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 speciation in cichlid fish in
Lake Victoria
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Figure 24.12
Normal light
Monochromatic
orange light
P. pundamilia
P. nyererei
EXPERIMENT
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 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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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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Figure 24.13
EUROPE
Yellow-bellied
toad, Bombina
variegata
Fire-bellied
toad range
Hybrid zone
Yellow-bellied
toad range
Fire-bellied toad, Bombina bombina
Fre
qu
en
cy o
f
B. v
ari
eg
ata
-sp
ecif
ic a
lle
le
Yellow-bellied
toad range
Hybrid
zone
Fire-bellied
toad range
Distance from hybrid zone center (km)40
0.99
0.9
0.5
0.1
0.01
30 20 10 0 10 20
Figure 24.13a
EUROPE
Fire-bellied
toad rangeHybrid zone
Yellow-bellied
toad range
Figure 24.13b
Fre
qu
en
cy o
f
B.
va
rie
ga
ta-s
pecif
ic a
llele
Yellow-bellied
toad range
Hybrid
zone
Fire-bellied
toad range
Distance from hybrid zone center (km)
40
0.99
0.9
0.5
0.1
0.01
30 20 10 0 10 20
Figure 24.13c
Fire-bellied toad, Bombina bombina
Yellow-bellied toad, Bombina variegata
• 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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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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Figure 24.14-1
Gene flow
PopulationBarrier to
gene flow
Figure 24.14-2
Gene flow
PopulationBarrier to
gene flow
Isolated
population
diverges
Figure 24.14-3
Gene flow
PopulationBarrier to
gene flow
Isolated
population
diverges
Hybrid
zone
Hybrid
individual
Figure 24.14-4
Gene flow
PopulationBarrier to
gene flow
Isolated
population
diverges
Hybrid
zone
Hybrid
individual
Possible
outcomes:
Reinforcement
OR
OR
Fusion
Stability
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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Figure 24.15
Females choosing between
these males:Females choosing between
these males:
Sympatric pied male
Sympatric collared male
Allopatric pied male
Allopatric collared male
(none)
Female mate choice Female mate choice
Own
speciesOwn
species
Other
species
Other
species
Nu
mb
er
of
fem
ale
s
28
24
20
16
12
8
4
0
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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Figure 24.16
Pundamilia nyererei Pundamilia pundamilia
Pundamilia “turbid water,”
hybrid offspring from a location
with turbid water
Figure 24.16a
Pundamilia nyererei
Figure 24.16b
Pundamilia pundamilia
Figure 24.16c
Pundamilia “turbid water,” hybrid offspring from a
location with turbid water
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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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 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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Figure 24.17
(a) Punctuated
pattern
Time
(b) Gradual
pattern
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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Figure 24.18
Figure 24.19
H. annuus
gameteH. petiolarus
gamete
F1 experimental hybrid
(4 of the 2n = 34
chromosomes are shown)
EXPERIMENT
RESULTS
Chromosome 1
H. anomalus
Chromosome 2
H. anomalus
Experimental hybrid
Experimental hybrid
Figure 24.19a
H. annuus
gameteH. petiolarus
gamete
F1 experimental hybrid
(4 of the 2n = 34
chromosomes are shown)
EXPERIMENT
Figure 24.19b
RESULTS
Chromosome 1
H. anomalus
Chromosome 2
H. anomalus
Experimental hybrid
Experimental hybrid
• 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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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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• 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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Typical
Mimulus
lewisii
(a)
Typical
Mimulus
cardinalis
(c)
M. lewisii with an
M. cardinalis flower-color
allele
(b)
M. cardinalis with an
M. lewisii flower-color
allele
(d)
Figure 24.20
From Speciation to Macroevolution
• Macroevolution is the cumulative effect of many
speciation and extinction events
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