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Biology
Sylvia S. Mader
Michael Windelspecht
Chapter 17
Speciation and
Macroevolution
Lecture Outline
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17.1 How New Species Evolve
• Macroevolution is best observed within
the fossil record
 Requires the origin of species, also called
speciation.
 Speciation is the final result of changes in
the gene pool’s allelic and genotypic
frequencies.
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How New Species Evolve
• Every species has its own evolutionary
history
• Species Concepts
 Refers to the different ways in which a
species is defined.
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How New Species Evolve
• Morphological species concept
 Based on analysis of diagnostic traits
distinguishing one species from another.
• Can be distinguished anatomically
• Method used by Linnaeus
• Most species are described this way
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How New Species Evolve
• The evolutionary species concept
distinguishes species from one another
based on morphological (structural) traits
 Critical traits for distinguishing species are
called diagnostic traits
• The phylogenetic species concept
relies on the identification of species
based on common ancestry.
 A common ancestor for two or more
different groups.
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Evolutionary
Species
Concept
Orcinus orca
Hindlimbs too
reduced for walking
or swimming
Rodhocetus kasrani
Ambulocetus natans
Hindlimbs used
for both walking
on land and
paddling in water
Tetrapod with limbs
for walking
Pakicetus attocki
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How New Species Evolve
• Biological Species Concept
• Populations of the same species breed only
among themselves
• Experience reproductive isolation from other
such populations
• Very few species are actually tested for
reproductive isolation
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How New Species Evolve
• Reproductive isolating mechanisms inhibit
gene flow between species
• Two general types:
 (1) Prezygotic Isolating Mechanisms – prevent
mating attempts or make it unlikely that fertilization
will be successful
• Habitat Isolation - species occupy different habitats
• Temporal Isolation - each reproduces at a different time
• Behavioral Isolation - courtship patterns for recognizing
mates
• Mechanical Isolation - incompatible animal genitalia or plant
floral structures
• Gamete Isolation - gametes that meet do not fuse to
become a zygote
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Reproductive Barriers
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Prezygotic Isolating Mechanisms
Premating
Postzygotic Isolating Mechanisms
Mating
Habitat isolation
Species at same locale
occupy different habitats.
species 1
Temporal isolation
Zygote mortality
Mechanical isolation
Genitalia between
species are unsuitable
for one another.
Species reproduce at
different seasons or
different times of day.
species 2
Fertilization
Fertilization occurs, but
zygote does not survive.
Hybrid sterility
hybrid
offspring
Hybrid survives but is
sterile and cannot
reproduce.
Gamete isolation
Behavioral isolation
In animal species,
courtship behavior differs,
or individuals respond to
different songs, calls,
pheromones, or other
signals.
Sperm cannot reach
or fertilize egg.
F2 fitness
Hybrid is fertile, but F2 hybrid
has reduced fitness.
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Temporal Isolation
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Mating Activity
high
low
March 1
April 1
May 1
June 1
July 1
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Prezygotic Isolating
Mechanism
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© Barbara Gerlach/Visuals Unlimited
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How New Species Evolve
• Reproductive Isolating Mechanisms (cont.)
 Postzygotic Isolating Mechanisms - Prevent
hybrid offspring from developing or breeding
• Hybrid Inviability - hybrid zygote is not viable and
dies
• Hybrid Sterility - hybrid zygote develops into a
sterile adult
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Postzygotic
Isolating
Mechanism
Parents
horse
donkey
mating
fertilization
Usually
mules cannot
reproduce
(are sterile).
If mules do
produce an
offspring,
it usually is
sterile.
mule
(hybrid)
Offspring
top left: © Creatas/PunchStock RF; top right: © Photodisc Collection/Getty RF; bottom : © Jorg & Petra Wegner/Animals Animals
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17.2 Modes of Speciation
• Speciation:
 The splitting of one species into two, or
 The transformation of one species into a new
species over time
• Two modes:
 (1) Allopatric Speciation
• Two geographically isolated populations of one
species become different species over time
• Can be due to differing selection pressures in
differing environments
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Allopatric Speciation
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Ensatina eschscholtzi picta
1
Members of a northern ancestral
population migrated southward.
Ensatina eschscholtzi
oregonensis
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Subspecies are separated by
California’s Central Valley .Some
interbreeding between populations
does occur.
Central
Valley
Barrier
Ensatina eschscholtzi platensis
Ensatina eschscholtzi
xanthoptica
Ensatina eschscholtzi
croceater
Ensatina eschscholtzi
eschscholtzii
3
Evolution has occurred, and in the
south, subspecies do not
interbreed even though they live in
the same environment.
Ensatina eschscholtzi
klauberi
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Allopatric Speciation Among
Sockeye Salmon
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Lake male
River male
Lake female
River female
a. Sockeye salmon at Pleasure Point Beach, Lake Washington
b. Sockeye salmon in Cedar River .The river connects with
Lake Washington.
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Modes of Speciation
• Two modes (continued):
 (2) Sympatric Speciation
• One population develops into two or more
reproductively isolated groups
• No prior geographic isolation
• In plants, sympatric speciation often involves
polyploidy (a chromosome number beyond the
diploid [2n] number)
– Tetraploid hybridization in plants
» Results in self fertile species that are reproductively
isolated from either parental species
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Modes of Speciation
 (2) Sympatric Speciation
• A polyploid plant can reproduce with itself, but cannot
reproduce with the 2n population because not all the
chromosomes would be able to pair during meiosis.
• Two types of polyploidy are known:
– Autoploidy - diploid plant produces diploid gametes due to
nondisjunction during meiosis.
» If diploid gamete fuses with a haploid gamete, a triploid plant
results.
» A triploid (3n) plant is sterile and cannot produce offspring
because the chromosomes cannot pair during meiosis.
– Alloploidy - more complicated process than autoploidy
» Requires two different but related species of plants
» Hybridization is followed by doubling of the chromosomes.
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Autoploidy
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2n = 14
2n = 10
Clarkia concinna
Clarkia virgata
hybrid
doubling of chromosome number
2n = 24
Clarkia pulchella
(C. pulchella): © J. L. Reveal; (C. concinna): © Gerald & Buff Corsi/Visuals Unlimited; (C. virgata): ©: Dr. Dean Wm. Taylor/Jepson Herbarium, UC Berkeley
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Modes of Speciation
• Adaptive Radiation
 Occurs when members of a species invade several
new geographically separate environments
 The populations become adapted to the different
environments
 Many new species evolve from the single ancestral
species
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Adaptive Radiation in
Hawaiian Honeycreepers
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* Lesser Koa finch
Palila
Laysan
finch
* Greater
Koa finch
Ou
* Kona
finch
Maui parrot bill
Akiapolaau
* Kauai
akialoa
Nukupuu
* Akialoa
Genus Loxops
Anianiau
(lesser
amakihi)
Great
amakihi
(green
solitaire)
* Extinct species or subspecies
Alauwahio
(Hawaiian
creeper)
Akepa
Amakihi
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Modes of Speciation
• Convergent Evolution
 Occurs when a similar biological trait evolves in two
unrelated species as a result of exposure to similar
environments.
• Traits evolving in this manner are termed analogous traits.
– Similar function, but different origin
– Ex: bird wing vs. bat wing
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Convergent Evolution of
Africa Lake Fish
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Lake Tanganyika
Lake Malawi
Lake Tanganyika
Lake Malawi
Reprinted by permission from Macmillan Publishers Ltd on behalf of Cancer Research UK: RC Albertson, TD
Kocher (2006) Genetic and developmental basis of cichlid trophic diversity. Heredity vol. 97 (3) pp. 211-221
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17.3 Principles of
Macroevolution
• Macroevolution
 Evolution at the species or higher level of
classification
 Some evolutionists support a gradualistic
model
• Evolution at the species level occurs gradually
• Speciation occurs after populations become
isolated
• Each group continues its own evolutionary pathway
• The gradualistic model suggests that it is difficult to
indicate when speciation occurred
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Principles of Macroevolution
• Macroevolution
 Some paleontologists support the punctuated
equilibrium model
• Species can appear quite suddenly
– Species then remain essentially unchanged phenotypically
during a period of stasis (sameness) until they undergo
extinction.
• This model states that periods of equilibrium are
punctuated by speciation
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Gradualistic and Punctuated
Equilibrium Models
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new
species 1
new
species 1
ancestral
species
ancestral
species
transitional link
ancestral
species
new
species 2
Time
a. Gradualistic model
stasis
new
species 2
Time
b. Punctuated equilibrium
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Principles of Macroevolution
• Developmental Genes and Macroevolution
 Genes can bring about radical changes in body
shapes and organs.
• Gene expression can influence development
– A change in gene expression could stop a developmental
process or continue it beyond its normal time.
• Using modern technology, researchers discovered
genes whose differential expression can bring about
changes in body shapes and organs.
– Pax6 in eye development
– Tbx5 in limb development
– Hox genes in development of overall shape
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Pax6 Gene and Eye
Development
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(Left): © Carolina Biological Supply/Photo Researchers, Inc.; (Center): © Vol. OS02/PhotoDisc/Getty Images; (Right): © Aldo Brando/Peter Arnold, Inc.
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Study of Pax6 Gene
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Courtesy Walter Gehring, reprinted with permission from Induction of Ectopic Eyes by Target Expression of the Eyeless Gene in Drosophila, G. Halder, P. Callaerts, Walter J. Gehring, Science Vol. 267, © 24
March 1995 American Association for the Advancement of Science
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Hox6 Genes
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(Both): © A. C. Burke, 2000
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Principles of Macroevolution
• Macroevolution is not goal-oriented
 The evolution of the horse (Equus)
• Studied since the 1870s
• Model for gradual, straight-line evolution with the
modern-horse as its goal
• Three trends were particularly evident during the
evolution of the horse:
– Increase in overall size
– Toe reduction
– Change in tooth size and shape
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Principles of Macroevolution
• Macroevolution is not goal-oriented
 Discovery of more fossils has led to recognition
that:
• The lineage of a horse is complicated by the
presence of many ancestors with varied traits
– The direct ancestor of Equus is not known
– Each ancestral species was adapted to its
environment
• Speciation, diversification, and extinction are
common occurrences in the fossil record
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Simplified Family Tree of Equus
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2 MYA
4 MYA
Equus
Neohipparion
Hipparion
12 MYA
Dinohippus
15 MYA
Megahippus
Merychippus
17 MYA
23 MYA
25 MYA
35 MYA
Miohippus
40 MYA
Palaeotherium
45 MYA
50 MYA
Hyracotherium
55 MYA
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