Transcript There are several ways to define a species

Chapter 24
The Origin of Species
PowerPoint Lectures for
Campbell Biology: Concepts & Connections, Seventh Edition
Reece, Taylor, Simon, and Dickey
© 2012 Pearson Education, Inc.
Lecture by Edward J. Zalisko
Figure 14.0_1
Chapter 14: Big Ideas
Defining Species
Mechanisms
of Speciation
Introduction
 Many species of cormorants around the world can
fly.
 Cormorants on the Galápagos Islands cannot fly.
 How did these flightless cormorants get to the
Galápagos Islands?
 Why are these flightless cormorants found
nowhere else in the world?
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Figure 14.0_2
Figure 14.1
There are several ways to define a species
 The biological species concept defines a
species as
– a group of populations,
– whose members have the potential to interbreed in
nature, and
– produce fertile offspring.
– Therefore, members of a species are similar because
they reproduce with each other.
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There are several ways to define a species
 Reproductive isolation
– prevents members of different species from mating with
each other,
– prevents gene flow between species, and
– maintains separate species.
– Therefore, species are distinct from each other because
they do not share the same gene pool.
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Figure 14.2A
Eastern Meadowlark
Western Meadowlark
There are several ways to define a species
 The biological species concept can be problematic.
– Some pairs of clearly distinct species occasionally
interbreed and produce hybrids.
– For example, grizzly bears and polar bears may interbreed and
produce hybrids called grolar bears.
– Melting sea ice may bring these two bear species together more
frequently and produce more hybrids in the wild.
– Reproductive isolation cannot usually be determined for
extinct organisms known only from fossils.
– Reproductive isolation does not apply to prokaryotes or
other organisms that reproduce only asexually.
– Therefore, alternate species concepts can be useful.
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Figure 14.2C
Grizzly bear
Polar bear
Hybrid “grolar” bear
There are several ways to define a species
 The morphological species concept
– classifies organisms based on observable physical traits
and
– can be applied to
– asexual organisms and
– fossils.
– However, there is some subjectivity in deciding which traits to
use.
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There are several ways to define a species
 The ecological species concept
– defines a species by its ecological role or niche and
– focuses on unique adaptations to particular roles in a
biological community.
– For example, two species may be similar in appearance
but distinguishable based on
– what they eat or
– where they live.
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There are several ways to define a species
 The phylogenetic species concept
– defines a species as the smallest group of individuals that
shares a common ancestor and thus
– forms one branch of the tree of life.
– Biologists trace the phylogenetic history of a species by
comparing its
– morphology or
– DNA.
– However, defining the amount of difference required to
distinguish separate species is a problem.
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Reproductive barriers keep species separate
 Reproductive barriers
– serve to isolate the gene pools of species and
– prevent interbreeding.
 Depending on whether they function before or after
zygotes form, reproductive barriers are categorized
as
– prezygotic or
– postzygotic.
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Figure 14.3A
Individuals of different species
Prezygotic Barriers
Habitat isolation
Temporal isolation
Behavioral isolation
Mechanical isolation
Gametic isolation
Fertilization
Postzygotic Barriers
Reduced hybrid viability
Reduced hybrid fertility
Hybrid breakdown
Viable, fertile offspring
14.3 Reproductive barriers keep species separate
 Five types of prezygotic barriers prevent mating or
fertilization between species.
1. In habitat isolation, two species live in the same general
area but not in the same kind of place.
2. In temporal isolation, two species breed at different times
(seasons, times of day, years).
Video: Giraffe Courtship Ritual
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Figure 14.3B
Habitat Isolation
2 species of garter snakes Thamnophis
A lives in mainly in water
B lives on land
Figure 14.3C
Temporal Isolation
Eastern spotted
skunk – breeds in
late winter
Western
spotted skunk
– breeds in the
fall.
14.3 Reproductive barriers keep species separate
 Prezygotic Barriers, continued
3. In behavioral isolation, there is little or no mate
recognition between females and males of different
species ex. Courtship rituals in birds
4. In mechanical isolation, female and male sex organs are
not compatible. Ex. Insects have unique and complex
males structures that will fit in female structures of the
same species, snails that have spirals in different
directions prevent from genital openings to align.
5. In gametic isolation, female and male gametes are not
compatible. Ex. Pollen of same species will allow
fertilization, release of sperm in water in aquatic species.
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Figure 14.3E
Genital openings (black
and white arrows) not
aligned
Figure 14.3F
Reproductive barriers keep species separate
 Postzygotic barriers operate after hybrid zygotes
have formed. Chromosome incompatibility, 3 types
1. In reduced hybrid viability, most hybrid offspring do not
survive.
2. In reduced hybrid fertility, hybrid offspring are vigorous
but sterile.
3. In hybrid breakdown,
– the first-generation hybrids are viable and fertile but
– the offspring of the hybrids are feeble or sterile.
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Figure 14.3G
Horse
Donkey
Mule
MECHANISMS
OF SPECIATION
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In allopatric speciation, geographic isolation leads
to speciation
 In allopatric speciation, populations of the same
species are geographically separated, isolating their
gene pools.
– the Grand Canyon and Colorado River separate two species of
antelope squirrels, and
– the Isthmus of Panama separates 15 pairs of snapping shrimp.
 Isolated populations will no longer share changes in
allele frequencies caused by
– natural selection,
– genetic drift, and/or
– mutation.
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Figure 14.4A
North rim
South rim
A. harrisii
A. leucurus
Figure 14.4B
A. formosus
A. nuttingi
ATLANTIC OCEAN
Isthmus of Panama
PACIFIC OCEAN
A. panamensis
A. millsae
Figure 14.5B
Pollinator choice in
typical monkey flowers
Pollinator choice after
color allele transfer
Typical M. lewisii
(pink)
M. lewisii with
red-color allele
Typical M. cardinalis
(red)
M. cardinalis with
pink-color allele
Sympatric speciation takes place without
geographic isolation
 Sympatric speciation occurs when a new species
arises within the same geographic area as a parent
species.
 How? Gene flow between populations may be
reduced by
– polyploidy,
– habitat differentiation, or
– sexual selection.
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Sympatric speciation takes place without
geographic isolation
 Many plant species have evolved by polyploidy in
which cells have more than two complete sets of
chromosomes.
 Sympatric speciation can result from polyploidy (cells
containing more than 2N chromosomes when sister
chromatids or homologous chromosomes don’t separate
during meiosis) and fertilization
– within a species (by self-fertilization) or
– between two species (by hybridization).
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Figure 14.6A_s3
1
3
2
Parent
species
2n = 6
Selffertilization
Tetraploid
cells
4n = 12
Diploid
gametes
2n = 6
Viable, fertile
tetraploid
species
4n = 12
Figure 14.6B_s1
Species A
2n = 4
Species B
2n = 6
Gamete
n=2
Gamete
n=3
Figure 14.6B_s2
Chromosomes
cannot pair
Species A
2n = 4
Gamete
n=2
1
Sterile hybrid
n=5
Species B
2n = 6
Gamete
n=3
Can reproduce
asexually
2
Figure 14.6B_s3
Chromosomes
cannot pair
Species A
2n = 4
Gamete
n=2
3
1
Sterile hybrid
n=5
Species B
2n = 6
Gamete
n=3
Can reproduce
asexually
Viable, fertile
hybrid species
2n = 10
2
Errors in cell
division
EVOLUTION CONNECTION: Most plant
species trace their origin to polyploid
speciation
 Plant biologists estimate that 80% of all living plant
species are descendants of ancestors that formed
by polyploid speciation.
 Hybridization between two species accounts for
most of these species.
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EVOLUTION CONNECTION: Most plant
species trace their origin to polyploid
speciation
 Polyploid plants include
– cotton,
– plums,
– oats,
– apples,
– potatoes,
– sugarcane,
– bananas,
– coffee, and
– peanuts,
– bread wheat.
– barley,
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14.7 EVOLUTION CONNECTION: Most plant
species trace their origin to polyploid
speciation
 Wheat
– has been domesticated for at least 10,000 years and
– is the most widely cultivated plant in the world.
 Bread wheat, Triticum aestivum, is
– a polyploid with 42 chromosomes and
– the result of hybridization and polyploidy.
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Figure 14.7

AA
BB
Wild Triticum
(14 chromosomes)
Domesticated
Triticum monococcum
(14 chromosomes)
1
Hybridization
AB
Sterile hybrid
(14 chromosomes)
2
Cell division error
and self-fertilization
DD
AABB
T. turgidum
Emmer wheat
(28 chromosomes)
Wild
T. tauschii
(14 chromosomes)
3
Hybridization
ABD
Sterile hybrid
(21 chromosomes)
4
Cell division error
and self-fertilization
AABBDD
T. aestivum
Bread wheat
(42 chromosomes)
Isolated islands are often showcases of speciation
 Most of the species on Earth are thought to have
originated by allopatric speciation.
 Isolated island chains offer some of the best
evidence of this type of speciation.
 Multiple speciation events are more likely to occur in
island chains that have
– physically diverse habitats,
– islands far enough apart to permit populations to evolve
in isolation, and
– islands close enough to each other to allow occasional
dispersions between them.
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Isolated islands are often showcases of speciation
 The evolution of many diverse species from a
common ancestor is adaptive radiation.
 The Galápagos Archipelago
– is located about 900 km (560 miles) west of Ecuador,
– is one of the world’s great showcases of adaptive
radiation,
– was formed naked from underwater volcanoes,
– was colonized gradually from other islands and the South
America mainland, and
– has many species of plants and animals found nowhere
else in the world.
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Isolated islands are often showcases of speciation
 The Galápagos islands currently have 14 species of
closely related finches, called Darwin’s finches,
because Darwin collected them during his aroundthe-world voyage on the Beagle.
 These finches
– share many finchlike traits,
– differ in their feeding habits and their beaks, specialized
for what they eat, and
– arose through adaptive radiation.
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Figure 14.8
Cactus-seed-eater (cactus finch)
Tool-using insect-eater (woodpecker finch)
Seed-eater (medium ground finch)
SCIENTIFIC DISCOVERY: A long-term field
study documents evolution in Darwin’s
finches
 Peter and Rosemary Grant have worked
– for more than three decades,
– on medium ground finches, and
– on tiny, isolated, uninhabited Daphne Major in the
Galápagos Islands.
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SCIENTIFIC DISCOVERY: A long-term field
study documents evolution in Darwin’s
finches
 Medium ground finches and cactus finches
occasionally interbreed. Hybrids
– have intermediate bill sizes,
– survive well during wet years, when there are plenty of
soft, small seeds around,
– are outcompeted by both parental types during dry
years, and
– can introduce more genetic variation on which natural
selection acts.
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Figure 14.9
Arrival of
new species
Large beaks can
crack large
seeds
Competitor species,
G. magnirostris
Mean beak size
Larger
Smaller beaked
G. fortis can feed
on small seeds
Severe
drought
Severe
drought
Smaller
1975
1980
1985
1990
Year
1995
2000
2005
Hybrid zones provide opportunities to study
reproductive isolation
 What happens when separated populations of
closely related species come back into contact with
each other?
 Biologists try to answer such questions by studying
hybrid zones, regions in which members of different
species meet and mate to produce at least some
hybrid offspring.
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14.10 Hybrid zones provide opportunities to study
reproductive isolation
 Over time in hybrid zones
– reinforcement may strengthen barriers to reproduction,
such as occurs in flycatchers, or
– fusion may reverse the speciation process as gene flow
between species increases, as may be occurring among
the cichlid species in Lake Victoria.
 In stable hybrid zones, a limited number of hybrid
offspring continue to be produced.
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Figure 14.10A
Newly formed
species
Three
populations
of a species
3
Hybrid
zone
2
1
4
Gene
flow
Gene flow
Population
Barrier to
gene flow
Hybrid
individual
Figure 14.10B
Allopatric
populations
Sympatric
populations
Male
collared
flycatcher
Male
pied
flycatcher
Pied flycatcher
from allopatric
population
Pied flycatcher
from sympatric
population
Speciation can occur rapidly or slowly
 There are two models for the tempo of speciation.
1. The punctuated equilibria model draws on the fossil
record, where species
– change most as they arise from an ancestral species and then
– experience relatively little change for the rest of their existence.
2. Other species appear to have evolved more gradually.
Animation: Macroevolution
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Figure 14.11
Punctuated pattern
Gradual pattern
Time
You should now be able to
1. Distinguish between microevolution and speciation.
2. Compare the definitions, advantages, and
disadvantages of the different species concepts.
3. Describe five types of prezygotic barriers and three
types of postzygotic barriers that prevent
populations of closely related species from
interbreeding.
4. Explain how geologic processes can fragment
populations and lead to speciation.
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You should now be able to
5. Explain how reproductive barriers might evolve in
isolated populations of organisms.
6. Explain how sympatric speciation can occur, noting
examples in plants and animals.
7. Explain why polyploidy is important to modern
agriculture.
8. Explain how modern wheat evolved.
9. Describe the circumstances that led to the adaptive
radiation of the Galápagos finches.
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You should now be able to
10. Describe the discoveries made by Peter and
Rosemary Grant in their work with Galápagos
finches.
11. Explain how hybrid zones are useful in the study
of reproductive isolation.
12. Compare the gradual model and the punctuated
equilibrium model of evolution.
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Figure 14.UN01
Zygote
Gametes
Prezygotic barriers
• Habitat isolation
• Temporal isolation
• Behavioral isolation
• Mechanical isolation
• Gametic isolation
Postzygotic barriers
• Reduced hybrid
viability
• Reduced hybrid
fertility
• Hybrid breakdown
Viable,
fertile
offspring
Figure 14.UN02
Original population
a.
b.
Figure 14.UN03
Species
may interbreed
in a
a.
outcome may be
b.
c.
when
d.
when
when
are
a few
hybrids
continue to
be produced
reproductive
barriers
are
f.
e.
keeps
species
separate
and
speciation is
reversed
Figure 14.UN03_1
Species
may interbreed
in a
a.
outcome may be
b.
c.
d.
Figure 14.UN03_2
b.
c.
when
d.
when
when
are
a few
hybrids
continue to
be produced
reproductive
barriers
are
f.
e.
keeps
species
separate
and
speciation is
reversed