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Chapter 14 The Origin of Species
 Bowerbirds, native to New Guinea and Australia,
are named for the structure, called a bower, that
the male weaves from twigs and grasses to attract
females.
 After building his bower, the male collects objects
such as fruits, seeds, insect parts, rocks, flowers,
and leaves and arranges them artfully by color and
type.
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Figure 14.01
Females are dull colored (as
are males) and tour the bowers
of local males, inspecting each
while its owner courts her with
a song and dance.
Vogelkop bowerbird photograph by Barrie Britton
DEFINING SPECIES
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14.1 The origin of species is the source of
biological diversity
 Microevolution is the change in the gene pool of a
population from one generation to the next.
 Speciation is the process by which one species
splits into two or more species.
– Every time speciation occurs, the diversity of life
increases.
– The many millions of species on Earth have all arisen
from an ancestral life form that lived around 3.5 billion
years ago.
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14.2 There are several ways to define a species
 The word species is from the Latin for “kind” or
“appearance.”
 Although the basic idea of species as distinct lifeforms seems intuitive, devising a more formal
definition is not easy and raises questions.
– How similar are members of the same species?
– What keeps one species distinct from others?
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14.2 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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14.2 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 Similarity between two species: the eastern meadowlark (left) and western meadowlark (right). Similar
looking but different songs and mating behavior
Figure 14.2B
Diversity within one species
14.2 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 Hybridization between two species of bears
Grizzly bear
Polar bear
Hybrid “grolar” bear
14.2 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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14.2 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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14.2 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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14.3 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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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).
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Figure 14.3
Habitat isolation
(lack of opportunities to encounter each other)
The garter snake Thamnophis
atratus lives mainly in water.
The garter snake
Thamnophis sirtalis
lives on land.
Figure 14.3
Temporal isolation
(breeding at different times or seasons)
The eastern spotted skunk
(Spilogale putorius) breeds in
late winter.
The western spotted skunk
(Spilogale gracilis) 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.
4. In mechanical isolation, female and male sex organs are
not compatible.
5. In gametic isolation, female and male gametes are not
compatible.
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Figure 14.3
Behavioral isolation
(different courtship rituals)
The blue-footed booby
(Sula nebouxii) performs an
elaborate courtship dance.
The masked booby
(Sula dactylatra) performs
a different courtship ritual.
Figure 14.3
Mechanical isolation
(physical incompatibility of reproductive parts)
Heliconia latispatha is pollinated
by hummingbirds with short,
straight bills.
Heliconia pogonantha is
pollinated by hummingbirds
with long, curved bills.
Figure 14.3
Gametic isolation
(molecular incompatibility of eggs and sperm
or pollen and stigma)
Purple sea urchin
(Strongylocentrotus
purpuratus)
Red sea urchin
(Strongylocentrotus
franciscanus)
14.3 Reproductive barriers keep species separate
 Three types of postzygotic barriers operate after
hybrid zygotes have formed.
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 14.3
Reduced hybrid viability
(hybrid development or survival impaired
by interaction of parental genes)
Some salamander species can hybridize,
but their offspring do not develop fully or
are frail and will not survive long enough
to reproduce.
Figure 14.3
Reduced hybrid fertility
(vigorous hybrids that cannot
produce viable offspring)
A mule is the sterile hybrid
offspring of a horse and a donkey.
Figure 14.3
Hybrid breakdown
(viable and fertile hybrids with feeble
or sterile offspring)
The rice hybrids on the left and right
are fertile, but plants of the next
generation (middle) are sterile.
MECHANISMS
OF SPECIATION
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14.4 In allopatric speciation, geographic isolation
leads to speciation
 In allopatric speciation, populations of the same
species are geographically separated, isolating their
gene pools.
 Isolated populations will no longer share changes in
allele frequencies caused by
– natural selection,
– genetic drift, and/or
– mutation.
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14.4 In allopatric speciation, geographic isolation
leads to speciation
 Gene flow between populations is initially prevented
by a geographic barrier. For example
– the Grand Canyon and Colorado River separate two
species of antelope squirrels, and
– the Isthmus of Panama separates 15 pairs of snapping
shrimp.
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Figure 14.4A Allopatric speciation of geographically isolated antelope squirrels
(Ammospermophilus)
North rim
South rim
A. harrisii
A. leucurus
Figure 14.4B Allopatric speciation in snapping shrimp (Alpheus)
A. formosus
A. nuttingi
ATLANTIC OCEAN
Isthmus of Panama
PACIFIC OCEAN
A. panamensis
A. millsae
14.5 Reproductive barriers can evolve as
populations diverge
 How do reproductive barriers arise?
 Experiments have demonstrated that reproductive
barriers can evolve as a by-product of changes in
populations as they adapt to different environments.
 These studies have included
– laboratory studies of fruit flies and
– field studies of monkey flowers and their pollinators.
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Figure 14.5A Evolution of reproductive barriers in laboratory populations of fruit flies adapted to different food sources
Initial sample
of fruit flies
Starch medium
Maltose medium
Female
Starch
Maltose
22
9
8
20
Number of matings
in experimental groups
Results
Female
Population Population
#1
#2
Male
Pop#2 Pop#1
Maltose Starch
Male
Mating experiments
18
15
12
15
Number of matings
in starch control groups
Figure 14.5B Transferring an
allele between monkey
flowers changes flower color Pollinator choice in
and influences pollinator
typical monkey flowers
choice in Mimulus.
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
14.6 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 can reproductive isolation develop when
members of sympatric populations remain in contact
with each other?
 Gene flow between populations may be reduced by
– polyploidy,
– habitat differentiation, or
– sexual selection.
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14.6 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
– within a species (by self-fertilization) or
– between two species (by hybridization).
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Figure 14.6A Sympatric speciation by polyploidy within a single species
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 Sympatric speciation producing a hybrid polyploid from two different species
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
2
Viable, fertile
hybrid species
2n = 10
14.7 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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14.7 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 The evolution of bread wheat,
Triticum aestivum

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)
14.8 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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14.8 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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14.8 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)
14.10 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 (Fig 14.10A)
– reinforcement may strengthen barriers to reproduction,
such as occurs in flycatchers (Fig. 14.10B), or
– fusion may reverse the speciation process as gene flow
between species increases, as may be occurring among
the cichlid species in Lake Victoria (Fig. 14.10C).
 In stable hybrid zones, a limited number of hybrid
offspring continue to be produced.
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Figure 14.10A Formation of a hybrid zone
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 Reinforcement
of reproductive barriers
Allopatric
populations
Sympatric
populations
Male
collared
flycatcher
Male
pied
flycatcher
Pied flycatcher
from allopatric
population
Pied flycatcher
from sympatric
population
Figure 14.10C Fusion: males of Pundamilia nyererei and Pundamilia pundamilia contrasted with a hybrid from an area with
turbid water
Pundamilia nyererei
Pundamilia pundamilia
Hybrid: Pundamilia “turbid water”
14.11 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 =
gradualism.
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Figure 14.11 Two models for the tempo of speciation
Punctuated pattern
Gradual pattern
Time
14.11 Speciation can occur rapidly or slowly
 What is the total length of time between speciation
events (between formation of a species and
subsequent divergence of that species)?
– In a survey of 84 groups of plants and animals, the time
ranged from 4,000 to 40 million years.
– Overall, the time between speciation events averaged 6.5
million years.
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