Download 1-Ch24-1OriginofSpecies

Ch. 24
The Origin of Species
AP Biology
2007-2008
So…what is a species?
 Biological species concept



defined by Ernst Mayr
population whose members can interbreed &
produce viable, fertile offspring
reproductively compatible
Distinct species:
songs & behaviors are different
enough to prevent interbreeding
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Eastern Meadowlark Western Meadowlark
How and why do new species originate?
 Species are created by a series of
evolutionary processes

populations become isolated
 geographically isolated
 reproductively isolated

isolated populations
evolve independently
 Isolation

allopatric
 geographic separation

sympatric
 still live in same area
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PREZYGOTIC-reproduction barriers
 Obstacle to mating or to fertilization if
mating occurs
geographic isolation
behavioral
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habitat isolation
temporal isolation
mechanical isolation
gametic isolation
Ammospermophilus spp
Geographic isolation
 Species occur in different areas
physical barrier
 allopatric speciation

 “other country”
Harris’s antelope
squirrel inhabits
the canyon’s
south rim (L). Just
a few miles away
on the north rim
(R) lives the
closely related
white-tailed
antelope squirrel
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Habitat isolation
 Species occur in same region, but occupy
different habitats so rarely encounter each other

reproductively isolated
2 species of garter snake,
Thamnophis, occur in
same area, but one lives in
water & other is terrestrial
lions & tigers could hybridize,
but they live in different habitats
and have different behaviors:
 lions are live in prides
 tigers are solitary
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This results in them not seeing
each other as suitable mates.
Temporal isolation
 Species that breed during different times of day,
different seasons, or different years cannot mix
gametes


reproductive isolation
sympatric speciation
 “same country”
Eastern spotted skunk
(L) & western spotted
skunk (R) overlap in
range but eastern mates
in late winter & western
mates in late summer
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sympatric speciation?
Behavioral isolation
 Unique behavioral patterns & rituals isolate species


identifies members of species
attract mates of same species •
 courtship rituals, mating calls
 reproductive isolation
Blue footed boobies mate only
after a courtship display
unique to their species
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Recognizing your
own species
courtship songs of sympatric
species of lacewings
courtship display of
Gray-Crowned Cranes, Kenya
firefly
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courtship displays
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Well, what can you say?
sympatric speciation?
Mechanical isolation
 Morphological differences can prevent
successful mating

reproductive isolation
Even in closely related
species of plants, the
flowers often have distinct
appearances that attract
different pollinators.
These 2 species of monkey
flower differ greatly in
shape & color, therefore
cross-pollination does not
happen.
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Plants
Mechanical isolation
Animals
 For many insects, male &
female sex organs of
closely related species do
not fit together, preventing
sperm transfer

lack of “fit” between sexual organs:
hard to imagine for us… but a big issue for insects with
different shaped genitals!
I can’t even imagine!
Damsel fly penises
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sympatric speciation?
Gametic isolation
 Sperm of one species may not be able to fertilize
eggs of another species

mechanisms
 biochemical barrier so sperm cannot penetrate egg
 receptor recognition: lock & key between egg & sperm
 chemical incompatibility
 sperm cannot survive in female reproductive tract
Sea urchins release sperm
& eggs into surrounding
waters where they fuse &
form zygotes. Gametes of
different species— red &
purple —are unable to fuse.
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POSTZYGOTIC- reproduction barriers
 Prevent hybrid offspring from
developing into a viable, fertile adult
reduced hybrid viability
 reduced hybrid fertility
 hybrid breakdown

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zebroid
sympatric speciation?
Reduced hybrid viability
 Genes of different parent species may
interact & impair the hybrid’s development
Species of salamander
genus, Ensatina, may
interbreed, but most
hybrids do not complete
development & those
that do are frail.
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Reduced hybrid fertility
 Even if hybrids are vigorous
they may be sterile

chromosomes of parents may differ in number
or structure & meiosis in hybrids may fail to
produce normal gametes
Mules are vigorous,
but sterile
Horses have 64
chromosomes
(32
pairs)
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Donkeys have 62
chromosomes
Mules have 63 chromosomes! (31 pairs)
sympatric speciation?
Hybrid breakdown
 Hybrids may be fertile & viable in first
generation, but when they mate offspring
are feeble or sterile
In strains of cultivated rice,
hybrids are vigorous but
plants in next generation are
small & sterile.
On
path to separate species.
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Allopatric and Sympatric Speciation
 Reproductive Isolation is the key!

Allopatric speciation
 geographic separation
 populations experience
different mutations/adaptations
 over time the two genomes
diverge to the point that
reproductive isolation has
occurred.
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
Sympatric Speciation
 Organisms still live in same area
 Can occur by polyploidy speciation
(usually in plants)
 Habitat differentiation – a subpopulation
exploits a habitat or resource not utilized by
the parent population
 Sexual selection
ex: fig. 24.12
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
Hybrid Zones
 Populations start diverging genetically.
 A hybrid zone occurs where interbreeding is possible
and gene flow can resume.
 The outcome depends on the strength of the isolating
factor: either reinforcing the isolation, genetically
fusing the populations or creating stable areas of
hybrid populations.
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
Hybrid Zones
Female hybrid
Enquiry Moment:
Male hybrid
Polar bear and grizzly bear
hybridization can occur and
produce fertile offspring.
What does this say about bear
evolution and the selection
factors involved?
Grizzly
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Polar Bear
And, discuss the possible
outcome of the Hybrid Zone if
climate change is occurring
with continued shrinkage of
the polar ice pack before the
next Glaciation Event occurs?
Polyploidy
Species A
2n = 6
 Polyploidy is the
presence of
extra sets of
chromosomes
due to accidents
during cell
division
Normal
gamete
n=3
Meiotic error;
chromosome number not
reduced from 2n to n
Unreduced gamete
with 4 chromosomes
Hybrid with
7 chromosomes
Normal
gamete
n=3
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Species B
2n = 4
Unreduced gamete
with 7 chromosomes
New species:
viable fertile hybrid
(allopolyploid) 2n = 10
Polyploidy
 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
 Many important crops (oats, cotton,
potatoes, tobacco, and wheat) are
polyploids
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Rate of Speciation
 Current debate:
Does speciation happen gradually or
rapidly

Gradualism
 Charles Darwin
 Charles Lyell

Punctuated equilibrium
 Stephen Jay Gould
 Niles Eldredge
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Niles Eldredge
Curator
American Museum of Natural History
Gradualism
 Gradual divergence
over long spans of
time

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assume that big
changes occur as
the accumulation
of many small ones
Punctuated Equilibrium
 Rate of speciation is
not constant
rapid bursts of
change
 long periods of little
or no change
 species undergo
rapid change when
they 1st bud from
parent population

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Time
Stephen Jay Gould (1941-2002)
 Harvard paleontologist &
evolutionary biologist
punctuated equilibrium
 prolific author

 popularized evolutionary thought
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Example: Punctuated Equilibrium- Slide 1
Species A lives in this region as the
climatic conditions favor the environmental
factors it is adapted to survive within.
Original mouse
Species – Species A
Adapted to do best on the wetter, more tropical eastern
side of the island under current conditions.
Example: Punctuated Equilibrium- Slide 2
Over a few tens of thousands of years,
climatic changes expand the wetter,
dryer regions suitable for Species A into the
Central Valley and Northeast Coast. Over
time, Species A increases in numbers and
range to expand into these regions.
Note: since the suitable environment is
expanding, there are no significant selection
forces to cause genetic differences to build
up.
Original mouse
Species – Species A
Adapted to do best on the wetter, more tropical eastern
side of the island under current conditions.
Example: Punctuated Equilibrium- Slide 3
Climatic changes swing back. The northern
coast becomes dryer/colder while the
central valley stays warm, but dryer. Over
time, most in these populations are not
suited to survive the conditions and die out.
If the proper mutations occur in the
northern coastal area some individuals will
vary with larger body size and darker fur, both
adaptations to retain body heat and survive
colder conditions. These individuals will have
higher reproductive success. If they survive, it
will create an isolated pocket of these mice.
Species A continues on in numbers in its
southeastern range.
Original mouse
Species – Species A
Adapted to do best on the wetter, more tropical eastern
side of the island under current conditions.
Example: Punctuated Equilibrium- Slide 4
Note: The Original Species A is back to its
original range.
The north coastal mice are surviving, but
mutations, natural selection forces are
creating genetic changes making them larger
and darker.
The central valley mice are undergoing
selection forces that favor drought survival. It
may or may not produce visual changes, but
you can tell when an overall drought it’s the
island, as the Central Valley mice will survive,
reproduce in higher numbers compared to
Species A.
Original mouse
Species – Species A
Example: Punctuated Equilibrium- Slide 5
Note:
• The Original Species A is unchanged.
• The north coastal mice are undergoing
genetic changes making them larger
and darker.
• The central valley mice are undergoing
genetic changes making them more
drought resistant.
• They are separated physically in different
habitats.
• All three variants are surviving
simulataneously, populations are
“branching off.” One species IS NOT
replacing the other.
Original mouse
Species – Species A
If enough genetic changes occur
that prevent successful
reproduction, then they have
become reproductively isolated
and can be considered as
different species!
Any Questions??
AP Biology
2007-2008