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AP Biology
So…what is a species?
Biological species concept
defined by Ernst Mayr
population whose members can interbreed &
produce viable, fertile offspring
reproductively compatible
Western Meadowlark Eastern Meadowlark
Distinct species: songs & behaviors are different enough to prevent interbreeding
AP Biology
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
AP Biology
Obstacle to mating or to fertilization if mating occurs
PREZYGOTIC-reproduction barriers
behavioral isolation
geographic isolation habitat isolation temporal isolation
mechanical isolation gametic isolation
AP Biology
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
Ammospermophilus spp
AP Biology
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
This results in them not seeing
each other as suitable mates.
AP Biology
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
AP Biology
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
sympatric speciation?
AP Biology firefly courtship displays
courtship display of
Gray-Crowned Cranes, Kenya
courtship songs of sympatric
species of lacewings
Recognizing your
own species
AP Biology
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.
Plants
sympatric speciation?
AP Biology
Mechanical isolation
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!
Damsel fly penises
Animals
I can’t even imagine!
AP Biology
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.
sympatric speciation?
AP Biology
Prevent hybrid offspring from
developing into a viable, fertile adult
reduced hybrid viability
reduced hybrid fertility
hybrid breakdown
zebroid
POSTZYGOTIC- reproduction barriers
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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.
sympatric speciation?
AP Biology
Mules are vigorous,
but sterile
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
Donkeys have 62
chromosomes
(31 pairs)
Horses have 64
chromosomes
(32 pairs) Mules have 63 chromosomes!
AP Biology
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.
sympatric speciation?
AP Biology
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.
AP Biology
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
AP Biology
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.
AP Biology
Hybrid Zones
Female hybrid Male hybrid
Grizzly Polar Bear
Enquiry Moment:
Polar bear and grizzly bear
hybridization can occur and
produce fertile offspring.
What does this say about bear
evolution and the selection
factors involved?
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?
AP Biology
Polyploidy 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
Polyploidy is the
presence of
extra sets of
chromosomes
due to accidents
during cell
division
AP Biology
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
AP Biology
Niles Eldredge
Curator
American Museum of Natural History
Rate of Speciation
Current debate:
Does speciation happen gradually or
rapidly
Gradualism
Charles Darwin
Charles Lyell
Punctuated equilibrium
Stephen Jay Gould
Niles Eldredge
AP Biology
Gradualism
Gradual divergence
over long spans of
time
assume that big
changes occur as
the accumulation
of many small ones
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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 Time
AP Biology
Stephen Jay Gould (1941-2002)
Harvard paleontologist & evolutionary biologist
punctuated equilibrium
prolific author popularized evolutionary thought
Example: Punctuated Equilibrium- Slide 1
Original mouse Species – Species A Adapted to do best on the wetter, more tropical eastern side of the island under current conditions.
Species A lives in this region as the climatic conditions favor the environmental factors it is adapted to survive within.
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
Original mouse Species – Species A Adapted to do best on the wetter, more tropical eastern side of the island under current conditions.
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.
Example: Punctuated Equilibrium- Slide 4
Original mouse Species – Species A
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.
Example: Punctuated Equilibrium- Slide 5
Original mouse Species – Species A
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.
If enough genetic changes occur that prevent successful reproduction, then they have become reproductively isolated and can be considered as different species!
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