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(a) Similarity between different species
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(b) Diversity within a species
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Prezygotic barriers
Habitat
isolation
Temporal
isolation
Behavioral
isolation
Mechanical
isolation
Postzygotic barriers
Gametic
isolation
MATING
ATTEMPT
(a)
(c)
(d)
(e)
(f)
Reduced
hybrid
viability
Reduced
Hybrid
hybrid
breakdown
fertility
VIABLE,
FERTILE
OFFSPRING
FERTILIZATION
(g)
(h)
(i)
(l)
(j)
(b)
(k)
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2
Grizzly bear (U. arctos)
Hybrid “grolar bear”
Polar bear (U. maritimus)
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3
(a) Allopatric speciation: forms (b) Sympatric speciation: a subset
forms a new species without
a new species while
geographic separation.
geographically isolated.
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4
(a) Under high predation:
(b) Under low predation:
body shape that enables
body shape that favors
rapid bursts of speed
long, steady swimming
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5
A. formosus
A. nuttingi
ATLANTIC
OCEAN
Isthmus of Panama
PACIFIC
OCEAN
A. panamensis
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A. millsae
6
Experiment
Initial population
of fruit flies
(Drosophila
pseudoobscura)
Some flies raised
on starch medium
Some flies raised
on maltose medium
Mating experiments
after 40 generations
Results
22
9
8
20
Number of matings
in experimental group
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Starch
Starch
population 2 population 1
Maltose
Starch
Starch
population 1 population 2
Male
Starch
Starch
Female
Maltose
Male
Female
18
15
12
15
Number of matings
in control group
7
Cell
division
error
2n  6
Tetraploid cell
4n  12
2n
New species
(4n)
Gametes produced
by tetraploids
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Species A
2n  6
Species B
2n  4
Meiotic error; chromosome
number not reduced from 2n to n
Normal
gamete
n3
Unreduced gamete
with 4 chromosomes
Hybrid with
7 chromosomes
Normal
gamete
n3
Unreduced gamete
with 7 chromosomes
New species:
viable fertile hybrid
(allopolyploid)
2n  10
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9
Experiment
Normal light
Monochromatic
orange light
P. pundamilia
P. nyererei
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10
Fire-bellied
toad range
Hybrid zone
Fire-bellied toad, Bombina
bombina
Yellow-bellied toad, Bombina
variegata
Frequency of
B. variegata-specific allele
Yellow-bellied
toad range
0.99
Hybrid
zone
0.9
Yellow-bellied
toad range
0.5
0.1
0.01
40
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Fire-bellied
toad range
30
20
10
0
10
20
Distance from hybrid zone center (km)
11
Isolated
population
diverges.
Hybrid
zone
Possible
outcomes:
Reinforcement
Fusion
Gene flow
Population
Barrier to
gene flow
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Hybrid
individual Stability
12
Pundamilia nyererei
Pundamilia pundamilia
Pundamilia “turbid water,”
hybrid offspring from a
location with turbid water
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(a) Punctuated model
Time
(b) Gradual model
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(a) Mimulus lewisii
(b) M. lewisii with
M. cardinalis allele
(c) Mimulus cardinalis (d) M. cardinalis with
M. lewisii allele
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