1. In the Galápagos Islands Darwin discovered plants and animals found nowhere else on Earth
Figure 24.1 How did this flightless bird come to live on the isolated Galápagos Islands?

2. (a) Similarity between different species
Fig. 24-2
(a) Similarity between different species
Figure 24.2 The biological species concept is based on the potential to interbreed rather than on physical similarity
(b) Diversity within a species

3. Gene Flow Between Populations
EXPERIMENT
Example of a gene tree for population pair A-B
Allele
Population
Gene flow event 1 B
Allele 1 is more closely related to
alleles 2, 3, and 4 than to
alleles 5, 6, and 7.
Inference: Gene flow occurred. 2 A 3 A 4 A 5 B
Alleles 5, 6, and 7 are more closely
related to one another than to
alleles in population A.
Inference: No gene flow occurred. 6 B 7 B
RESULTS
Pair of
populations
with detected
gene flow
Estimated minimum
number of gene flow
events to account for
genetic patterns
Distance between
populations (km)
Figure 24.3 Does gene flow occur between widely separated populations?
A-B 5
340
K-L 3
720
A-C
2–3
1,390
B-C 2
1,190
F-G 2
760
G-I 2
1,110
C-E
1–2
1,310

4. Reproductive Barriers Between Species
Prezygotic Barriers
Postzygotic Barriers
Habitat
Isolation
Temporal
Isolation
Behavioral
Isolation
Mechanical
Isolation
Gametic
Isolation
Reduced
Hybrid Viability
Reduced Hybrid
Fertility
Hybrid
Breakdown
Individuals of
different
species
Mating
attempt
Viable,
fertile
offspring
Fertilization
(a)
(c)
(e)
(f)
(g)
(h)
(i)
(l)
(d)
(j)
(b)
Figure 24.4 Reproductive barriers
(k)

5. Water-dwelling Thamnophis
Habitat Isolation
Figure 24.4 Reproductive barriers
Water-dwelling Thamnophis

6. Terrestrial Thamnophis
Habitat Isolation
Figure 24.4 Reproductive barriers
Terrestrial Thamnophis

7. Courtship ritual of blue-footed boobies
Behavioral Isolation Occurs Without Appropriate Mating Rituals
Figure 24.4 Reproductive barriers
Courtship ritual of blue-footed boobies

8. Mechanical Isolation:
Figure 24.4 Reproductive barriers
Mechanical Isolation:
Bradybaena with shells spiraling in opposite directions

9. Mule: sterile hybrid offspring between horse and donkey
PostZygotic Reproductive Barrier
Figure 24.4 Reproductive barriers
Mule: sterile hybrid offspring between horse and donkey

10. Speciation (a) Allopatric speciation (b) Sympatric speciation
Fig 24.5 Two main modes of speciation
(a) Allopatric speciation
(b) Sympatric speciation

11. Allopatric Speciation
A. harrisi
A. leucurus
Figure 24.6 Allopatric speciation of antelope squirrels on opposite rims of the Grand Canyon

12. Continental Drift Caused Allopatric Speciation
Mantellinae
(Madagascar only):
100 species
Rhacophorinae
(India/Southeast
Asia): 310 species
Other Indian/
Southeast Asian
frogs
100 80 60 40 20 1 2 3
Figure 24.7 Allopatric speciation in frogs
Millions of years ago (mya) 1 2 3
India
Madagascar
88 mya
65 mya
56 mya

13. Sympatric Speciation via Polyploidy is Common in Plants
Failure of cell
division after
chromosome
duplication gives
rise to tetraploid
tissue.
Gametes
produced
are diploid..
Offspring with
tetraploid
karyotypes may
be viable and
fertile.
Fig Sympatric speciation by autopolyploidy in plants

14. Sympatric Speciation - Polyploidy --> Allopolyploid
Species B
2n = 4
Unreduced
gamete
with 4
chromosomes
Unreduced
gamete
with 7
chromosomes
Hybrid
with 7
chromosomes
Meiotic
error
Normal
gamete
n = 3
Viable fertile
hybrid
(allopolyploid)
2n = 10
Figure One mechanism for allopolyploid speciation in plants
Normal
gamete
n = 3
Species A
2n = 6

15. Hybrid Zones
EUROPE
Fire-bellied
toad range
Hybrid zone
Fire-bellied toad,
Bombina bombina
Yellow-bellied
toad range
Yellow-bellied toad,
Bombina variegata
0.99
0.9
Fig A narrow hybrid zone for B. variegata and B. bombina in Europe
Allele frequency (log scale)
0.5
0.1
0.01 40 30 20 10 10 20
Distance from hybrid zone center (km)

16. Hybrid Zones Over Time Possible outcomes: Reinforcement Fusion into
Isolated population
diverges
Hybrid
zone
Reinforcement
of gene flow barrier. OR
Fusion into
one species.
Gene flow
Hybrid OR
Figure Formation of a hybrid zone and possible outcomes for hybrids over time
Barrier to
gene flow
Population
(five individuals
are shown)
Stability:
Continued formation
of hybrid individuals.

17. Patterns in Speciation
Punctuated Equilibrium
pattern
Change / Time
Gradualism pattern
Figure Two models for the tempo of speciation

18. Speciation Review Allopatric speciation Sympatric speciation
Original population
Fig. 24-UN1
Allopatric speciation
Sympatric speciation

19. Speciation Ancestral species: AA BB DD Triticum monococcum (2n = 14)
Wild
Triticum
(2n = 14)
Wild
T. tauschii
(2n = 14)
Product:
Fig. 24-UN2
AA BB DD
T. aestivum
(bread wheat)
(2n = 42)

20. You should now be able to:
Define and discuss the limitations of the four species concepts.
Describe and provide examples of prezygotic and postzygotic reproductive barriers.
Distinguish between and provide examples of allopatric and sympatric speciation.
Explain how polyploidy can cause reproductive isolation.
Define the term hybrid zone and describe three outcomes for hybrid zones over time.