Speciation

1. Speciation
Chapter 18

2. Barriers to Gene Flow
• Whether or not a physical barrier deters
gene flow depends upon:
 Organism’s mode of dispersal or locomotion
 Duration of time organism can move

3. Speciation & Natural Selection
 Natural selection can lead to speciation
 Speciation can also occur as a result of other
microevolutionary processes
 Genetic drift
 Mutation

4. Morphology & Species
 Morphological traits may not be useful in
distinguishing species
 Members of same species may appear different
because of environmental conditions
 Morphology can vary with age and sex
 Different species can appear identical

5. Variable Morphology
Grown in water Grown
on land
Figure 18.2
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6. Biological Species Concept
“Species are groups of interbreeding natural
populations that are reproductively isolated from
other
such groups.”
Ernst Mayr

7. Reproductive Isolation
 Cornerstone of the biological species concept
 Speciation is the attainment of reproductive
isolation
 Reproductive isolation arises as a
by-product of genetic change

8. Genetic Divergence
 Gradual accumulation of differences in the gene
pools of populations
 Natural selection, genetic drift, and mutation can
contribute to divergence
 Gene flow counters divergence

9. Genetic Divergence
time A time B time C time D
daughter
species
Figure 18.3
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parent species

10. Reproductive Isolating
Mechanisms
 Prezygotic isolation
 Mating or zygote formation is prevented
 Postzygotic isolation
 Takes effect after hybrid zygotes form
 Zygotes may die early, be weak, or be sterile

11. Prezygotic Isolation
Ecological Isolation
Temporal Isolation
Behavioral Isolation
Mechanical Isolation
Gametic Mortality

12. Postzygotic Mechanisms
 Zygotic mortality
 Hybrid inviability
 Hybrid sterility

13. Mechanisms of Speciation
 Allopatric speciation
 Sympatric speciation
 Parapatric speciation

14. Allopatric Speciation
 Speciation in geographically isolated
populations
 Some sort of barrier arises and prevents gene
flow
 Effectiveness of barrier varies with species

15. Extensive Divergence Prevents
Inbreeding
 Species separated by geographic barriers will
diverge genetically
 If divergence is great enough it will prevent
inbreeding even if the barrier later disappears

16. Archipelagos
 Island chains some distance from continents
 Galapagos Islands
 Hawaiian Islands
 Colonization of islands followed by genetic
divergence sets the stage for speciation

17. Speciation
on an
Archipelago
1
2
3
4
1
2
3
4
1
2
A few individuals of a
species on the mainland
reach isolated island 1.
Speciation follows genetic
divergence in a new habitat.
Later in time, a few
individuals of the new
species colonize nearby
island 2. In this new
habitat, speciation follows
genetic divergence.
Speciation may also
follow colonization of
islands 3 and 4. And it
may follow invasion of
island 1 by genetically
different descendents
of the ancestral species.
Figure 18.6
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18. Hawaiian Islands
 Volcanic origins, variety of habitats
 Adaptive radiations:
 Honeycreepers - In absence of other bird species,
they radiated to fill numerous niches
 Fruit flies (Drosophila) - 40% of fruit fly species are
found in Hawaii

19. Hawaiian Honeycreepers
FOUNDER SPECIES
Figure 18.7
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20. Speciation without a Barrier
 Sympatric speciation
 Species forms within the home range of the parent
species
 Parapatric speciation
 Neighboring populations become distinct species
while maintaining contact along a common border

21. Speciation by Polyploidy
 Change in chromosome number
(3n, 4n, etc.)
 Offspring with altered chromosome number
cannot breed with parent population
 Common mechanism of speciation in flowering
plants

22. Parapatric Speciation
Adjacent
populations evolve
into distinct species
while maintaining
contact along a
common border
BULLOCK’S
ORIOLE
BALTIMORE
ORIOLE
HYBRID ZONE
Figure 18.10
Page 299

23. We’re All Related
 All species are related by descent
 Share genetic connections that extend back in
time to the prototypical cell

24. Patterns of Change
in a Lineage
 Cladogenesis
 Branching pattern
 Lineage splits, isolated populations diverge
 Anagenesis
 No branching
 Changes occur within single lineage
 Gene flow throughout process

25. Evolutionary Trees
new species
branch point
(a time of
divergence,
speciation)
a single
lineage
branch point
(a time of
divergence,
speciation)
a new
species
a single
lineage
extinction
(branch
ended
before
present)
dashed line
(only sketchy
evidence of
presumed
evolutionary
relationship)
Figure 18.11
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26. Gradual Model
 Speciation model in which species emerge
through many small morphological changes that
accumulate over a long time period
 Fits well with evidence from certain lineages in
fossil record

27. Punctuation Model
 Speciation model in which most changes in
morphology are compressed into brief period
near onset of divergence
 Supported by fossil evidence in some lineages

28. Adaptive Radiation
 Burst of divergence
 Single lineage gives rise to many new
species
 New species fill vacant adaptive zone
 Adaptive zone is “way of life”

29. Adaptive Radiation
Figure 18.12
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30. Extinction
 Irrevocable loss of a species
 Mass extinctions have played a major
role in evolutionary history
 Fossil record shows 20 or more large-
scale extinctions
 Reduced diversity is followed by adaptive
radiation

31. Who Survives?
 Species survival is to some extent random
 Asteroids have repeatedly struck Earth,
destroying many lineages
 Changes in global temperature favor lineages
that are widely distributed