1. Speciation
How species evolve

2. SPECIES – BIOLOGICAL CONCEPT
Population or groups of populations whose members have the potential to interbreed and produce fertile offspring with each other, but not with members of other species
Emphasizes reproductive isolation

3. Speciation
Process by which one species splits into two or more species, accounts for both the unity and diversity of life.

4. Reproductive Barriers
Depending on whether they function before or after zygotes form, reproductive barriers are categorized as
prezygotic or
postzygotic.

6. Habitat isolation (lack of opportunities to encounter each other)
Figure 14.3_1 Reproductive barriers keep species separate (part 1)
The garter snake
Thamnophis atratus
lives mainly in water.
The garter snake
Thamnophis sirtalis
lives on land.

7. Temporal isolation (breeding at different times or seasons)
The eastern spotted
skunk (Spilogale
putorius) breeds in late
winter.
The western spotted
skunk (Spilogale
gracilis) breeds in
the fall.
Figure 14.3_2 Reproductive barriers keep species separate (part 2)

8. Behavioral isolation (different courtship rituals)
Figure 14.3_3 Reproductive barriers keep species separate (part 3)
The blue-footed booby
(Sula nebouxii) performs
an elaborate courtship
dance.
The masked booby (Sula
dactylatra) performs a
different courtship ritual.

9. Video: Blue-Footed Boobies Courtship Ritual

10. Video: Albatross Courtship Ritual

11. Video: Giraffe Courtship Ritual

12. Mechanical isolation (physical incompatibility of reproductive parts)
Figure 14.3_4 Reproductive barriers keep species separate (part 4)
Heliconia pogonantha
is pollinated by
hummingbirds with
long, curved bills.
Heliconia latispatha is
pollinated by hummingbirds
with short, straight bills.

13. Gametic isolation (molecular incompatibility of eggs and sperm or pollen and stigma)
Figure 14.3_5 Reproductive barriers keep species separate (part 5)
Purple sea urchin
(Strongylocentrotus
purpuratus)
Red sea urchin
(Strongylocentrotus
franciscanus)

14. Reduced hybrid viability (hybrid development or survival impaired by interaction of parental genes)
Some species of salamander can hybridize,
but their offspring do not develop fully or,
like this one, are frail and will not survive
long enough to reproduce.
Figure 14.3_6 Reproductive barriers keep species separate (part 6)

15. Reduced hybrid fertility (vigorous hybrids that cannot produce viable offspring)
The hybrid offspring of a horse and a donkey is a mule, which is robust but sterile.
Figure 14.3_7 Reproductive barriers keep species separate (part 7)

16. Reduced hybrid fertility (vigorous hybrids that cannot produce viable offspring)
The rice hybrids on the left
and right are fertile, but plants
of the next generation (middle)
are sterile.
Figure 14.3_8 Reproductive barriers keep species separate (part 8)

17. Modes of speciation
Geographically separated from other populations, a small population may become genetically unique as its gene pool is changed by
natural selection,
mutation, or
genetic drift.

18. ALLOPATRIC
Populations segregated by geographic barrier
Geographic barrier = ocean, mtn. Range, etc.
Conditions favoring
o Small population at fringe
o Better chance gene pool already different
o Different selection factors
Adaptive radiation – emergence of numerous species from common ancestor introduced into environment

19. SYMPATRIC
New species arise within range of parent species
Reproductive isolation without geographic barrier
Plants – polyploidy (multiple sets of chromosomes)
Animals – group fixed on resources not used by parent population

20. GENETIC CHANGE
Adaptive divergence
2 parts of population adapt to different environments
Each accumulates different gene pools
Reproductive barriers develop
Usually secondary to change in gene pool
May be side effect of sexual selection

22. Remember: Species do not evolve because of need.
Biological diversity exists and the environment selects.
Evolution is not deliberate; it is reactive.
Species do not deliberately change. There is no plan.

23. Models for speciation 1 species gradually evolves into new species •
GRADUALISM
PUNCTUATED EQUILIBRIUM
1 species gradually evolves into new species
Represents microevolution Big changes occur through the accumulation of small changes •
Long periods of stasis (equilibrium) punctuated by episodes of speciation
Species undergo most changes when first branch from parent; then change little
Species develop in spurts of rapid change
Not slow and gradual

24. Tempo of speciation
The time interval between speciation events varies from a few thousand to tens of millions of years.
Punctuated pattern
Gradual pattern
Time

25. Animation: Macroevolution

26. You should now be able to
Distinguish between microevolution and speciation.
Compare the definitions, advantages, and disadvantages of the different species concepts.
Describe five types of prezygotic barriers and three types of postzygotic barriers that prevent populations of closely related species from interbreeding.
Explain how geologic processes can fragment populations and lead to speciation.

27. You should now be able to
Explain how reproductive barriers might evolve in isolated populations of organisms.
Explain how sympatric speciation can occur, noting examples in plants and animals.
Explain how sexual selection can lead to speciation.
Explain why isolated islands are often sites of speciation.

28. You should now be able to
Describe the circumstances that led to the adaptive radiation of the Galápagos finches.
Explain how hybrid zones are useful in the study of reproductive isolation.
Compare the gradual model and the punctuated equilibrium model of evolution.