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 Page 294

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 parent species Figure 18.3 Page 294
daughter species
time A
time B
time C
time D
Figure 18.3 Page 294

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 Archipelago1
A few individuals of a species on the mainland reach isolated island 1. Speciation follows genetic divergence in a new habitat. 3 2 4
Later in time, a few individuals of the new species colonize nearby island 2. In this new habitat, speciation follows genetic divergence. 1 2
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. 1 3 2
Figure 18.6 Page 297 4

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
Figure 18.7 Page 297
FOUNDER SPECIES

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 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 Figure 18.11 Page 300
new species
branch point (a time of divergence, speciation)
a single lineage
a new species
extinction (branch ended before present)
dashed line (only sketchy evidence of presumed evolutionary relationship)
Figure Page 300

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 Page 301

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