1. Chapter 24
The Origin of Species

2. What You Need to Know:
The difference between microevolution and macroevolution.
The biological concept of a species.
Prezygotic and postzygotic barriers that maintain reproductive isolation in natural populaitons.
How allopatric and sympatric speciation are similar and different.
How autopolyploid or an allopolyploid chromosomal change can lead to sympatric speciation.
How punctuated equilibrium and gradualism describe two different tempos of speciation.

3. Do Now # 8 Problem 1:
In humans, having the Rh blood antigen is dominant over not having the Rh antigen. In the United States, 84% of the population is Rh positive. How many individuals are homozygous. (Assume it is Hardy-Weinberg)

4. Homework 7 Problem 2:
One in every 1700 Caucasian babies that are born have cystic fibrosis. Cystic fibrosis is an autosomal recessive disorder (c).
What is the frequency of the recessive allele?
How many of every 1700 babies are heterozygous?

5. Speciation = origin of species
Microevolution: changes within a single gene pool
Macroevolution: evolutionary change above the species level
cumulative effects of speciation over long periods of time

6. Biological Species Concept
Species = population or group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring
Reproductively compatible
Reproductive isolation = barriers that prevent members of 2 species from producing viable, fertile hybrids

7. Types of Reproductive Barriers
Prezygotic Barriers:
Impede mating/fertilization
Types:
Habitat isolation
Temporal isolation
Behavioral isolation
Mechanical isolation
Gametic isolation
Postzygotic Barriers:
Prevent hybrid zygote from developing into viable adult
Types:
Reduced hybrid viability
Reduced hybrid fertility
Hybrid breakdown

8. Types of Reproductive Barriers
REDUCED HYBRID
VIABILITY
REDUCED HYBRID
FERTILITY
HYBRID BREAKDOWN

9. Types of Reproductive Barriers
REDUCED HYBRID
VIABILITY
REDUCED HYBRID
FERTILITY
HYBRID BREAKDOWN

10. Other definitions of species:
Morphological – by body shape, size, and other structural features
Ecological – niche/role in community
Phylogenetic – share common ancestry, branch on tree of life

11. Two main modes of speciation

12. Two main modes of speciation:
Allopatric Speciation
“other” “homeland”
Geographically isolated populations
Caused by geologic events or processes
Evolves by natural selection & genetic drift
Eg. Squirrels on N/S rims of Grand Canyon
Sympatric Speciation
“together” “homeland”
Overlapping populations within home range
Gene flow between subpopulations blocked by:
polyploidy
sexual selection
habitat differentiation
Eg. polyploidy in crops (oats, cotton, potatoes, wheat)

13. Allopatric speciation of antelope squirrels on opposite rims of the Grand Canyon

14. Sympatric Speciation by Polyploidy
Autopolyploid: extra sets of chromosomes
Failure of cell division (2n  4n)
Eg. Strawberries are 4n, 6n, 8n, 10n (decaploid)!
Allopolyploid: 2 species produce a hybrid
Species A (2n=6) + Species B (2n=4)  Hybrid (2n=10)
2n = 6
4n = 12 4n 2n
Autopolyploid Speciation

15. Allopolyploidy

16. Adaptive Radiation
Many new species arise from a single common ancestor
Occurs when:
A few organisms make way to new, distant areas (allopatric speciation)
Environmental change  extinctions  new niches for survivors
Eg. Hawaiian archiepelago
Founding Parents

17. Adaptive Radiation: Hawaiian plants descended from ancestral tarweed from North America 5 million years ago
KAUAI
5.1
million
years
OAHU
3.7
HAWAII
0.4
1.3
MAUI
MOLOKAI
LANAI
Argyroxiphium sandwicense
Dubautia linearis
Dubautia scabra
Dubautia waialealae
Dubautia laxa N

18. Hybrid Zones Incomplete reproductive barriers
Possible outcomes: reinforcement, fusion, stability

19. Hybrid zones
Where divergent allopatric populations come back and interbreed
Biologists look for patterns to study reproductive isolation

20. Fig
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)

21. Reinforcement: Strengthening Reproductive Barriers
The reinforcement of barriers occurs when hybrids are less fit than the parent species that is reproduce less successfully.
Where reinforcement occurs, reproductive barriers should be stronger for sympatric than allopatric species

22. Fig
Sympatric male
pied flycatcher
Allopatric male
pied flycatcher 28
Pied flycatchers 24
Collared flycatchers 20 16
Number of females
Figure Reinforcement of barriers to reproduction in closely related species of European flycatchers 12 8 4
(none)
Females mating
with males from:
Own
species
Other
species
Own
species
Other
species
Sympatric males
Allopatric males

23. Fusion: Weakening Reproductive Barriers
If hybrids are as fit as parents, there can be substantial gene flow between species
If gene flow is great enough, the parent species can fuse into a single species

24. Pundamilia pundamilia
Fig
Pundamilia nyererei
Pundamilia pundamilia
Figure The breakdown of reproductive barriers
Pundamilia “turbid water,”
hybrid offspring from a location
with turbid water

25. Stability: Continued Formation of Hybrid Individuals
Hybrids continue to be produced between the two species in the area of their overlap, but the gene pools of both parent species remain distinct.

26. Tempo of Evolution Gradualism Common ancestor Slow, constant change
Punctuated Equilibium
Eldridge & Gould
Long period of stasis punctuated by short bursts of significant change