1. Anagenesis vs. Cladogenesis
One or more species arising from
one original species
Changes within a species

2. Defining and Identifying Species
There have been many different attempts to define “species”:
The biological species concept is based on reproductive isolation.
The morphological species concept is based on identifying evolutionarily independent lineages based on appearance of organisms.
The phylogenetic species concept is based on reconstructing the evolutionary history of populations.

3. Biological Species Concept
A species includes all populations that have the potential to interbreed and produce fertile offspring.
Example: A horse and a donkey can mate and produce a mule, but mules are sterile, so horses and donkeys are separate species
Problems with the biological species concept?

4. Morphological Species Concept
Species are defined as groups with measurably different anatomical features.
Useful for fossil species
Problems with the Morphospecies species concept?
Cryptic Species
Short-toed treecreeper
Certhia brachydactyla
Common treecreeper
Certhia familiaris

5. Phylogenetic Species Concept
Species are defined as the smallest monophyletic group in a tree diagram representing populations.
Problems with the Genealogical species concept? A B C D E F G H I J
Cryptic Species

6. Why should we care about defining species?
The case of the red wolf
The case of the dusky seaside sparrow
1980- last 6 sparrows were male

7. Ancestral ground finch
How do species arise?
Speciation is a splitting event that creates two or more distinct species from a single ancestral group.
Large ground finch
Small ground finch
Ancestral ground finch
Figure: 23.1
Caption:
When lack of gene flow isolates populations, they tend to diverge. This is because mutation, drift, and selection act on the populations independently. As a result, they acquire distinctive characteristics. For example, the large ground finch and medium ground finch shown here are derived from the same ancestral population. This ancestral population split into two populations that were isolated by lack of gene flow. Because the populations began evolving independently, they acquired the distinctive characteristics observed today.

8. Allopatric speciation
One large population is split into two populations that are separated by a physical barrier (no gene flow)
A population can colonize a new habitat (dispersal).
A new physical barrier can split a widespread population into two or more isolated groups (a vicariance event).
The 2 populations diverge from each other due to drift, selection, mutation
The two populations become so different that when they come back into contact with each other, they can no longer reproduce with each other --> they are separate species
Two different populations of the same species
Gene flow
Population 1 in
environment 1
Population 2 in
environment 2

9. Allopatric speciation
One large population is split into two populations that are separated by a physical barrier (no gene flow)
A population can colonize a new habitat (dispersal).
A new physical barrier can split a widespread population into two or more isolated groups (a vicariance event).
The 2 populations diverge from each other due to drift, selection, mutation
The two populations become so different that when they come back into contact with each other, they can no longer reproduce with each other --> they are separate species
Gene flow
Population 1 in
environment 1
Population 2 in
environment 2
Two different species

10. DISPERSAL AND COLONIZATION
1. Start with one continuous population. Then, a colonist floats to an island on a raft.
Island
Continent
2. Finish with two populations isolated from one another.
Figure: 23.8 left
Caption:
These diagrams illustrate the difference between dispersal and vicariance. When dispersal occurs, colonists establish a population in a novel location. In vicariance, a widespread population becomes fragmented into isolated subgroups.

11. Dispersal: Hawaiian fruit flies
Figure: 23.4
Caption:
Water snakes occupy most of the larger islands in Lake Erie, along with mainland habitats on either side of the lake. On island habitats, unbanded individuals like the one in the top photo are quite common. All or most of the water snakes found in mainland habitats are strongly banded, such as the individual in the bottom photo.  Question Banded snakes are most common on the island closest to shore. Why?

12. VICARIANCE
1. Start with one continuous population. Then a chance event occurs that changes the landscape (river changes course).
River
2. Finish with two populations isolated from one another.
River changes course
Figure: 23.8 right
Caption:
These diagrams illustrate the difference between dispersal and vicariance. When dispersal occurs, colonists establish a population in a novel location. In vicariance, a widespread population becomes fragmented into isolated subgroups.

14. What causes populations to diverge from one another?
Genetic drift
Random changes in allele frequencies that typically occur in small populations
Natural selection
Populations/groups may experience selection for different traits
Sexual Selection
Different traits may lead to reproductive success in different pops
Gene flow
No!!! Gene flow (genetic exchange) between populations or groups will keep them similar to one another and prevent them from diverging

15. Gene flow vs. selection and drift
Hence, in order for 2 populations or groups of the same species to become different from one another the strength of selection and drift must outweigh the level of gene flow
If gene flow > selection & drift --> no divergence
If gene flow < selection & drift --> divergence

16. Reproductive Isolation
Two populations or groups become different species once they are reproductively isolated from one another
When 2 populations or groups are reproductively isolated, there is no longer any gene flow between them
Therefore, 2 different species have different gene pools

17. Is there selection for reproductive isolation?
When populations have partially diverged, they may come back into contact with each other --> secondary contact
If the 2 populations have not diverged very much --> they may merge back together
If the 2 populations have diverged significantly, their hybrid offspring may struggle to survive and reproduce
In the second case, individuals that do not mate with individuals of the other type will have higher fitness
Selection for traits that prevent interbreeding of populations = reinforcement

18. An Example of reinforcement: 2 species of flycatcher
Male pied flycatcher
Male collared flycatcher
When the 2 species are in allopatry (i.e., apart): the
2 species look very much alike

19. When the 2 species are found in sympatry, the males of the species look very different from each other
Male collared flycatcher
Male pied flycatcher
Hybrid offspring have 70% lower survival rates
Females will preferentially mate with males that are most different
from the other species --> males that look different have higher fitness

20. Conclusion
When there is the potential to interbreed and that interbreeding produces hybrids with lower fitness…
Selection favors traits that prevent the interbreeding, i.e. selecting favors reproductive isolating mechanisms

21. Reproductive Isolation
Reproductive isolating mechanisms:
Prezygotic mechanisms make it very unlikely that interbreeding will even take place.
Postzygotic mechanisms cause hybrids to become sterile or to fail to develop properly. +
egg
sperm
zygote

22. Prezygotic Isolating Mechanisms
Temporal Isolation- species breed at different times
Ex: The western spotted skunk and the eastern spotted skunk overlap in distribution. Even though these species are very similar, the western spotted skunk breeds in late summer and the eastern in late winter.
Ecological Isolation
Ex: two species of garter snake (genus Thamnophis) live in the same area, but one species lives mainly in the water, and the other is mainly terrestrial.
Behavioral Isolation
Ex: fireflies - if the male flashes its light in the right pattern, he gets the girl. Different species have different light flashing patterns, and only the female of the same species recognizes the pattern as being courtship behavior.

23. Prezygotic Isolating Mechanisms
Mechanical Isolation
This is when the male and female sex organs are not compatible, so fertilization cannot occur.
Gametic Isolation
Ex: many marine species broadcast their eggs and sperm in the water. The gametes of more than one species can encounter each other, but they generally will not fuse.

24. Postzygotic Isolating Mechanisms
Hybrid Inviability: hybrids (in this case, the offspring of two different species) cannot survive embryonic development, or are weak and without human intervention, do not survive to reproduce.
Ex: some species of frog (genus Rana) that overlap in habitat use sometime hybridize. Their offspring usually die during development. A few frail offspring sometimes survive development, but die shortly after they “hatch” into tadpoles.
Hybrid Sterility: This is when the offspring of two parent can't reproduce. They are sterile.
Ex: like the horse and donkey producing a mule, which is sterile.
Hybrid breakdown: this is when the first generation of hybrids can reproduce, but the next generation of hybrids is sterile or the individuals cannot survive.
Ex: different cotton species can produce fertile hybrid offspring, but the hybrid’s offspring die as seeds or grow into weak defective plants

25. Sympatric speciation occurs in groups that occupy the same geographic area. Gene flow is occurring
Within one population, 2 groups start to diverge from each other
Natural selection can cause this to occur in spite of gene flow.
Polyploidy, a type of mutation, can also cause sympatric speciation.
The two groups become separate species

26. Apple Maggot Flies: The evolution of a new species
Rhagoletis pomonella
Rhagoletis lays its eggs
in fruit to develop -->
Original host = hawthorn trees
Recently, the species R. pomonella shifted from its native host to introduced, domestic apples (Malus pumila)
Now: two genetically distinct groups
that do not interbreed

27. Speciation by Polyploidy is most common in plants

28. When these gametes combine, most offspring
Diploid parent
Tetraploid parent
(Two copies of
each chromosome)
(Four copies of
each chromosome)
Meiosis
Mating
Haploid gametes
Diploid gametes
(One copy of each chromosome)
(Two copies of each chromosome)
Triploid zygote
Figure: 23.6
Caption:
The mating diagrammed here shows why normal and polyploid individuals are reproductively isolated. The example illustrates just two chromosomes, although most organisms have at least four.
Meiosis
(Three copies of
each chromosome)
When these gametes combine, most offspring
have incorrect number of chromosomes.

29. Defining and Identifying Species
Different populations may diverge due to microevolutionary processes (mutation, selection, drift)
Once two groups of organisms are incapable of interbreeding, they are no longer exchanging genes--> 2 separate species (origin of new species = macroevolution)
Lack of gene flow makes a species independent.
Species are distinct types of organisms because they represent evolutionarily independent groups.