Anagenesis vs. Cladogenesis
Changes within a species
One or more species arising from
one original species
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.
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?
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
Phylogenetic Species Concept

Species are defined as the smallest monophyletic group in a tree diagram
representing populations.
A
B
C
D
E
F
G
Cryptic Species

Problems with the Genealogical species concept?
H
I
J
Why should we care about defining species?

The case of the red wolf

The case of the dusky seaside sparrow
How do species arise?
Speciation is a splitting event that creates two or more
distinct species from a single ancestral group.
Small ground finch
Large ground finch
Ancestral ground finch
Allopatric speciation
1.
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).
2.
The 2 populations diverge from each other due to drift, selection, mutation
3.
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
Allopatric speciation
1.
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).
2.
The 2 populations diverge from each other due to drift, selection, mutation
3.
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
species
Gene flow
Population 1 in
environment 1
Population 2 in
environment 2
DISPERSAL AND COLONIZATION
Island
1. Start with
one continuous
population.
Then, a colonist
floats to an
island on a raft.
Continent
2. Finish with
two populations
isolated from
one another.
Dispersal: Hawaiian fruit flies
VICARIANCE
River
River
changes
course
1. Start with
one continuous
population.
Then a chance
event occurs
that changes
the landscape
(river changes
course).
2. Finish with
two populations
isolated from
one another.
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
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
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
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
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
When the 2 species are found in sympatry, the males of
the species look very different from each other
Male pied flycatcher
Male collared 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
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
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
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.
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.
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.
•

Hybrid Sterility: This is when the offspring of two parent can't reproduce. They are
sterile.
•

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.
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
Sympatric speciation occurs in groups that occupy the
same geographic area. Gene flow is occurring
1. Within one population, 2 groups start to diverge from each
other
2.
•
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
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
Speciation by Polyploidy is most common in plants
Diploid parent
Tetraploid parent
(Two copies of
each chromosome)
Meiosis
(Four copies of
each chromosome)
Mating
Haploid gametes
Diploid gametes
(One copy of each chromosome)
(Two copies of each chromosome)
Triploid zygote
Meiosis
(Three copies of
each chromosome)
When these gametes combine, most offspring
have incorrect number of chromosomes.
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.