BIOE 109
Summer 2009
Lecture 11-Part II
Speciation
What is speciation?
What is speciation?
• in Darwin’s words, speciation is the “multiplication of
species”.
What is speciation?
• in Darwin’s words, speciation is the “multiplication of
species”.
• according to the BSC, speciation occurs when
populations evolve reproductive isolating mechanisms.
What is speciation?
• in Darwin’s words, speciation is the “multiplication of
species”.
• according to the BSC, speciation occurs when
populations evolve reproductive isolating mechanisms.
• the barriers may act to prevent fertilization – this is prezygotic isolation.
What is speciation?
• in Darwin’s words, speciation is the “multiplication of
species”.
• according to the BSC, speciation occurs when
populations evolve reproductive isolating mechanisms.
• the barriers may act to prevent fertilization – this is prezygotic isolation.
• may involve changes in location or timing of breeding, or
courtship.
What is speciation?
• in Darwin’s words, speciation is the “multiplication of
species”.
• according to the BSC, speciation occurs when
populations evolve reproductive isolating mechanisms.
• the barriers may act to prevent fertilization – this is prezygotic isolation.
• may involve changes in location or timing of breeding, or
courtship.
• barriers also occur if hybrids are inviable or sterile – this
is post-zygotic isolation.
Modes of Speciation
Modes of Speciation
1. Allopatric speciation
Modes of Speciation
1. Allopatric speciation
• reproductive isolation occurs in complete geographic
isolation.
Modes of Speciation
1. Allopatric speciation
• reproductive isolation occurs in complete geographic
isolation (no gene flow).
Geographic isolation can rise from
dispersal or vicariance
Modes of Speciation
1. Allopatric speciation
• reproductive isolation occurs in complete geographic
isolation (no gene flow).
Example: Hawaiian Drosophila
Hawaiian
Drosophila
D. suzukii
D. microthrix
D. nigribasis
Speciation by island-hopping
Modes of Speciation
2. Parapatric speciation
Modes of Speciation
2. Parapatric speciation
• reproductive isolation occurs without complete
geographic isolation (some gene flow).
Modes of Speciation
2. Parapatric speciation
• reproductive isolation occurs without complete
geographic isolation (some gene flow).
Example: ring species of salamanders (Ensatina) in CA
Ensatina salamanders
Ring species – evidence for parapatric
speciation
Ring species – evidence for parapatric
speciation
Modes of Speciation
3. Sympatric speciation
Modes of Speciation
3. Sympatric speciation
• reproductive isolation evolves with complete
geographic overlap.
Modes of Speciation
3. Sympatric speciation
• reproductive isolation evolves with complete
geographic overlap.
Example: the apple maggot fly, Rhagoletis pomonella?
Apple maggot fly
Hawthorn fly
Speciation due to host specialization in this case
Modes of speciation: summary
Allopatric
peripatric
parapatric sympatric
Original population
Initial step of speciation
Barrier
formation
New
niche
New
niche
Genetic
polymorphism
In isolation
In isolation
In adjacent
niche
Within the
population
Evolution of reproductive
isolation
What evolutionary processes are involved
in speciation?
What evolutionary processes are involved
in speciation?
1. Natural selection
What evolutionary processes are involved
in speciation?
1. Natural selection
• driven by different abiotic conditions (e.g.,
temperature, altitude) and biotic conditions (e.g.,
competitors, parasites).
What evolutionary processes are involved
in speciation?
1. Natural selection
• driven by different abiotic conditions (e.g.,
temperature, altitude) and biotic conditions (e.g.,
competitors, parasites).
2. Sexual selection
What evolutionary processes are involved
in speciation?
1. Natural selection
• driven by different abiotic conditions (e.g.,
temperature, altitude) and biotic conditions (e.g.,
competitors, parasites).
2. Sexual selection
• both female choice and male-male competition can
promote rapid divergence (e.g., Hawaiian Drosophila).
What evolutionary processes are involved
in speciation?
1. Natural selection
• driven by different abiotic conditions (e.g.,
temperature, altitude) and biotic conditions (e.g.,
competitors, parasites).
2. Sexual selection
• both female choice and male-male competition can
promote rapid divergence (e.g., Hawaiian Drosophila).
• sexual antagonistic selection too!
Male-male competition in Hawaiian Drosophila
Establish territory
On a lek by head
butting
Fight over display
Territory by
grappling
What evolutionary processes are involved
in speciation?
3. Random genetic drift
What evolutionary processes are involved
in speciation?
3. Random genetic drift
• may involve founder effects and genetic bottlenecks.
What evolutionary processes are involved
in speciation?
3. Random genetic drift
• may involve founder effects and genetic bottlenecks.
• alleles that are neutral in one environment may not be
neutral in another!
Some generalities
1. The magnitude of pre-zygotic and post-zygotic
isolation both increase with the time.
Some generalities
1. The magnitude of prezygotic and postzygotic
isolation both increase with the time.
• in Drosophila, it takes about 1.5 to 3 million years for
complete isolation to evolve.
Some generalities
1. The magnitude of prezygotic and postzygotic
isolation both increase with the time.
• in Drosophila, it takes about 1.5 to 3 million years for
complete isolation to evolve.
• in marine bivalves, it may take 4 to 6 million years!
Some generalities
1. The magnitude of prezygotic and postzygotic
isolation both increase with the time.
• in Drosophila, it takes about 1.5 to 3 million years for
complete isolation to evolve.
• in marine bivalves, it may take 4 to 6 million years!
2. Among recently separated groups, pre-zygotic
isolation is generally stronger than post-zygotic
isolation.
Some generalities
3. In the early stages of speciation, hybrid
sterility or inviability is almost always seen in
the heterogametic sex.
Some generalities
3. In the early stages of speciation, hybrid
sterility or inviability is almost always seen in
the heterogametic sex.
• for example, D. simulans and D. mauritiana female
hybrids are completely viable yet male hybrids are
completely sterile!
Some generalities
3. In the early stages of speciation, hybrid
sterility or inviability is almost always seen in
the heterogametic sex.
• for example, D. simulans and D. mauritiana female
hybrids are completely viable yet male hybrids are
completely sterile!
• this is called Haldane’s rule.
J.B.S. Haldane (1892-1964)
What causes post-zygotic isolation?
What causes postzygotic isolation?
• the underlying mechanism is called DobzhanskyMuller incompatibility:
Dobzhansky and Muller were incompatible!
“Balanced”
school
“Classical”
school
What causes postzygotic isolation?
• the underlying mechanism is called DobzhanskyMuller incompatibility:
Ancestral Pop:
A1A1B1B1
What causes postzygotic isolation?
• the underlying mechanism is called DobzhanskyMuller incompatibility:
Ancestral Pop:
A1A1B1B1

Derived Pops:
A2A2B1B1

A1A1B2B2
What causes postzygotic isolation?
• the underlying mechanism is called DobzhanskyMuller incompatibility:
Ancestral Pop:
A1A1B1B1

Derived Pops:

A2A2B1B1

Hybrids:
A1A1B2B2

A1A2B1B2
 fitness
Differences between plant and animal
speciation
Differences between plant and animal
speciation
• in plants, polyploidization is a major mode of
speciation.
Differences between plant and animal
speciation
• in plants, polyploidization is a major mode of
speciation.
• polyploidization refers to the retention of extra sets of
chromosomes (i.e., tetraploids, octoploids, etc.)
Differences between plant and animal
speciation
• in plants, polyploidization is a major mode of
speciation.
• polyploidization refers to the retention of extra sets of
chromosomes (i.e., tetraploids, octoploids, etc.)
• there are two types of polyploids: autopolyploids
and allopolyploids.
Differences between plant and animal
speciation
• autopolyploids add chromosomal sets from the same
species:
Differences between plant and animal
speciation
• autopolyploids add chromosomal sets from the same
species:
Species 1 x Species 1
(2N = 4)
(2N = 4)

Species 2
(4N = 8)
Differences between plant and animal
speciation
• autopolyploids add chromosomal sets from the same
species:
Species 1 x Species 1
(2N = 4)
(2N = 4)

Species 2
(4N = 8)
• allopolyploids combine chromosomal sets from
different species:
Differences between plant and animal
speciation
• autopolyploids add chromosomal sets from the same
species:
Species 1 x Species 1
(2N = 4)
(2N = 4)

Species 2
(4N = 8)
• allopolyploids combine chromosomal sets from
different species:
Species 1 x Species 2
(2N = 4)
(2N = 6)

Species 3
(2N = 10)
Secondary contact and reinforcement
Secondary contact and reinforcement
• secondary contact occurs when two formerly
allopatric populations meet.
Secondary contact and reinforcement
• secondary contact occurs when two formerly
allopatric populations meet.
Three outcomes are possible:
Secondary contact and reinforcement
• secondary contact occurs when two formerly
allopatric populations meet.
Three outcomes are possible:
1. No interbreeding occurs
Secondary contact and reinforcement
• secondary contact occurs when two formerly
allopatric populations meet.
Three outcomes are possible:
1. No interbreeding occurs
• isolating mechanisms in place – speciation
completed.
Secondary contact and reinforcement
• secondary contact occurs when two formerly
allopatric populations meet.
Three outcomes are possible:
1. No interbreeding occurs
• isolating mechanisms in place – speciation
completed.
2. Introgression
Secondary contact and reinforcement
• secondary contact occurs when two formerly
allopatric populations meet.
Three outcomes are possible:
1. No interbreeding occurs
• isolating mechanisms in place – speciation
completed.
2. Introgression
• no isolating mechanisms in place – populations
merge completely.
Secondary contact and reinforcement
3. Partial interbreeding occurs
Secondary contact and reinforcement
3. Partial interbreeding occurs
• some isolating mechanisms in place – a hybrid zone
forms (but hybrids are less fit).
Secondary contact and reinforcement
3. Partial interbreeding occurs
• some isolating mechanisms in place – a hybrid zone
forms (but hybrids are less fit).
• reinforcement should occur to “complete” the
process by the evolution of additional pre-zygotic
barriers.
Evidence for reinforcement in Drosophila
Evidence for reinforcement in Drosophila
• Coyne & Orr (1997) compared sister species of
Drosophila that were either allopatric or sympatric.
Evidence for reinforcement in Drosophila
• Coyne & Orr (1997) compared sister species of
Drosophila that were either allopatric or sympatric.
For each species pair they estimated:
Evidence for reinforcement in Drosophila
• Coyne & Orr (1997) compared sister species of
Drosophila that were either allopatric or sympatric.
For each species pair they estimated:
1. The degree of pre-mating isolation from mate choice
experiments.
Evidence for reinforcement in Drosophila
• Coyne & Orr (1997) compared sister species of
Drosophila that were either allopatric or sympatric.
For each species pair they estimated:
1. The degree of premating isolation from mate choice
experiments.
2. The degree of genetic divergence using allozymes.
Evidence for reinforcement in Drosophila
Ecological speciation in sticklebacks
Ecological speciation in sticklebacks
Ecological speciation in sticklebacks
Ecological speciation in sticklebacks
Ecological speciation in sticklebacks
1. Colonization by marine
stickleback ~10,000 years ago
Ecological speciation in sticklebacks
1. Colonization by marine
stickleback ~10,000 years ago
2. Adaptation to freshwater
environment
Ecological speciation in sticklebacks
1. Colonization by marine
stickleback ~10,000 years ago
2. Adaptation to freshwater
environment
3. Secondary invasion by marine
stickleback
Ecological speciation in sticklebacks
3. Secondary invasion by marine
stickleback
Ecological speciation in sticklebacks
3. Secondary invasion by marine
stickleback
4. Evolution of limnetic and
benthic sticklebacks
Evidence for secondary invasion hypothesis
Evidence for secondary invasion hypothesis
1. Only low elevation lakes possess limnetic and benthic
species pairs.
Evidence for secondary invasion hypothesis
1. Only low elevation lakes possess limnetic and benthic
species pairs.
2. Cores from lakes with limnetic and benthic species pairs
show evidence of salt water influx (e.g, clams, etc.).
Evidence for secondary invasion hypothesis
1. Only low elevation lakes possess limnetic and benthic
species pairs.
2. Cores from lakes with limnetic and benthic species pairs
show evidence of salt water influx (e.g, clams etc.).
3. Higher elevation lakes have neither limnetic and benthic
species pairs nor evidence of salt water influx.
What types of genes are involved in
speciation?
Example: desat-2 in D. melanogaster
What types of genes are involved in
speciation?
Example: desat-2 in D. melanogaster
• D. melanogaster has radiated out of Africa with
humans and lives all over the world (in our garbage
cans).
What types of genes are involved in
speciation?
Example: desat-2 in D. melanogaster
• D. melanogaster has radiated out of Africa with
humans and lives all over the world (in our garbage
cans).
• female flies from Africa (A) possess a different
cuticular hydrocarbon than cosmopolitan females
(C).
What types of genes are involved in
speciation?
Example: desat-2 in D. melanogaster
• D. melanogaster has radiated out of Africa with
humans and lives all over the world (in our garbage
cans).
• female flies from Africa (A) possess a different
cuticular hydrocarbon than cosmopolitan females
(C).
• difference due to a different position of a single
double bond.
2. desat-2 in D. melanogaster
• the desat-2 mutation also affects mate choice.
2. desat-2 in D. melanogaster
• the desat-2 mutation also affects mate choice.
• when A females are placed with A and C males, they
only mate with the former.
2. desat-2 in D. melanogaster
• the desat-2 mutation also affects mate choice.
• when A females are placed with A and C males, they
only mate with the former.
• this modified hydrocarbon affects female smell – in
effect they wear a different “perfume”.
2. desat-2 in D. melanogaster
• the desat-2 mutation also affects mate choice.
• when A females are placed with A and C males, they
only mate with the former.
• this modified hydrocarbon affects female smell – in
effect they wear a different “perfume”.
• the A females are not courted very intensely by C
males.