Lecture 22 Speciation

Lecture 22
Speciation
“These facts seemed to me to throw some light on the origin of
species — that mystery of mysteries”.
C. Darwin – The Origin
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.
• these 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.
• these 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.
• these 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 postzygotic isolation.
Modes of Speciation
Modes of Speciation
1.  Allopatric speciation
2.  Peripatric speciation
3.  Parapatric speciation
4.  Sympatric speciation
Modes of Speciation
1.  Allopatric speciation
2.  Peripatric speciation
3.  Parapatric speciation
4.  Sympatric speciation
Ecological v.s. Mutation-order
Modes of Speciation
1. Allopatric speciation
Allopatric Speciation
‘‘The phenomenon of disjunction, or complete geographic
isolation, is of considerable interest because it is almost
universally believed to be a fundamental requirement for
speciation.’’ Endler (1977)
Allopatric Speciation
Large ground finch
Small ground finch
Modes of Speciation
1.  Allopatric speciation
Allopatric speciation is the outcome of isolation and
divergence.
–  Isolation is created by reductions in gene flow.
–  Divergence is created when mutation, genetic drift, and
selection act on populations separately.
Modes of Speciation
1.  Allopatric speciation
Allopatric speciation is the outcome of isolation and
divergence.
–  Isolation is created by reductions in gene flow.
–  Divergence is created when mutation, genetic drift, and
selection act on populations separately.
Isolation may be caused by dispersal or vicariance.
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
Speciation by island-hopping
Modes of Speciation
2. Peripatric speciation
Modes of Speciation
3. Parapatric speciation
Modes of Speciation
3. Parapatric speciation
• reproductive isolation occurs without complete
geographic isolation (some gene flow).
Modes of Speciation
3. 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
4. Sympatric speciation
Modes of Speciation
4. Sympatric speciation
• reproductive isolation evolves with complete
geographic overlap.
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.
Soapberry bug
Beak length correlates with fruit size.
Balloon vine
(native species)
Flat-podded
golden rain tree
(non-native species)
Short-beaked population
growing on non-native
plants
12
Frequency
8
Long-beaked population
growing on native plants
4
0
2
3
6
7
8
9
Beak length (mm)
10
11
12
11
12
8
Non-native plant
(small fruit)
4
Native plant
(large fruit)
0
2
3
6
7
8
9
Fruit radius (mm)
10
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).
• antagonistic sexual selection too!
Male-male competition in Hawaiian Drosophila
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!
Ecological speciation in sticklebacks
Ecological speciation in sticklebacks
Dolph Schluter
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?
Sensory drive in Victoria Cichlids
Ole Seehausen
Some generalities
1. The magnitude of prezygotic and postzygotic
isolation both increase with the time.
Some generalities
1. The magnitude of prezygotic and postzygotic
isolation both increase with the divergence 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, prezygotic
isolation is generally stronger than postzygotic
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 postzygotic isolation?
What causes postzygotic isolation?
• the underlying mechanism is called DobzhanskyMuller incompatibility:
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 prezygotic
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 premating 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