Lecture 21 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
Modes of Speciation
1. Allopatric speciation
2. Peripatric speciation
3. Parapatric speciation
4. Sympatric speciation
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) 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 Allopatric Speciation
Large ground finch Small ground finch
Adaptive radiations Galapagos finches Hawaiian honeycreepers Madagascar Vangas Cone snails – Cape Verde Islands
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
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: greenish warblers (Himalayas)
Trevor Price Ring species – evidence for parapatric speciation
Modes of Speciation
3. Parapatric speciation
• reproductive isolation occurs without complete geographic isolation (some gene flow).
Example: ring species of salamanders (Ensatina) in CA
Ring species – evidence for parapatric speciation
Ensatina salamanders
Modes of Speciation
4. Sympatric speciation
Modes of Speciation
4. Sympatric speciation
• reproductive isolation evolves with complete geographic overlap.
Requirements for sympatric speciation
1. Sympatric distribution
2. History of allopatry is unlikely
3. Monophyletic sister taxa
4. Reproductive isolation
5. Pre-zygotic isolation
Diploid parent Tetraploid parent
(Two copies of (Four copies of each chromosome) Meiosis 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 Flat-podded (native species) golden rain tree (non-native species)
Short-beaked population 12 growing on non-native Long-beaked population plants growing on native plants 8
4
0 2 3 6 7 8 9 10 11 12 Beak length (mm) Frequency
8 Non-native plant Native plant 4 (small fruit) (large fruit) 0 2 3 6 7 8 9 10 11 12 Fruit radius (mm)
Barluenga et al. Nature 439, 719
Nicaragua Cichlids Barluenga et al. Nature 439, 719
Nicaragua Cichlids
What evolutionary processes are involved in speciation?
What evolutionary processes are involved in speciation?
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).
Next generation sequencing – RAD tags Local adaptation – loci under selection 3000 Histogram of Fst_2Fst outliers – 7471 loci 600 500 400 300 Frequency 3 Sdev. 200 216 loci 100 0
0.0 0.2 0.4 0.6 0.8 1.0 FstFst_2 Island of Speciation Selected locus Sea level = upper limit of expected neutral divergence Tightly- linked neutral loci
(Fst) Sea floor = purely neutrally evolving regions Genetic divergence Genetic Loosely- linked neutral loci Trenches = loci under balancing selection 5. Adaptation 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 divergence 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 divergence 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. Pre-zygotic isolation
Pre-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 postzygotic isolation? What causes postzygotic isolation?
• the underlying mechanism is called Dobzhansky-
Muller incompatibility:
What causes postzygotic isolation?
• the underlying mechanism is called Dobzhansky-
Muller incompatibility:
Ancestral Pop: A1A1B1B1
What causes postzygotic isolation?
• the underlying mechanism is called Dobzhansky-
Muller incompatibility:
Ancestral Pop: A1A1B1B1
ß à
Derived Pops: A2A2B1B1 A1A1B2B2
What causes postzygotic isolation?
• the underlying mechanism is called Dobzhansky-
Muller incompatibility:
Ancestral Pop: A1A1B1B1
ß à
Derived Pops: A2A2B1B1 A1A1B2B2
à ß
Hybrids: A1A2B1B2 i 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 → Species 2 (2N = 4) (2N = 4) (4N = 8)
Differences between plant and animal speciation
• autopolyploids add chromosomal sets from the same species:
Species 1 x Species 1 → Species 2 (2N = 4) (2N = 4) (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 → Species 2 (2N = 4) (2N = 4) (4N = 8)
• allopolyploids combine chromosomal sets from different species:
Species 1 x Species 2 → Species 3 (2N = 4) (2N = 6) (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.
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