11/10/2018
SPECIATION
Masters of Science in Fish Biology and Genetics
FBG-510, Evolutionary Biology
MOHAMMAD AMZAD HOSSAIN, MSc (UBX, EHU, ULG)
Assistant Professor
Department of Fish Biology and Genetics
Faculty of Fisheries
Sylhet Agricultural University, Sylhet-3100
Email: mamzad.fbg@sau.ac.bd
What is speciation?
•Darwin’s words, speciation is the “multiplication of
species”.
• BSC- speciation occurs when populations
reproductive isolating mechanisms.
evolve
• 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.
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Modes of Speciation
1. Allopatric speciation
• reproductive isolation occurs in complete geographic
isolation (no gene flow).
Hawaiian
Drosophila
D. suzukii
D. microthrix
D. nigribasis
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Speciation by island-hopping
Modes of Speciation
2. Parapatric speciation
• reproductive isolation occurs without complete
geographic isolation (some gene flow).
Example: ring species of salamanders (Ensatina) in CA
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Ensatina salamanders
Ring species – evidence for parapatric
speciation
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Ring species – evidence for parapatric
speciation
Ring species
a connected series of neighboring populations that can
interbreed with relatively closely related populations, but
for which there exist at least two "end" populations in
the series that are too distantly related to interbreed.
Often such non-breeding-though-genetically-connected
populations co-exist in the same region thus creating a
"ring". Ring species illustrate what happens over time as
populations genetically diverge, and are special because
they represent in living populations what normally
happens over time between long deceased ancestor
populations and living populations.
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Modes of Speciation
3. Peripatric:
In peripatric speciation, new species are formed in
isolated, small peripheral populations which are
prevented from exchanging genes with the main
population. It is related to the concept of a founder
effect, since small populations often undergo
bottlenecks. Genetic drift is often proposed to play a
significant role in peripatric speciation
Observed instances
Mayr bird fauna
The Australian bird Petroica multicolor
Reproductive isolation occurs in populations of Drosophila subject to population
bottlenecking.
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Modes of Speciation
4. 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
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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?
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!
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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
• may involve founder effects and genetic bottlenecks.
• alleles that are neutral in one environment may not be
neutral in another!
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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.
• 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)
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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
• in plants, polyploidization is a major mode of
speciation.
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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:
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)
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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
• 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.
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Ecological speciation in sticklebacks
Ecological speciation in sticklebacks
1. Colonization by marine
stickleback ~10,000 years ago
2. Adaptation to freshwater
environment
3. Secondary invasion by marine
stickleback
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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
• 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.
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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.
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