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Speciation
Speciation can be analyzed as a three-step
process:
• An initial step that isolates populations;
• A second step that results in the divergence of
characteristics between populations;
• A final step that results in reproductive
isolation (re-enforcement)
Mechanisms of Isolation
• Allopatric speciation is the formation of a new species
in isolated geographical areas
• Populations often become isolated through dispersal and
colonization
Mechanisms of Isolation cont.
• Populations
can also become geographically
isolated as the result of vicariance events (=splitting
of a population’s former range into 2 or more
isolated patches)
Geographic Isolation via Dispersal and Colonization
• This often occurs as the result of founder events
• The founding population experiences drift and differences in
mutation and selection as the result of being in different
environment
• In the process of adapting to a different environment, the
population becomes genetically differentiated
• When populations have changed enough to be recognizably
different, but not to be reproductively isolated, they are usually
referred to as races or varieties
• Thus, the first recognizable step in allopatric
speciation is when populations have become
genetically differentiated in the process of adapting
to different environments
Dispersal and Colonization: Example involving Hawaiian Drosophila
• Many of the Hawaiian Drosophila are endemic to
particular islands of the archipelago and their
diversification is thought to be due to founder events
• Based on the manner in which the islands were
formed, the founder event hypothesis makes two
predictions:
1) closely related species should be found on adjacent
islands;
2) phylogenetic relations among the flies should correspond
to the manner in which the islands were formed
Mitochondrial DNA sequence data for 5 species of flies
supports these predictions
• The most recent species are found on the youngest islands and the
sequence of branches for the phylogenetic hypothesis of flies is
consistent with the order of island formation
Geographic Isolation through Vicariance
• A comparison of mitochondrial DNA sequences of snapping shrimp
on either side of the isthmus of Panama (pairs of closely related
morphospecies) indicated that species pairs from either side of the
land bridge are each other’s closest relatives
Changes in Chromosomes as a Barrier to Gene
Flow
Chromosomal mutations that lead to polyploidization can
result in reproductive isolation between populations
because if the incompatibilities between gametes with
different number of chromosomes
Mechanisms of Divergence
The Role of Genetic Drift
• The impact of drift on small populations is not always
through a loss of overall genetic variation
• When a population is reduced to a small size, only rare
alleles are lost due to drift
• Genetic variation in the population can actually increase
as a result of bottlenecking
• This still results in a rapid genetic divergence of the
isolated population due to epistatic component of genetic
variance being converted to additive variance
Mechanisms of Divergence cont.
The Role of Natural Selection
Example: The apple maggot fly, Rhagoletis
pomonella
• Traditionally associated hawthorne fruit
trees
• More recently associated with apple trees
Apple
fruit
Hawthorne tree
Rhagoletis pomonella study cont.
• When given a choice flies show a strong preference for
their original host
• Because mating occurs on the plant, host preference
results in strong nonrandom mating
Questions:
•Were flies associated with different
hosts distinct races/species? Can
natural selection based on host
preference for different food create
different races/species of flies?
Rhagoletis pomonella study cont.
• Electrophoretic analysis indicated that the flies are
genetically differentiated
• The flies are isolated on different host species:
• This prediction has been confirmed by following
marked individuals in the field – matings between
hawthorn and apple flies account for only 6% of the
total observed
Rhagoletis pomonella study cont.
• To cause the degree of divergence that has been observed
in this system, NS must overwhelm this level of gene flow
• Feder and others hypothesized that NS for divergence may
be triggered by marked differences when the apple and
hawthorn fruits ripen
• Consequently, hawthorn fly larvae experience cool temperatures
prior to the onset of winter, while apple fly larvae experience
warms temperatures before pupating
Rhagoletis pomonella study cont.
• The hawthorn race individuals that were exposed to a month
of warm days as pupae, and that survived to develop into adults
during spring, had allozyme frequencies similar to those found
in apple flies
Rhagoletis pomonella study cont.
• There appears to be an increase in fitness from
switching to apple trees:
• Escape from parasitoids: The average level of wasp
parasitism is 70% less on apple than on hawthorne
• Escape from intraspecific competition due to the
large size of apple fruit
• Escape from interspecific competition
Mechanisms of Divergence cont.
The Role of Mutation
• For speciation, the primary role of point mutations, gene
duplications, and chromosomal inversions is to provide
the raw materials for drift and selection once gene flow is
reduced
Sympatric speciation is the formation of new species in
sympatry; reproductive isolation without geographic
isolation
• Genetic modeling suggests that populations can diverge
with low to moderate gene flow provided that 2 conditions
are satisfied:
1) selection for divergence is strong;
2) mate choice must be correlated with the factor that is
promoting divergence
Sympatric speciation cont.
Genetic Analysis of Cichlid Fishes
• DNA analysis indicates that each lake with fish was colonized
independently
• DNA analysis also
shows that the two
species in each lake are
more closely related to
each other than they
are to any of the
species in the other
lakes.
The two species in
each lake are
reproductively
isolated; neither mates
with the other.
Parapatric speciation
• The establishment of reproductive isolation between
adjacent populations; formation of contiguous races and
species
1. Strong selection for
divergence is thought to
cause gene frequencies in the
continuous population to
diverge along the gradient
2. The formation of
parapatrically distributed
races could be accomplished
by diversifying selection
3. If the dissimilar habitats
are sharply delineated, there
could be strong selection
against the transmission of
genes responsible for the
local adaptations across such
a boundary
4. Hybrid individuals or their
offspring would be ill adapted
in either environment; prezygotic isolation would be
favored, resulting in
speciation
Reinforcement (reproductive isolation)
• The third stage in speciation may occur if diverged
populations come back into contact and have an opportunity
to interbreed
• Possible fates of the hybrids: 1) they may survive and
interbreed with the parental populations and eliminate the
divergence; 2) may have new characteristics and become a
distinct population
• Dobzhansky indicated that if populations diverged
sufficiently while allopatric, they may no longer be
compatible - post-zygotic isolated mechanisms
• Selection should favor assortative mating, and that
this reinforcement would finalize the speciation
process
• Selection would favor mutations in populations that
would prevent mating from occurring – prezygotic
isolating mechanisms
Post-zygotic Isolating Mechanisms
1. Hybrid Mortality (Inviability)
Incompatibility in parental genes results in the breakdown of
genetic and cellular mechanisms that regulate development
2. Hybrid Sterility
Reproductive isolation in which hybrid zygotes are sterile
3. Hybrid (F2) Breakdown
F1 hybrids are normal, vigorous, and fertile, but the F2 contain
inviable or sterile individuals
Pre-zygotic Isolating Mechanisms
• Under the circumstances of post-zygotic isolation, selection
would favor assortative mating
• It would favor some mechanism that would decrease the
possibility of the 2 populations mating in the first place - prezygotic isolating mechanisms (reinforcement)
1. Habitat Isolation
Two species may never come into contact with one another
because they occur in different habitats in the same
geographical area
Thamnophis sp.
2. Temporal (Seasonal)
Isolation
Potential mates do not
meet because mating or
cross fertilization takes
place at different times of
the year or different times
of the day
B. fowleri
B. americanus
3. Ethological Isolation
Populations are isolated by different and incompatible
behavior before mating
Also, species may
come into contact with
one another, but they
never mate because
there is no sexual
attraction between
males and females
Blue-footed boobies
4. Mechanical Isolation
Closely related species may attempt
to mate, but fail to consummate the
act because they are anatomically
incompatible
5. Gametic Isolation
In organisms with external fertilization, male and female
gametes may not be attracted to one another
In organisms with internal fertilization, although mating
occurs and the gametes may meet, they do not form a zygote
due to gametic mortality
Hybridization and Hybrid Zones
Hybridization
• Reinforcement should occur when hybrids have reduced fitness, but
what if they have increased fitness?
Hybrid Zones
• A hybrid zone is a region where interbreeding
between divergent populations occurs and hybrids
are frequent
• Three possible outcomes dictate their size, shape
and longevity
Hybrid Zones cont.
1. Under some circumstances, there may be no measurable
differences in the fitness of hybrids and purebred lines
• When this is the case, the hybrid zone is usually wide
• The width of the zone is a function of: how far
individuals from each population disperse and how long
the zone has existed
Hybridization and Hybrid Zones cont.
2. Under some circumstances, the resulting hybrids have
lower fitness that the parental offspring
• The fate of the hybrid zone under this scenario depends
on the strength of selection against them
Hybridization and Hybrid Zones cont.
3. Under other circumstances, the resulting hybrids can
have a higher fitness than the purebred offspring in newly
colonized environments or certain restricted habitats
• When the hybrids are more fit, the fate of the hybrid zone
depends on the extent of the environments in which hybrids
have an advantage
Rates of Speciation
• The Rift Lakes of East Africa (Lakes Victoria, Tanganyika, and
Malawi) contain s diverse assemblage of freshwater cichlid fishes
(e.g., 1000 of the 1300 species that are known worldwide)
Cichlids cont.
• Biologists have suggested that the cichlid faunas of these
lakes are the result of rapid speciation
Cichlids cont.
• Phylogeny estimates from DNA sequence data suggest that the
cichlid fauna of each lake may have descended from a common
ancestor (descended from a single population)
Cichlids cont.
• Lake Victoria almost completely dried up some 10-12,000
years ago, suggesting that the approximately 300 species of
this Lake descended from a common ancestor roughly 10,000
years ago
What is responsible for these rapid rates of evolution?
• After the original colonization, periods of drought produced
repeated vicariance events that isolated populations in lakes
• In addition, the ecology and mating systems of these fishes
provided numerous opportunities for divergent selection on
mate choice characteristics, habitat choice, and feeding
strategies