Speciation

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Speciation
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• We can separate speciation into 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)

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Mechanisms of Isolation

• Speciation process often begins when populations
  become isolated and gene flow is reduced
• Allopatric speciation is the formation of a new
  species in isolated geographical areas
• Populations often become isolated through
  dispersal and colonization
• Populations can also become geographically
  isolated as the result of vicariance events
  (splitting of a populations former range into 2
  or more isolated patches), including formation of
  mountains ranges, river formation, drying and the
  subsequent fragmentation of forests, lava flows,
  etc.

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Geographic Isolation via Dispersal and
Colonization

• This often occurs as the result of founder events
  
• One consequence of a founder event is that
  colonizing individuals are isolated from the
  original ancestral population
• 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 undergoes different
  genetic substitutions and gradually becomes
  genetically differentiated
• As the result of accumulated genetic differences,
  distinguishable morphological and physiological
  differences may arise
• When populations have changed enough to be
  recognizably different, but not to be
  reproductively isolated, they are usually
  referred to as races or varieties
• The first recognizable step in allopatric
  speciation is when populations have become
  genetically differentiated in the process of
  adapting to different environments

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Dispersal and Colonization Example involving
Hawaiian Drosophila

• Hawaii contains a diverse assemblage of
  drosophilids
• Many of the Hawaiian Drosophila are endemic to
  particular islands of the archipelago leading
  hypothesis for their diversification is 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
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Results

• Mitochondrial DNA sequence data for 4 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

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• Geographic Isolation through Vicariance
• An example of a vicariance event separating
  populations relates to the isthmus of Panama
  the land bridge that developed between North and
  South America some 3 million years ago
• A comparison of mitochondrial DNA sequences of
  snapping shrimp on either side of the isthmus
  (e.g., 7 pairs of closely related morphospecies,
  with one member of each pair found on either side
  of the isthmus) indicated that species pairs from
  either side of the land bridge are each others
  closest relatives

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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
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• 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 for
  genetic diversity to be dramatically reduced the
  population would have to be extremely small
• Most models indicate that genetic variation in
  the population can actually increase as a result
  of bottlenecking
• This can still results in a rapid genetic
  divergence of the isolated population due to
  epistatic component of genetic variance being
  converted to additive variance

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• Mechanisms of Divergence cont.
• The Role of Natural Selection
• Example The apple maggot fly, Rhagoletis
  pomonella
• This fly is common throughout the midwest and
  northeastern US
• Traditionally associated hawthorne fruit trees
• Adults reproduce on the fruit, and females lay
  eggs in the fruit
• When the fruit ripens and falls off the tree,
  the larvae burrow in the ground and emerge next
  spring and summer to repeat the life cycle
• Interestingly, these flies have more recently
  become associated with apple trees probably when
  they were introduced from Europe some 300 years
  ago

Hawthorne tree
Apple fruit
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• 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
• 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?
• Interestingly, host-associated populations of
  flies are not physically isolated from one
  another
• Electrophoretic analysis indicated that the
  flies are genetically differentiated exhibited
  significant allele frequency differences at 6
  enzyme loci
• It appears that rather than being isolated by
  dispersal or vicariance, the flies are isolated
  on different host species

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• Rhagoletis pomonella study cont.
• Results imply that selection has favored
  distinct habitat preferences in apple and
  hawthorn flies.
• 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

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• 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

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• Sympatric and Parapatric Speciation
• A big debate in evolutionary biology has been
  whether physical isolation between populations is
  necessary prerequisite for populations to diverge
  or whether selection for divergence can overwhelm
  gene flow thereby triggering speciation
• Sympatric speciation is the formation of new
  species in sympatry reproductive isolation
  without geographic isolation
• Thus, the difference between allopatry and
  sympatry is a distance factor
• 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

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• 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 pre-zygotic
  isolation would be favored, resulting in
  speciation

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• Reinforcement (reproductive isolation)
• The third stage in speciation may occur if
  diverged populations come back into contact and
  have an opportunity to interbreed
• There are a number of 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 isolating
  mechanisms - isolating mechanisms that prevent
  hybrid zygotes from developing into viable,
  fertile adults
• 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

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Post-zygotic Isolating Mechanisms 1. Hybrid
Mortality (Inviability) Incompatibility in
parental genes results in the breakdown of
genetic and cellular mechanisms that regulate
development Cessation of development and death
before the individual reaches adulthood 2.
Hybrid Sterility Reproductive isolation in which
hybrid zygotes are sterile Cannot reproduce and
their genes cant flow between species Probable
causes a) abnormal development of the gonads b)
failure of parental genes to produce gametes with
correct numbers of homologous chromosomes
(polyploidy) 3. Hybrid (F2) Breakdown F1
hybrids are normal, vigorous, and fertile, but
the F2 contain inviable or sterile individuals
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• 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 - pre-zygotic isolating
  mechanisms (reinforcement)
• There are 5 main types habitat isolation,
  temporal isolation, ethological isolation,
  mechanical isolation, gametic isolation

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1. Habitat Isolation Two species may never come
into contact with one another because they occur
in different habitats in the same geographical
area 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 3.
Ethological Isolation Populations are isolated by
different and incompatible behavior before
mating e.g., incompatible special signals or
courtship displays that attract mates Also,
species may come into contact with one another,
but they never mate because there is no sexual
attraction between males and females
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4. Mechanical Isolation Closely related species
may attempt to mate, but fail to consummate the
act because they are anatomically
incompatible Common in plants - physical
differences in the flowering parts Sometimes
observed in animals, whereby differences in the
genital structure of animals can also lead to
unsuccessful mating 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
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• Hybridization and Hybrid Zones
• Hybridization
• Reinforcement should occur when hybrids have
  reduced fitness, but what if they have increased
  fitness?

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• 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

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• 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, with the hybrids having their
  highest frequency at the center of the zone and
  decreasing frequencies with increasing distance
• The width of the zone is a function of how far
  individuals from each population disperse and how
  long the zone has existed
• Zones are wider if individuals disperse far and
  the populations are in contact for long periods

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• 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
• If selection is strong and reinforcement occurs
  then the hybrid zone is narrow and short lived
• If selection is weak, then the zone is wider and
  longer-lived

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• 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
• If hybrids have higher fitness in environments
  outside the range of the parental species, then a
  new species may form
• If hybrids have an advantage at the boundary of
  each parental species range, then a stable hybrid
  zone may form
• The idea being that hybrid individuals have
  intermediate characteristics and have a fitness
  advantage in the transitional habitats

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• 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)
• Many have very specific feeding strategies and
  habitat preferences
• Many of elaborate colors and behavioral studies
  suggest that sexual selection is intense
• For a long time, biologists have suggested that
  the cichlid faunas of these lakes are the result
  of rapid speciation

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• Cichlids cont.
• For a long time, biologists have suggested that
  the cichlid faunas of these lakes are the result
  of rapid speciation

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• Cichlids cont.
• The longstanding idea was that radiation that
  occurred in each of the lakes was the result of
  many different founder populations
• However, 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)
• The phylogenetic hypotheses that have been
  generated to date clearly separate the species
  found in each lake
• Thus far, no lineage seems to have descendents
  in more than one lake

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• Cichlids cont.
• If the phylogenetic data are correct then
  speciation rates for these lakes has been faster
  than previously envisioned
• There is evidence to suggest that Lake Victoria
  almost completely dried up some 10-12,000 years
  ago
• This would suggest 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?
• The original colonization event was apparently
  from a single founder species
• 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