Speciation

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Speciation

• Evolution of new types

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Clinal variation in house sparrows
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How does discontinuity arise from gradual pattern
and process?

• continuity of gene pools through migration
• continuity of gene pools across generations
• gradual changes in form across environmental
  gradients
• continuous intermediates of form (continuous
  characters bell-shaped distributions) within
  populations at any time

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Darwins biggest challenge

• Promoted gradual change over long periods of time
• Later seen as changes in frequencies of many
  genes, each of small effect
• How can new species arise?

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What are we trying to do by naming species?

• Classify systematically (organize)
• Use terms that match discrete groups of similar
  organisms
• Reflect the process of creating new kinds
• Reflect the history of descent with modification
• Have a system useful for most organisms (all???)

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Classification challenges

• Typological vs. population thinking
• Names based on appearance, vs.
• Names based on processes of sharing genes
• Phenetic Species Concept
• Biological Species Concept

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Species the smallest units that are
evolutionarily independent, but what criteria can
be applied rigorously?

• Morphospecies
• Biological Species
• Phylogenetic Species
• ESU (Evolutionary significant unit)

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Biological species concept

• Ernst Mayr (1942), textbook definition
• Reproductively isolated
• Advantages
• Related to evolutionary independence, a key
  element for diversification
• For a set of species, a clear testable
  criterion
• Disadvantages (issues of domain,
  practicality)
• Irrelevant to asexual species
• Cannot be applied to fossil species
• Isolation by distance potential untestable

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Phenetic Species Concept

• Individuals of a species are phenotypically more
  alike than groups of individuals from other
  species.
• Advantages
• Can be applied to all organisms
• Does not require mating behavior or phylogenies
• Disadvantages
• Defining more alike can be arbitrary
• Characters may not be ecologically important
• Fossil species or rare species may be cryptic

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Phylogenetic Species

• Smallest monophyletic group that can be clearly
  distinguished by diagnostic traits
• Advantages
• Independent of reproductive mode
• Quantifiable
• Includes evolutionary information
• Disadvantages
• Difficult to reconstruct phylogenies
• Diagnostic traits may be trivial, leading to
• Unrealistically large number of species?

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Basic Questions about the process of speciation

• How do populations become evolutionarily
  (genetically) independent?
• What mechanisms promote divergence?
• What mechanisms inhibit divergence?
• How rapidly can speciation occur?
• Are these processes reversible? (What if
  populations reestablish contact?)

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What promotes genetic independence?

• Isolation in space (allopatry)
• Reproductive Isolation
• Prezygotic (lack of mating)
• ecological or seasonal separation
• behavioral (mate preference)
• mechanical or physiological (genitalia or gametes
  incompatible)
• Postzygotic (lack of hybrid success)
• death
• sterility
• hybrid breakdown

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Geographic Isolation usually is primary

• Dispersal vs. vicariance mechanisms

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Vicariance changing geology
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Evidence for vicariance creating isolation of
large populations

• Knowlton (snapping shrimp)
• Vicariance event completion of land connection
  between N. S. America
• mtDNA indicates sister species
• Reproductive isolation between sister species

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Snapping shrimp cladogram based on mtDNA
(Knowlton)P/C Pacific/Caribbean
populationSister species share the same number
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Geographic isolation through dispersal
Galapagos Islands finches, lizards, tortoises,
Opuntia, Senecio, marine iguanas and more!
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Not all reproductive isolation is geographical

• Mating prevented locally by
• Premating barriers
• Ecological
• Behavioral
• Postmating, prezygotic
• Mechanical
• Gametic
• Postzygotic
• Hybrid inviability, sterility
• Hybrid reduction in fitness

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Premating
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Prezygotic, mechanical
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Given isolation of demes (allopatry), why would
species evolve genetic isolation?

• or, why are hybrids unsuccessful?
• Classes of hypotheses
• incidental
• selection for isolation, per se

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Genetic Isolation Can have rapid isolation
through chromosomal mechanisms

• Allopolyploidy hybridization
• Autopolyploidy
• estimated 4 - 10 of dicot and monocot
  speciations

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Genetic Isolation Epistasis

• Genic speciation or the Dobzhansky-Muller
  Model
• 2 allopatric populations experience independent
  substitutions
• Would a gene from one population work as well in
  the other?
• Consider interactions of genes the action of an
  allele depends on the rest of the genetic
  background
• An allele should be favored in its original
  background (so hybrids should be less fit)