Speciation and Hybridization

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Speciation and Hybridization
Avise (2004) Chapter 7 - Speciation
history - Speciation Concepts -
Reproductive isolation - Genetics of
Speciation - (Hybridization)
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Speciation and Hybridization
Wed. March 7, Fri. March 9 - Speciation Speciation
Reading for Monday March 12, 900 950 am
discussion http//www.mun.ca/biology/dinnes/B4250
/Biol4250.html Wed. March 14, Fri. March 9
Hybridization Hybridization reading for Monday
March 19, 900 950 am discussion
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Speciation and Hybridization
..the origin of species that mystery of
mysteries
I look at the term species, as one arbitrarily
given for the sake of convenience
http//www.btinternet.com/glynhughes/squashed/dar
win.htm
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Speciation and Hybridization
Ernst Mayr 1904 2005 Systematics and the
origin of Species (1942) Animal Species and
Evolution (1963)
Are species realities of nature or are they
simply theoretical constructs of the human mind?
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Speciation and Hybridization
Theodosius Dobzhansky (1900-1975) Genetics and
the Origin of Species (1937) Genetics of the
Evolutionary Process (1970)
Discrete groups are encountered among animals as
well as plants nobodyclaims that there is a
continuum between eagles and crows
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Speciation and Hybridization
George Ledyard Stebbins, Jr. (1906-2000) Variation
and Evolution in Higher Plants (1950)

• Plants less distinct?
• - hybridization
• polyploidy
• mating system variation
• - agamospermy (apomixis)

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Speciation and Hybridization

• N. Eldredge and S. J. Gould (1972)
• Punctuated equilibria an alternative to
  phylogenetic gradualism
• observed gaps in the fossil record
• long periods of stasis punctuated by
• short periods of rapid speciation

1942 - 2002
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Speciation and Hybridization
Richard C. Lewontin The Genetic Basis of
Evolutionary Change (1974)
Allozyme variation within and among species
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Speciation and Hybridization
MJD White (1910-1983) Modes of Speciation (1978)
Cytogenetics - chromosomes
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Speciation and Hybridization
Jerry Coyne Allen Orr Speciation
(2004)
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Speciation and Hybridization
Summary most biologists agree that species are
real Coyne and Orr (2004)
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Speciation

• Modes of Speciation
• Species concepts

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Speciation

• Modes of Speciation
• allopatric
• peripatric
• parapatric
• sympatric

Gene flow
little
some
high
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Speciation Concepts

• While much of the debate seems more
  philosophical than scientific, the issue is
  important, for we cannot study how species form
  until we determine what they are.
• (Coyne and Orr, 2004)

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Speciation Concepts

• The species problem is not a scientific problem
  at all, merely one about choosing and
  consistently applying a convention about how to
  use a word. So, we should settle on our favorite
  definition, use it, and get on with the science.
  (Brookfield, 2002)

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Speciation Concepts

• Avise (2004) Box 7.1
• BSC Biological species concept
• ESC Evolutionary species concept
• PSC Phylogenetic species concept
• RSC Recognition species concept
• CSC Cohesion species concept
• CP Concordant principles

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Speciation Concepts

• Biological Species Concept (BSC)
• Species are systems of populations gene
  exchange between these systems is limited or
  prevented by reproductive isolating mechanisms or
  perhaps by a combination of several such
  mechanisms Dobzhansky 1937
• What is the specific status of closely related
  forms in allopatry?

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Reproductive Isolation

• RIBs Reproductive isolating barriers
• RIMs Reproductive isolating mechanisms
• May develop as a by-product of genetic divergence
  in allopatry
• Test for biological species status retain
  separate identity in sympatry
• How much genetic divergence required for
  speciation?
• How much genetic exchange disqualifies separate
  species status?
• Genetic divergence and speciation gradual
  processes
• - intermediate stage ? genetic divergence
  and genetic exchange

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Reproductive Isolation

• RIBs Reproductive isolating barriers
• RIMs Reproductive isolating mechanisms
• Box 7.2
• Prezygotic - ecological or habitat isolation
• - temporal isolation
• - ethological isolation
• - mechanical isolation
• - gamete incompatibility

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Reproductive Isolation

• Postzygotic
• 1. Intrinsic (independent of the Env.)
• - F1 inviability
• - F1 sterility
• - hybrid breakdown (F2 and
• backcrosses)
• 2. Extrinsic (depends on Env.)
• - ecological inviability
• - behavioural sterility

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Male and female sing duet
Lacewing morphologically indistinguishable species
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(No Transcript)
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Reproductive Isolation

• Evolutionary origins of RIBs and their genetic
  consequences
• BSC RIBs considered due to intrinsic biological
  factors rather than extrinsic limits to
  reproduction simply due to geographic separation.

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Reproductive Isolation

• Issues
• Speciation sympatrically or allopatrically
• followed by secondary
  contact
• RIBs arise in direct response to selection for
  homotypic matings (reinforcement)
• non-selective by-products of genomic
  differentiation for other reasons

or
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Speciation

• Speciation process
• conversion of genetic variability within a
  species to between species differences
• For BSC RIBs define species. Therefore cannot
  just use magnitude of genetic differentiation to
  identify species

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Speciation

• Usefulness of Genetic markers
• Distinct genetic signatures related to
  demographic events during speciation
• Genetic differentiation at different stages of
  RIB acquisition temporal aspect of speciation
  process
• Assess magnitude and pattern of genetic exchange
  among related forms understand the intensity
  and nature of RIBs

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Speciation

• How much genetic change accompanies speciation?
• Morgan species differ by a number of small
  genetic differences
• de Vries (Goldschmidt) macromutations
• species differ small of genes with large
  effect saltational speciation

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Speciation

• Sudden speciation
• - rapid development of RIBs with minimal genetic
  change
• Box 7.3 polyploidization
• chromosomes rearrangements
• changes in mating systems