Module 6 Mechanisms of evolution Lecture 6 Modes of speciation

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Module 6 Mechanisms of evolution Lecture 6Modes
of speciation

• 1. Allopatric speciation
• Species arise following the geographical
  isolation of a sub-population
• 2. Sympatric speciation
• Speciation among a subset of organisms living
  together

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Allopatry

• Two models
• Colonization model
• where a group of colonists evolve into a new
  species
• Extinction model
• where the extinction of intermediates isolates
  populations which may then undergo divergent
  evolution

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1. Colonization model

• Adaptive radiation on an island chain Fig.24.9
• A scenario where four new species evolves from
  the founding species
• An example of cladogenesis or anagenesis?
• Significance of group size and habitat
  differences?

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2. Extinction model Mouse Deer Fig. 24.6 four
subspecies. Extinction might result in cessation
of gene flow (?) and speciation
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Sympatric speciation

• Mostly in plants from polyploidy
• Non-disjunction of chromosomes during mitosis or
  meiosis, gives,
• viable, fertile offspring with multiples of
  chromosomes, however,
• the polyploid cannot interbreed with the parental
  forms and therefore represents a new species
• De Vries Evening Primrose Oenothera
• O.lamarkiana (2n14) ? O. gigas (4n28)
• Wheat, oats, potatoes are polyploids

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Two processes autopolyploidy and allopolyploidy

• Autopolyploidy results from ploidy involving only
  one species as in the Evening Primrose
• Allopolyploidy involves the hybridisation of two
  species
• Since polyploidy is an unusual event asexual
  reproduction would usually be needed for the
  polyploid to breed.

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Sympatric speciation by autopolyploidy Fig.24.10a
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Sympatric speciation by allopolyploidy Fig.24.10b
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The evolution of wheatby alloploidy Fig. 24.11

• A series of happy accidents
• In this A, B and D each represents a set of
  chromosomes (n7 chromosomes)
• Note that meiotic error here is in fact mitotic
  non-disjunction followed by asexual reproduction
• The present day wheat is a hexaploid (42).

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The pace of evolution

• Gradualism
• proposed by Darwin and others
• some fossil series suggest evolution progressed
  in small steps

• Punctuated equilibrium
• proposed by Gould and others
• some series show periods of rapid change
  separated by periods of little or no speciation
• evolution in large steps
• And/or??

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Two models for the pace of evolution Fig.24.13
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Reconciling Gradualism with Punctuated Equilibrium

• Darwin puzzled over lack of intermediate forms in
  some series.
• Fossil series span rock strata representing many
  millions of years of sedimentation
• A particular species may be judged to have
  evolved from its immediate ancestor over 50 000
  years and then remain unchanged for 5 million
  years
• Gradualists acknowledge that the pace of
  evolution differed between these two periods, but
  assert that it was still gradual i.e. in small
  steps, over what is quite a long period (50 000
  years).
• However, these steps may not be represented in
  the series. Fossilization is an unlikely event.

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And the future for human kind?

• Too pessimistic our future depends on cultural
  rather than organic evolution