The Geographic Mosaic Theory of Coevolution

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The Geographic Mosaic Theory of Coevolution

• Advanced by John N.Thompson in his 1994 book
  entitled
• The Coevolutionary Process

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Geographic Mosaic Theory

• States that most interactions exhibit an
  evolutionary dynamic geographic mosaic
• Formed by the interpopulational differences in
  outcome, adaptation and specialization

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Geographic Mosaic Theory

• For example, natural selection on the
  relationship between a pair or group of species
  may favor directional selection in one population
  and polymorphism in another
• It may favor escalation of antagonism in some
  environments and reduced antagonism or mutualism
  in others

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Geographic Mosaic Theory

• Parasitism, grazing, predation, competition,
  symbiotic mutalisms, and nonsymbiotic mutalisms
  all show dynamic patterns of specialization that
  are molded in different ways both locally and
  geographically by natural selection through the
  community context in which the interactions occur

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Geographic Mosaic Theory

• The geographic mosaic approach bridges studies of
  local populations and of whole species, it also
  incorporates many aspects of the ecology and
  genetics of species that weve discovered in
  recent decades

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An Example

• Herre, E.A. 1993. Population structure and the
  evolution of virulence in nematode parasites of
  fig wasps. Science 2591442-1445.

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1st Some Theoretical Predictions

• This first example deals with the conditions that
  favor the evolution of reduced antagonism
• One of the conditions favoring reduced antagonism
  in long-term interactions is high partner
  fidelity and low availability of alternative
  partners
• For internal parasites the availability of
  alternative partners depends upon both the modes
  and the rates of transmission between hosts

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Theoretical Predictions

• Low transmission rates and strictly vertical
  transmission (parent to offspring) generally
  appear to favor parasite genotypes that are less
  detrimental in their effects on host survival
• High transmission rates and horizontal
  transmission (between unrelated individuals in a
  population) favor parasite genotypes that
  maximize their growth rate in the host and
  thereby remain highly antagonistic to their host

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Herres (1993) work on fig wasps and their
nematode parasites provide strong support for
this view as well as support for the Geographic
Mosaic Theory
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Fig Wasps

• The opportunity for horizontal transmission of
  the nematode parasite is dependent upon the
  number of fig wasp females that normally enter a
  fig inflorescence
• Each fig species is pollinated by its own species
  of fig wasp
• Depending upon the fig species and the
  population, one or more gravid female fig wasps
  enter each enclosed inflorescence

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Fig Wasps

• The females pollinate and lay eggs within the
  flowers and then die within the fig
• The offspring develop within the seeds, eclose as
  adults, mate, and then the females fly off to lay
  their eggs in another fig inflorescence

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Nematodes

• In the vicinity of the Panama Canal, each of
  eleven species of fig wasp is parasitized by a
  distinct species of nematode
• The nematodes lay their eggs within the figs and
  the eggs hatch synchronously with the emergence
  of adult female fig wasps
• The young nematodes crawl onto a fig wasp, enter
  the body cavity, and are transported with the
  female as she flies off in search of a fig where
  she lays her eggs

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Nematodes

• As the nematodes grow, they eventually consume
  the adult female, emerge from her body, and renew
  the cycle by laying their eggs in the fig

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Transmission

• If only one fig wasp lays her eggs within a fig,
  the nematodes must rely solely on the offspring
  of that female as hosts for the next generation.
  That is, transmission of the nematodes is
  strictly vertical, from a fig wasp female to her
  offspring
• If, however, several fig wasps oviposit into the
  same fig, the nematodes have the opportunity to
  attack the offspring of several females
  (horizontal transmission can take place).

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Herres (1993) Findings

• That vertical transmission has favored nematodes
  that have no effect on the number of offspring
  produced by their host
• By comparison, horizontal transmission has
  favored nematodes that are more destructive to
  their hosts

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Among the 11 species he studied, there was a near
linear relationship between virulence and the
proportion of figs colonized by fig wasp females
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These results suggest that the direction of
evolution depends partially upon the structure of
populations, which will affect factors such as
partner fidelity and transmission
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How Gene Flow Affects Geographic Mosaics

• S.E. Forde, J.N. Thompson and B.J.M. Bohannan.
  2004. Adaptation varies through space and time in
  a coevolving host-parasitoid interaction. Nature
  431841-844.

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Gene Flow Geographic Mosaic

• Show that gene flow across a spatially structured
  landscape alters coevolution of parasitoids and
  their hosts and that the resulting patterns can
  fluctuate in both space and time
• Study used E. coli and a T7 bacteriophage

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Gene Flow Geographic Mosaic

• Results provide evidence that gene flow can alter
  the dynamics of coevolution
• and
• That fragmentation of the natural landscape can
  alter the evolutionary processes that enable
  organisms to adapt to changing local conditions
  slowing coevolutionary changes