A Review of Sympatric Speciation with an Emphasis on Rhagoletis pomonella.

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A Review of Sympatric Speciation with an Emphasis
on Rhagoletis pomonella.
by Dan Eltringham
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Speciation, in brief

• Speciation-The process by which one species
  becomes two species during evolution.
• This occurs when groups within a species become
  reproductively isolated.
• There are different ways in which speciation can
  occur, defined by how the species are
  reproductively isolated.

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Speciation, in brief

• Allopatric Speciation- describes speciation when
  populations are geographically isolated.
• Sympatric Speciation- describes speciation when
  populations are not geographically isolated.
• Allochronic Speciation- describes a speciation in
  which the reproductive barrier is temporal
  instead of geographical.

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Speciaton visualizations
-------TIME-------
No geographic barriers.
Allopatric Speciation
Allochronic Speciation
Sympatric Speciation
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Sympatric Speciation

• The validity of sympatric speciation has been a
  topic of some incongruity in the scientific
  community.
• There are few examples of sympatric speciation.
• It may be difficult to determine if species
  developed in proximity to one another in the
  past.
• There is no obvious barrier to interbreeding
  between populations.
• If there is no barrier there is nothing to halt
  individuals from interbreeding with each other.

Why?
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Sympatric Speciation

• Sympatric speciation is now considered valid
  because
• Speciation appears to be occuring in Rhagoletis
  pomonella in two populations that are not
  geographically isolated.
• It has been observed that Rhagoletis populations
  have been in physical proximity to each other
  since one of the host races developed.
• Barriers can be chronological or ecological as
  well as physical.

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What is Rhagoletis pomonella?

• Rhagoletis pomonella is a fruit fly.
• Related to Drosophila melanogaster only in that
  they are both flies.
• This organism has been the subject of several
  studies concerning sympatric speciation.

Forbes
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What makes this fly important?

• Rhagoletis pomonella populations are one of the
  few in which sympatric speciation is believed to
  have been observed.
• This fly is also important because it is an
  agricultural pest.

European and Mediterranean Plant Protection
Organization
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How do we know?

• There are two distinct groups of Rhagoletis
  pomonella, known as host races.
• Populations of Rhagoletis have been determined to
  be different from one another (Feder et al. 1998,
  McPheron et al.1998).
• Genetic differences are correlated to food
  preference (Forbes et al. 2005).

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Mechanisms of Sympatric Speciation

• Disruptive Selection-A selective pressure selects
  for two extreme phenotypes.
• This mechanism is frequently criticized due to
  lack of examples and there being little
  likelihood judging from selection patterns in
  other species (Gourbiere 2004).
• Allochronic Speciation
• Seasonal Asynchrony-Varieties of hawthorn and
  apple plants have different fruiting seasons.
• Habitat Avoidance- Dambroski et al. 2005 observed
  heritable habitat avoidance in both host races of
  Rhagoletis.

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A Recent Development

• Some researchers were intrigued by the
  possibility of an allopatric aspect to the
  ongoing speciation in Rhagoletis pomonella.
• Rull et al. (2006) did research in Mexico where
  they believed that allopatric conditions existed
  for populations of Rhagoletis.
• They discovered that there was variation in the
  Rhagoletis populations, although the Mexican
  population doesnt appear to be allopatric to the
  United States population.
• They may have discovered a cline.

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What do we know?
Studies Reveal
Unknowns

• The host races of Rhagoletis are genetically
  different.
• Host-avoidance is based on olfactory cues (Forbes
  et al. 2005).
• Rhagoletis populations are still similar enough
  to breed with each other (Dambroski et al. 2005).
• Populations of Rhagoletis are not generally in
  geographic isolation.

• The importance of reproductive isolation on
  genetic integrity.
• Rhagoletis may exist in a cline, suggesting
  parapatric speciation.
• There may be an allopatric factor in the
  speciation of Rhagoletis.
• The mechanisms of sympatric speciation.

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Conclusion

• Sympatric speciation is a growing field.
• As in many things in biology, there appear to be
  few absolutes.
• As research continues, how sympatric speciation
  plays a part in evolution in general may be
  revealed.

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