Species Concepts and Speciation

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Species Concepts and Speciation
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Non-Evolutionary Approaches I

• Typological Species Concept species are a 'type'
  of organismPlato
• Existence of a limited number of universals
• No regard to variation in a species
• "Species are as many as were created in the
  beginning by the Infinite." (Linnaeus, 1758)

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Non-Evolutionary Approaches II

• Essentialism
• Similar to typological thinking
• Species consist of individuals sharing the same
  essence
• Species are separated by sharp discontinuity
• Each species is constant through time
• Limits to variation within a species

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Non-Evolutionary Approaches III

• Nominalistic
• Only Individuals exist
• There are no real universals
• Popular in France in late 1800s
• Nature produces individuals and nothing moreno
  species.
• Species are only mental constructs

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Species Concepts Based in Evolution

• The lineage species concept.
• A single species forms an evolutionary lineage
  separated from other species.
• They are not in genetic contact with other
  species
• Therefore, the concepts you may have learned are
  actually criteria for determining the split
  between species.

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Biological Species Criterion (Concept)

• Species are groups of actually or potentially
  interbreeding natural populations, which are
  reproductively isolated from other such groups
  (Mayr, 1940).

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Problems with the BSC

• What do you do with asexual organisms, and what
  do you do with organisms that occasionally form
  hybrids with one another? Other difficulties
  include
• What is meant by potentially interbreeding
• Fossils---for example, it is not really possible
  (or very meaningful!) to figure out whether a
  trilobite living 300 million years ago would have
  interbred with its ancestor living 310 million
  years ago.

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Evolutionary species criterion (concept)

• Evolutionary species concept A species is a
  single lineage of ancestor-descendant populations
  which maintain its identity from other such
  lineages and which has it own evolutionary
  tendencies and historical fate (Simpson, 1961
  Wiley, 1981).
• Difficult to apply, so we use the following

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Phylogenetic species criterion (concept)

• A species is the smallest diagnosable cluster of
  individual organisms within which there is a
  parental pattern of ancestry and descent
  (Cracraft 1983).

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Other criteria (concepts)

• Cohesion species concept A species is the most
  inclusive group of organisms having the potential
  for genetic and/or demographic exchangeability.
  (Templeton, 1989)
• Ecological species concept A species is a set of
  organisms exploiting (or adapted to) a single
  niche (Ridley 1993).
• Recognition species concept A species is the
  most inclusive population of individual
  biparental organisms which share a common
  fertilization system. (as defined by Paterson,
  1985 in Templeton, 1989).
• Isolation species concept Species are systems of
  populations the gene exchange between these
  systems is limited or prevented by a reproductive
  isolating mechanism or perhaps by a combination
  of several such mechanisms. (as defined by
  Dobzhansky 1970 in Templeton, 1989)

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Now that we know some concepts, how does
speciation occur?

• Geographic Isolation
• A physical barrier (mountain, river etc.) reduces
  gene flow
• Poor habitat separating organisms
• From http//evolution.berkeley.edu/evosite/evo101
  /VC1eSympatric.shtml

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Reduction of Gene Flow

• Physical barrier
• Or no contact at the edges of a large range

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On a phylogeny a node indicates a speciation even
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Modes of speciation I Allopatric

• In this mode of speciation, something extrinsic
  to the organisms prevents two or more groups from
  mating with each other regularly, eventually
  causing that lineage to speciate. Isolation might
  occur because of great distance or a physical
  barrier, such as a desert or river, as shown
  below.

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Modes of speciation II Peripatric

• An isolated group is severed from the original
  population
• Rare genes move to fixation
• If these genes are associated with reproduction,
  sexual selection takes over, then speciation has
  occurred as they may not mate with the original
  population

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Modes of speciation III Parapatric

• In parapatric speciation there is no specific
  extrinsic barrier to gene flow.
• The population is continuous, but nonetheless,
  the population does not mate randomly.
• Individuals are more likely to mate with their
  geographic neighbors than with individuals in a
  different part of the populations range.
• In this mode, divergence may happen because of
  reduced gene flow within the population and
  varying selection pressures across the
  populations range.

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We may be observing the first steps of
parapatric speciation in the grass species
Anthoxanthum odoratum
Evolved different flowring times due to natural
selection from waste
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Modes of speciation IV Sympatric

• Unlike the previous modes, sympatric speciation
  does not require large-scale geographic distance
  to reduce gene flow between parts of a
  population.
• How could a randomly mating population reduce
  gene flow and speciate? Merely exploiting a new
  niche may automatically reduce gene flow with
  individuals exploiting the other niche.
• This may occasionally happen when, for example,
  herbivorous insects try out a new host plant.