Plant Speciation

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Plant Speciation Evolution (PBIO 475/575)

• Species Concepts, Taxonomy and Evolutionary Models

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Species Concepts

• Morphological
• Species are distinguished as sets of populations
  separated by a detectable morphological
  discontinuity from other sets
• e.g., "Species are the smallest groups that are
  consistently and persistently distinct, and
  distinguishable by ordinary means" (Arthur
  Cronquist)

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Species Concepts

• Morphological (cont.)
• Employed implicitly by all studies at least as a
  beginning point, whether admitted explicitly or
  not
• Without other data on reproductive behavior,
  hybridization, ecology, etc., this is the only
  option
• Range of macro- and micromorphological traits
  available, depending on cleverness of
  investigator (most "portable")

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Species Concepts

• Morphological (cont.)
• Problems
• Phenotypic variation fits loosely with genetic
  variation and, occasionally, with reproductive or
  other isolation
• Very similar organisms may not be especially
  closely related genetically

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Species Concepts

• Morphological (cont.)
• Problems (cont.)
• Morphological variation might appear
  semi-continuous or "clinal" but sets of
  populations could be isolated in nature (no
  available data)
• Substantial cytogenetic or other "endophenotypic"
  variation (e.g., physiological) may not be
  expressed in morphology

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Species Concepts

• Biological
• "Species is a group of interbreeding populations
  which are reproductively isolated from other such
  groups" (Ernst Mayr)
• Few studies have empirically ascertained whether
  groups of populations are capable of
  interbreeding
• Note that capability for gene flow in lab is
  not realized gene flow in nature, but BSC
  requires laboratory demonstration

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Species Concepts

• Biological (cont.)
• A few studies have documented reproductive
  isolation between sets of populations called
  biological species many others have used
  circumstantial data (e.g., absence of hybrids in
  region of overlap, low pollen fertility in
  hybrids)
• Many use a morphological species concept in
  practice with circumstantial observations
  suggesting reproductive isolation
• Proponents argue that it holds for a great many
  groups (probably more true for animals than
  plants)

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Species Concepts

• Biological (cont.)
• Problems
• Hybridization and asexual speciation invalidates
  it
• Gene flow may not be frequent or substantial
  among populations of many species in nature, but
  any gene flow is rejected
• Crossing experiments in the laboratory do not
  accurately portray the ability to interbreed in
  nature
• Products of ecological speciation would not be
  recognized (e.g., all members of the white oak
  subgenus 1 species)

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Species Concepts

• Phylogenetic (in the broad sense)
• Several closely related concepts differing in how
  to distinguish taxa and clades
• diagnosability" of species based on one or more
  unique traits
• coalescence of populations (gene flow) and
  divergent populations

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Species Concepts

• Phylogenetic (cont.)
• Problems
• Require discrete characteristics--eliminates
  continuous variation from consideration
• Evolutionary pattern must be strictly
  bifurcating, with common ancestor and 2 or more
  descendants (sister species)
• progenitor-derivative pairs excluded
• reticulation (anything involving hybridization)
  excluded

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Species Concepts

• Phylogenetic (cont.)
• Problems (cont.)
• Some cladistic approaches recognize only
  monophyletic taxa (most recent common ancestor
  and all of its descendants)forbids founder and
  peripheral isolates models, etc.
• Dynamics at the population-species interface pose
  special problems of methodology, sampling and
  types of data necessary

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Species Concepts

• Evolutionary
• "An evolutionary species is a lineage an
  ancestral-descendant sequence of populations
  evolving separately from others and with its own
  unitary evolutionary role and tendencies" (George
  Simpson)
• Circumvents problems with application of
  biological species concept, permits some
  hybridization where this does not compromise
  "species integrity" overall

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Species Concepts

• Evolutionary (cont.)
• Inserts a time perspective, and stresses species
  as sets of populations with independent
  "evolutionary trajectories" from other sets of
  populations
• Allows for combination of morphological and
  utilitarian aspects of biological species
  concept may include phenotypic data, phenology,
  ecology, pollen fertility, chromosome
  numberscircumstantial data OK

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Species Concepts

• Ecological
• "A species is a lineage (or a closely related set
  of lineages) which occupies an adaptive zone
  minimally different from that of any other
  lineage in its range and which evolves separately
  from all lineages outside its range" (Van Valen)
• Permits hybridization, as long as "species
  integrity" is not compromised

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Species Concepts

• Ecological (cont.)
• Really examines recognition of evolutionary
  units from the plants perspective, rather than
  imposing a human construct on biodiversity
• Problems
• Assumes investigations (at least accumulation of
  circumstantial observations) on other aspects of
  organisms, including cytogenetics, ecology and
  physiology, as these involve potential isolation
  mechanisms
• So, not practical for floristicians but probably
  the most flexible species concept

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Restricted Concepts

• Phenetic
• Set of individuals with a high degree of overall
  morphological similarity, separated by a
  morphological gulf from all other sets of
  individuals
• May be only efficient approach with hybrid
  apomictic complexes to order diversity, e.g.,
  blackberries (Rubus), hawthorn (Crataegus)

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Restricted Concepts

• Phenetic (cont.)
• Problems
• No other evidence used in formation of "species
  often loses information where polyploidy or other
  processes are involved
• Doesnt always easily accommodate some
  qualitative traits
• If no broad morphological gulf can be detected,
  where is the line drawn?

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Restricted Concepts

• Genetic
• Groups of populations with high genetic identity
• Really a phenetic concept using genetic distance,
  where population-level markers are examined
• In some algae (e.g., Batrachospermales), where
  morphology provides few characters, divergence in
  DNA sequences used by some to infer species
  break points
• Also essential in bacteria and other groups

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Restricted Concepts

• Paleontological
• Relies on fossil data, involves a time scale
• Paleospecies, chronospecies, etc., to reflect the
  status of an evolutionary lineage at a particular
  time in the fossil record
• Problem--where the fossil record is good, there
  may be few or no abrupt discontinuities among
  transitional forms!

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Perspectives on Biological Diversity

• Taxonomy (incl. nomenclature)
• Characterizes the smallest consistently
  detectable (and "useable") units e.g.,
  species, subspecies, varieties, forms
• Taxa are consistently morphologically distinct,
  without us of special equipment (beyond a hand
  lens)
• Gives distinct taxa stable names

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Perspectives on Biological Diversity

• Taxonomy (cont.)
• Groups individuals into populations, populations
  into species, etc., based on overall similarity
  in an attempt to order biological diversity in a
  convenient and informative way
• Floristicians and monographers practice this
  approach by default cladists use this initially
  (even though they may be embarrassed to admit it)

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Perspectives on Biological Diversity

• Evolutionary inference
• Relates populations to species, species to
  lineages, etc.
• Evolutionary species are distinct when they
  retain their individuality or integrity and are
  on their own evolutionary trajectory from related
  taxa
• Attempts to explain the events and forces that
  have created and maintained sets of organisms

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Perspectives on Biological Diversity

• Evolutionary inference (cont.)
• Puts the history of these organisms into a
  geological or historical context
• Accounts for origination of an organism's biology
  and life history traits
• Evolutionary biologist and many systematists use
  this

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Perspectives on Biological Diversity

• Phylogenetic/cladistic classification
• Groups individuals into populations, populations
  into species, species into higher-level taxa
• Taxa (at least above the species level) must be a
  natural group (monophyletic), or clade, to be
  recognized at all species in some cladistic
  systems are those with at least one consistently
  diagnosable trait distinguishing them from
  relatives

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Perspectives on Biological Diversity

• Phylogenetic/cladistic classification (cont.)
• Attempts to put biological diversity within a
  larger phylogenetic framework of hypothetical
  ancestor and descendants (essentially, a
  classification)
• Used by many systematists, at least above the
  species level

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Conflicts Among Perspectives

• These 3 perspectives, or languages represent
  overlapping but non-correspondent views with
  different aims, using different types of data and
  analytical approaches
• Taxonomy clashes with evolutionary inference
• One single species concept (including the
  morphological one) cannot successfully explain
  biological diversity in every plant group, or
  even in most of them
• Taxonomy must use a "utilitarian"
  characterization of biodiversity based on
  exophenotypic traits but "endophenotypic" or
  molecular variation is ignored

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Conflicts Among Perspectives

• Taxonomy clashes with evolutionary inference
  (cont.)
• Plants evolve in numerous ways, some of which are
  probably not at all understood
• Taxonomy using gross morphological traits will
  obscure some evolutionary species e.g.,
  allopolyploids, asexual species, even
  ecologically or reproductively isolated species
• Unfortunately, absence of evolutionary data for
  most plant groups forces the use of a
  morphological (or phenetic) species concept ?
  underestimation of biological diversity

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Conflicts Among Perspectives

• Phylogenetic/cladistic classification conflicts
  with evolutionary inference
• Requirement of monophyly at the population level
  (by some cladists) or species level (by others)
  ignores founder effect and peripheral isolates
  speciation models
• Asexual and polytypic species concepts, and those
  invoking hybridization, violate theoretical
  assumptions

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Conflicts Among Perspectives

• Prerequisite assumptions reject most of the
  accepted speciation models
• Evolution proceeds parsimoniously (used in
  maximum parsimony criterion for choosing best
  phylogeny)
• Assumes that a trait evolves through the fewest
  number of stepwise changes
• But, we believe increasingly in the
  stochasticity or evolutionary processes, with
  many examples of phenotypic evolution NOT
  proceeding through the simplest series of steps

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Conflicts Among Perspectives

• Prerequisite assumptions (cont.)
• Evolution always results in sister species from a
  common ancestor (which then disappears)
• Cannot accommodate evolutionary taxa produced by
  most of the known speciation models works well
  mainly for classic allopatric model
• While this may work for animals, plants evolve by
  diverse mechanisms including hybridization,
  polyploidy and asexual meanscladistic concepts
  fail

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Growing Agreement

• More than one concept may be applied for
  different species within each group or lineage
  ("pluralistic" approach)
• One species concept may fail to adequately
  accommodate all evolutionary diversity, so may
  need to apply 2 or more for any given taxon to
  characterize detectable patterns of variation in
  nature

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Bibliography

• Baum, D. A. and M. J. Donoghue. 1995. Choosing
  among alternative "phylogenetic" species
  concepts. Systematic Botany 20560-573
• Davis, J. I. 1995. Species concepts and
  phylogenetic analysis--Introduction. Systematic
  Botany 20555-559
• Luckow, M. 1995. Species concepts Assumptions,
  methods and applications. Systematic Botany
  20589-605
• McDade, L. A. 1995. Species concepts and problems
  in practice Insight from botanical monographs.
  Systematic Botany 20606-622

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Bibliography

• Olmstead, R. G. 1995. Species concepts and
  plesiomorphic species. Systematic Botany
  20623-630
• Rieseberg, L. and L. Brouillet. 1994. Are many
  plant species paraphyletic? Taxon 4321-32
• Stuessy, T. F. 1990. Plant Taxonomy The
  systematic evaluation of comparative data.
  Columbia University Press, New York, New York.
  514 pp.