Principles of Classification

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Principles of Classification

• Classification and Speciation

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• Taxonomy is the science of classification.
• The only useful traits in taxonomy are those that
  are indicative of evolutionary descent.
• Homologies are structures shared by species
  because of common descent.
• Similar structures (analogies) may arise in
  different lineages not because of common descent,
  but because they are the product of similar
  functional demands.
• The process that produces analogies is called
  homoplasy.

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Taxonomic Concepts

• Homologies - structures shared through descent
  from a common ancestor.
• Analogies - structures used for the same function
  that developed independently and are not the
  result of common descent.
• Homoplasy - the process by which similarities
  can develop in different groups of organisms.

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Constructing Classifications and Interpreting
Evolutionary Relationships

• The two major schools of classification are
  evolutionary systematics and cladistics.
• Evolutionary systematics and cladistics have some
  features in common.
• Both schools trace evolutionary relationships and
  construct classifications.
• Both schools recognize that some features
  (characters) are more informative of evolutionary
  relationships than others.
• Both schools focus exclusively on homologies.

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Cladistic Taxonomy

• Groups species according to shared derived
  characteristics
• Primitive traits reflect the ancestral condition.
• Shared derived characteristics are shared traits
  that werent present before the group's
  appearance.

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• These two schools differ in how characters are
  chosen, which groups are compared, and how the
  results are ultimately interpreted.
• Cladistics is concerned with ancestral and
  derived characters.
• Ancestral traits (those traits shared through
  common distant ancestry) do not provide useful
  information in a cladistic analysis.
• Derived, or modified, traits are more informative
  when distinguishing evolutionary lineages because
  they are modified from the ancestral condition
  cladistics concentrates on these traits.
• Cladistics has been used to clarify the
  evolutionary relationship between modern birds
  and theropod dinosaurs.
• For example, some theropods share feathers (a
  derived trait) with modern birds.

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• Phylogenetic systematics uses a phylogenetic tree
  to illustrate evolutionary relationships, whereas
  cladistics uses a cladogram.
• A phylogenetic tree incorporates the dimension of
  time, but a cladogram does not.
• Phylogenetic trees use hypothesized
  ancestor-descendant relationships, but cladograms
  do not.
• Ultimately, most physical anthropologists use the
  information from cladistic analyses to produce
  phylogenetic trees.

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Evolutionary Trees Showing Development of
Vehicles
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Cladogram of Relationships of Birds, Dinosaurs,
and Other Terrestrial Vertebrates
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Evolutionary Relationshipsof Birds and Dinosaurs
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Definition of Species

• The biological species concept (see Chapter 1) is
  the definition accepted by most zoologists.
• Speciation is the macroevolutionary process that
  produces new species.

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Approaches to the Definition of Species
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Process of Speciation

• There are four different hypothesized modes of
  speciation allopatric, parapatric, sympatric,
  and instantaneous speciation.
• Allopatric speciation requires the complete
  geographic isolation of insipient species.
• Parapatric speciation requires partial
  reproductive isolation.
• Hybrid zones form between adjacent populations.
• Mate recognition and selective breeding serve to
  help complete the speciation process.

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• Sympatric speciation has been suggested as a
  theoretical model, but has yet to be supported by
  observations of populations.
• This model requires no reproductive isolation.
• Instantaneous speciation is thought to occur by
  chromosomal mutation.
• In plants, this mode has been observed to occur
  when problems in meiosis results in varieties
  with different chromosomal numbers.
• Large-scale chromosomal alterations leading to
  speciation have been difficult to demonstrate in
  mammals.

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Speciation Model Branching Evolution
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