Schools of thoughts in systematics

• Classification is the process of arranging taxa into groups of accordance with a plan and in conformity with a naming system
• The 3 largest classification groups

  Bacteria, archaea, eukarya
• What is the 6 kingdom?

  Phylum, Class, Order, Family, Genus, Species
• Classification used in the past by carl linnaeus

  Life - regnum animale, regnum vegetabile

  non-life - regnum lapideum
• Classification used in the past by John hogg

  Life - kingdom protista, kingdom plantae, kingdom animalia

  non life - regnum lapideum
• Classification used in the past by Herbert copeland

  life - empire prokaryota - kingdom monera

  - empire eukaryota - kingdom protista, kingdom plantae,

  kingdom animalia
• Five kingdom of classification based on Robert whittaker

  Monera, protists, fungi, plants, animals
• 6 kingdoms of life by Carl woese

  Domain eukarya - plantae, animalia, protista, fungi

  Domain bacteria - eubacteria

  Domain archae - archaebacteria
• 3 schools of systematics

  Evolutionary systematics, phenetics, cladistics
• Evolutionary systematics

  Oldest approach to reconstructing phylogeny - “Anything goes”

  • No explicit procedure to reconstructing phylogeny

  • Character states thought to be more evolutionarily or functionally

  significant are often heavily weighted over others

  • Diagram: Phylogenetic Tree
• Evolutionary tree (time and ancestors)

  Classical evolutionary taxonomists believed that

  different-looking descendants should be placed

  in different groups

  • The problem arises in the subjectivity of this

  judgment.
• Phenetics

  A response to the lack of objectivity of Evolutionary Systematics

  Attempts to classify organisms based on overall similarity (morphology or observable traits) regardless of their evolutionary relation

  Most influential contributors: Peter Sneath & Robert R. Sokal
• Phenetic techniques include various forms of clustering and ordination

  reduces the variationdisplayed byorganisms to a manageable level

  • Phenogram
• The method that groups organisms that share

  derived characters is called cladistics or

  phylogenetic systematics.

  Taxa that share many derived characters are

  grouped more closely together than those that

  do not.

  The relationships are shown in a branching

  hierarchical tree called a cladogram.
• The state which is in the outgroup is primitive and the one found only in the ingroup is derived.
• The outgroup comparison method is the primary one in use today.

  • The outside taxon is called the outgroup and the organisms being classified are the ingroup.
• The cladogram is constructed such that the number of changes from one character state to the next is minimized. The principle behind this is the rule of parsimony
• parsimony - any hypothesis that requires fewer assumptions is a more defensible hypothesis.
• The trees that result from cladistic analysis are relative statements of

  relationship and do not indicate ancestors or descendants.
• there are actually two reasons why organisms have similar characteristics and only one of them is due to evolutionary relatedness.
• Homologous feature is When two species have a similar characteristic because it was inherited by both from a common ancestor
• Analogous feature is When two species have a similar characteristic because of convergent evolution
• Convergent evolution is When unrelated species adopt a similar way of life, their body parts may take on similar functions and end up resembling one another
• a homology may be:

  recently derived

  ancient retained feature
• monophyletic group:

  • a taxon which forms a clade

  • contains all descendants of their closest common ancestor

  • typically characterized by shared derived characteristics (synapomorphies)

  • ex: mammals
• paraphyletic group:

  • includes some but not all descendants of a common ancestor

  • defined based on ancestral characteristics rather than shared derived characteristics.

  • ex: reptiles (excluding birds)
• polyphyletic group:

  • members don’t share a recent common ancestor

  • have evolved similar characteristics independent of each other

  • ex: warm-blooded animals (mammals and birds)

  • considered unnatural and misleading because they do not reflect true evolutionary relationships
• A clade is basal to another clade if it contains that other clade as a subset within it.
• A clade located within a clade is said to be nested within that clade.
• Two clades are sisters if they have an immediate common ancestor.
• Synapomorphy: A trait shared by 2 or more taxa derived from their most

  recent common ancestor.

  Symplesiomorphy: A characteristic shared by 2 or more taxa also found in their earliest common ancestor.

  Plesiomorphy: A plesiomorphy refers to the ancestral trait state

  Apomorphy: a specialized trait or character that is unique to a group or species

  Homoplasy: A character shared by a set of species but not present in

  their common ancestor.
• Among the three schools of thought, cladistics is the most widely accepted by the scientific community and systematics.