Systematics and the Phylogenetic Revolution

1
Systematics and the Phylogenetic Revolution

• Chapter 25

2
The Classification of Organisms

• Classification organisms first classified by
  Aristotle over 2,000 years ago
• Classification scheme of the Middle Ages was
  replaced with a binomial system by Linnaeus about
  250 years ago.
• binomial - two-part name for each species

3
Species Names

• Taxa - group of organisms at a particular level
  in a classification system (taxonomy)
• By convention
• first word of binomial name is genus and is
  always capitalized
• second word refers to particular species and is
  not capitalized
• together form scientific name, written in italics

4
The Taxonomic Hierarchy

• Species
• Genus
• Family
• Order
• Class
• Phylum
• Kingdom
• Domain

5
Evolutionary Relationships

• Systematics - reconstruction and study of
  evolutionary relationships
• construct phylogeny by looking at similarities
  and differences between species
• Cladistics - distinguishes ancestral from shared
  characters
• only shared derived characters are considered in
  determining evolutionary relationships

6
Cladistics

• A cladogram is constructed that depicts
  hypothesis of evolutionary relationships
• Species that share derived characters belong to a
  clade.
• Each node represents a hypothetical ancestral
  species.

7
Cladogram
8
Cladistics

• Outgroup comparison A species or group of species
  closely related to, but not a member of, the
  group under study is designated an outgroup.
• Character states exhibited by the outgroup are
  assumed ancestral, and other states are
  considered derived.

9

• Principle of parsimony favors the hypothesis that
  requires the fewest assumptions

10
Cladogram

• Systematics and classification
• A monophyletic group includes the most recent
  common ancestor of the group and all of its
  descendants.

11
paraphyletic group includes the most recent
common ancestor of the group, but not all of its
descendents.
12
A polyphyletic group does not include the most
recent common ancestor of all the members.
13
Analogy Versus Homology

• Examination of the characters on a cladogram can
  provide insight on how they evolved, how many
  times they evolved, and how complex characters
  evolved.
• Wings in birds and insects
• Eyes in mammals and squid

14
The Kingdoms of Life

• Most biologists use a six-kingdom system.
• Animalia
• Plantae
• Fungi
• Protista
• Archaebacteria
• Bacteria
• Domains - taxonomic level above kingdoms

15
Impact of Molecular Cladistics

• Traditional classification systems are based on
  similar traits, but do not take into account
  evolutionary relationships.
• no clear basis for determining relative
  importance of traits
• Systematic phylogenetics is based on evolutionary
  relationships using cladistics.
• Each clade has a single common ancestor.

16
Tree of Life
17
Domain Archaea

• Share key characteristics
• cells wall lack peptidoglycan
• possess unusual lipids and ribosomal RNA
• Three general categories
• methanogens - obtain energy using hydrogen gas to
  reduce carbon dioxide to methane gas

18
Domain Archaea

• extremophiles - grow under extreme conditions
• thermophiles (heat)
• halophiles (salt)
• pH tolerant
• pressure tolerant
• nonextreme archaebacteria - grow in same
  environment as eubacteria

19
Domain Bacteria

• most abundant organisms on earth
• most taxonomists recognize 12-15 major groups
• as different from archaebacteria as from
  eukaryotes

20
Domain Eukarya

• Appear in fossil record only about 1.5 billion
  years ago.
• complex cellular organization
• Fungi, Plants, and Animals are well-defined
  evolutionary groups.
• largely multicellular
• Diversity among protists is much greater than
  within or between Fungi, Plants, and Animals.

21
Domain Eukarya

• With few exceptions, all modern eukaryotic cells
  possess energy-producing organelles
  (mitochondria).
• Some protist phyla have also acquired
  chloroplasts and are photosynthetic.
• Mitochondria and chloroplasts are both believed
  to have entered early eukaryotic cells by
  endosymbiosis.

22
Domain Eukarya

• Key characteristics
• Compartmentalization
• Discrete compartments provide evolutionary
  opportunities for increased specialization within
  the cell.
• Multicellularity
• Activities of individual cells are coordinated
  and the cells themselves are in contact.

23
Domain Eukarya

• Key characteristics
• Sexuality
• alternate between syngamy and meiosis

24
Impact of Molecular Cladistics

• Molecular systematics labels protists as
  paraphyletic.

Insects may be a sister group of crustaceans.
25
Major Mammalian Groups
Eutherian mammals may be reclassified.