Download Classification of Organisms Classification of Organisms

The Tree of Life
Chapter 26
Classification of Organisms
More than 2000 years ago, Aristotle divided
living things into animals and plants
Later, basic units were called genera
-Felis (cats) and Equus (horses)
In the 1750s, Carolus Linnaeus instituted the
use of two-part names, or binomials
-Homo sapiens
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Classification of Organisms
Taxonomy is the science of classifying living
things
-A classification
level is called a
taxon
Scientific names
avoid the confusion
caused by common
names
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The Linnaean Hierarchy
Taxa are based on shared characteristics
-Domain (most shared)
-Kingdom
-Phylum
-Class
-Order
-Family
-Genus
-Species (least shared)
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Limitations of the Hierarchy
Many hierarchies are being re-examined
based on the results of molecular analysis
-Linnaean taxonomy does not take into
account evolutionary relationships
-Linnaean ranks are not equivalent
-Legume family (16,000 species)
-Cat family (36 species)
The phylogenetic and systematic revolution is
underway
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Grouping Organisms
Carl Woese proposed a six-kingdom system
Prokaryotes
Eukaryotes
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Grouping Organisms
Biologists are increasingly adopting a
three-domain phylogeny based on rRNA
studies
-Domain Archaea
-Domain Bacteria
-Domain Eukarya
Each of these domains forms a clade
Archaea and Eukarya are more closely related
to each other than to bacteria
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During evolution, microbes swapped genetic
information via horizontal gene transfer (HGT)
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Viruses
Are not organisms and so cannot
be placed in any of the kingdoms
Are literally “parasitic” chemicals
-DNA or RNA wrapped in protein
Can only reproduce
within living cells
Vary greatly in
appearance and size
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