systematics and the phylogenetic revolution
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Systematics and the Phylogenetic Revolution. Chapter 25. 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. - PowerPoint PPT PresentationTRANSCRIPT
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Systematics and the Phylogenetic Revolution
Chapter 25
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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
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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
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The Taxonomic Hierarchy
• Species• Genus• Family• Order• Class• Phylum• Kingdom• Domain
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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
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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.
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Cladogram
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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.
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• Principle of parsimony favors the hypothesis that requires the fewest assumptions
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Cladogram
• Systematics and classification– A monophyletic group includes the most
recent common ancestor of the group and all of its descendants.
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paraphyletic group includes the most recent common ancestor of the group, but not all of its descendents.
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A polyphyletic group does not include the most recent common ancestor of all the members.
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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
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The Kingdoms of Life
• Most biologists use a six-kingdom system.
– Animalia– Plantae– Fungi– Protista– Archaebacteria– Bacteria
• Domains - taxonomic level above kingdoms
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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.
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Tree of Life
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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
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Domain Archaea
• extremophiles - grow under extreme conditions
– thermophiles (heat)– halophiles (salt)– pH tolerant– pressure tolerant
• nonextreme archaebacteria - grow in same environment as eubacteria
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Domain Bacteria
• most abundant organisms on earth• most taxonomists recognize 12-15 major
groups• as different from archaebacteria as from
eukaryotes
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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.
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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.
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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.
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Domain Eukarya
• Key characteristics– Sexuality
alternate between syngamy and meiosis
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Impact of Molecular Cladistics
• Molecular systematics labels protists as paraphyletic.
Insects may be a sister group of crustaceans.
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Major Mammalian GroupsEutherian mammals may be reclassified.