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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.