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CHAPTER 26

PHYLOGENY AND SYSTEMATICS

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What are the evolutionary relationships between a human, a mushroom, and a flower?

A phylogeny based on DNA data reveals that animals (including humans) and fungi (including mushrooms) are more closely related to each other than to plants.

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OVERVIEW Phylogeny is the evolutionary history of a species or group

of species. The discipline of systematics classifies organisms and

determines evolutionary relationships of living and extinct organisms.

Systematists use fossil, molecular, and genetic data to infer evolutionary relationships

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I. Concept 26.1: Classification A. Taxonomy

The ordered division of organisms into categories based similarities and differences

-useful component of systematicsB. Binomial Nomenclature (two word name)

1. First proposed by Carolus Linnaeus in 18th century2. Two main characteristics:

a. Each species has a two-part name.b. Species are organized hierarchically into

broader and broader groups of organisms.

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3. The first part of the name is the genus.4. The second part, called the specific epithet, is

unique for each species within the genus.5. Ex: Human—Homo sapiens which means wise

manC. Hierarchical Classification

1. Groups species into increasingly broad taxonomic categories

2. Taxonomic groups from broad to narrow are:domain, kingdom, phylum, class, order, family,

genus, and species3. A taxonomic unit at any level of hierarchy is

called a taxon

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D. Phylogenetic Tree1. Diagram used to show

the evolutionary history of a group

of organisms2. Can provide important

information about similar

characteristics in closely related species

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II. Concept 26. 2: Morphological and Molecular DataA. To infer relationships, systematists gather information

about morphologies, genes, and biochemistry of living organisms

B. Organisms with similar morphologies or DNA sequences are likely to be more closely related than organisms with different structures or sequences

C. When constructing a phylogeny, systematists need to distinguish whether a similarity is the result of homology or analogy1. Homology is similarity due to shared ancestry

Ex: bones of a whale’s flipper and a tiger’s paw

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2. Analogy is similarity due to convergent evolutionConvergent evolution occurs when two organisms developed similarities as they adapted to similar environmental challenges—not because they evolved from a common ancestor

Ex: wing of a butterfly is analogous to wing of a bat because both are adaptations for flight

3. Analogous structures or molecular sequences that evolved independently are also called homoplasies4. Homology can be distinguished from analogy by comparing fossil

evidence and the degree of complexity5. The more complex two similar structures are, the more likely it is

that they are homologous

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D. Molecular systematics uses DNA and other molecular data to determine evolutionary relationships.The more alike the DNA sequences of two organisms, the more closely related they are evolutionarily.

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III. Concept 26.3: Constructing Phylogenetic TreesA. Cladistics

1. Cladistics groups organisms by common descent 2. Once homologous characters have been identified, they can be used to infer a phylogeny3. A cladogram depicts patterns of shared characteristics among taxa and forms the basis of a phylogenetic tree4. A clade is a group of species that includes an ancestral species and all its descendants

Not all groupings of organisms qualify as clades5. A valid clade is monophyletic, signifying that it consists

of the ancestor species and all its descendants

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MONOPHYLETIC GROUP (CLADE) Group I, consisting of 3

species (A, B, C) and their common ancestor (1), is a clade, also called a monophyletic group.

A monophyletic group consists of an ancestral species and all of its descendants

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PARAPHYLETIC GROUP Group II is paraphyletic. A paraphyletic grouping

consists of an ancestral species and some, but not all, of the descendants.

In this case Group II consists of an ancestral species (2) and some of its descendants (species D, E, F) but not all of them (missing species G).

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POLYPHYLETIC GROUP Group III is polyphyletic. A polyphyletic grouping

consists of various species that lack a common ancestor

In this case, species D, E, F, and G share common ancestor (2), but species C has a different ancestor (1).

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B. Shared Ancestral and Shared Derived Characters1. A shared ancestral character is found not only in the clade being analyzed but also in older clades

Ex: Backbone in vertebrates2. A shared derived character is unique to a particular clade.

Ex: Hair in mammals3. A character can be both ancestral and derived, depending on the context4. When inferring evolutionary relationships, it is useful to know in which clade a shared derived character first appeared

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Phylogenetic Tree

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IV. Concept 26.4: Genome Documentation(Not on exam)

A. Comparing nucleic acids or other molecules to infer relatedness is a valuable tool for tracing organisms’ evolutionary history

B. DNA that codes for ribosomal RNA changes relatively slowly and is useful for investigating branching points hundreds of millions of years ago

C. Mitochondrial (mt)DNA evolves rapidly and can be used to explore recent evolutionary events

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V. Concept 26.5: Molecular Clocks(not on exam) Molecular clocks are methods used to measure the

absolute time of evolutionary change based on the observation that some genes and other regions of the genome appear to evolve at constant rates.

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VI. Concept 26.6: Three-Domain SystemA. Supported by data from many sequenced genomesB. A Comparison of the Three Domains of Life

Characteristics Bacteria Archaea Eukarya

Nuclear Envelope No No Yes

Membrane-enclosed organelles No No Yes

Introns No Yes Yes

Histone proteins associated with DNA No Yes Yes

Circular Chromosome Yes Yes No

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You should now be able to:1. The taxonomic categories and how they indicate

relatedness2. How systematics is used to develop phylogenetic

trees3. The three domains of life including their similarities

and their differences