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Biology
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2. 18-2 Modern Evolutionary Classification
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3. Evolutionary Classification
Phylogeny is the study of evolutionary relationships among organisms.
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4. Evolutionary Classification
Biologists currently group organisms into categories that represent lines of evolutionary descent, or phylogeny, not just physical similarities.
The strategy of grouping organisms is based on evolutionary history and is called evolutionary classification.
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5. Evolutionary Classification
The higher the level of the taxon, the further back in time is the common ancestor of all the organisms in the taxon.
Organisms that appear very similar may not share a recent common ancestor.
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6. Evolutionary Classification
Different Methods of Classification
Crustaceans
Mollusk
Appendages
Conical Shells
Crab
Barnacle
Limpet
Crab
Barnacle
Limpet
Molted external skeleton
Early systems of classification grouped organisms together based on visible similarities. That approach might result in classifying limpets and barnacles together (left). Biologists now group organisms into categories that represent lines of evolutionary descent, or phylogeny, not just physical similarities. Crabs and barnacles are now grouped together (right) because they share several characteristics that indicate that they are more closely related to each other than either is to limpets. These characteristics include segmented bodies, jointed limbs, and an external skeleton that is shed during growth.
Tiny free-swimming larva
Segmentation
CLASSIFICATION BASED ON VISIBLE SIMILARITY
CLADOGRAM
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7. Classification Using Cladograms
Many biologists now use a method called cladistic analysis.
Characteristics that appear in recent parts of a lineage but not in its older members are called derived characters.
Clade- is a group of species that includes a single common ancestor and all descendants of that ancestor
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8. Classification Using Cladograms
Derived characters can be used to construct a cladogram, links groups of organisms by showing how evolutionary lines, or lineages, branched off from common ancestors.
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9. Classification Using Cladograms
A cladogram shows the evolutionary relationships between crabs, barnacles, and limpets.
Crustaceans
Mollusk
Crab
Barnacle
Limpet
Early systems of classification grouped organisms together based on visible similarities. That approach might result in classifying limpets and barnacles together. Biologists now group organisms into categories that represent lines of evolutionary descent, or phylogeny, not just physical similarities. Crabs and barnacles are now grouped together because they share several characteristics that indicate that they are more closely related to each other than either is to limpets. These characteristics include segmented bodies, jointed limbs, and an external skeleton that is shed during growth.
Molted external skeleton
Segmentation
Tiny free-swimming larva
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10. Similarities in DNA and RNA
The genes of many organisms show important similarities at the molecular level.
Similarities in DNA can be used to help determine classification and evolutionary relationships.
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11. Similarities in DNA and RNA
The genes of many organisms show important similarities at the molecular level.
Similarities in DNA can be used to help determine classification and evolutionary relationships.
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12. Similarities in DNA and RNA
DNA Evidence
The more similar the DNA of two species, the more recently they shared a common ancestor, and the more closely they are related in evolutionary terms.
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13. Molecular Clocks
A gene in an ancestral species
Molecular Clocks
A molecular clock uses DNA comparisons to estimate the length of time that two species have been evolving independently.
2 mutations
2 mutations
new mutation
new mutation
new mutation
Species
Species
Species A B C

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Section Quiz
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18-2
Grouping organisms together based on their evolutionary history is called
evolutionary classification.
traditional classification.
cladogram classification.
taxonomic classification.
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18-2
Traditional classification groups organisms together based on
derived characters.
similarities in appearance.
DNA and RNA similarities.
molecular clocks.
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18-2
In an evolutionary classification system, the higher the taxon level,
the more similar the members of the taxon become.
the more common ancestors would be found in recent time.
the fewer the number of species in the taxon.
the farther back in time the common ancestors would be.
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18-2
Classifying organisms using a cladogram depends on identifying
external and internal structural similarities.
new characteristics that have appeared most recently as lineages evolve.
characteristics that have been present in the group for the longest time.
individual variations within the group.
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18-2
To compare traits of very different organisms, you would use
anatomical similarities.
anatomical differences.
DNA and RNA.
proteins and carbohydrates.
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20. END OF SECTION