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Biology
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2. 18-1 Finding Order in Diversity
Photo credit: ©Gary Randall/Visuals Unlimited
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3. 18-1 Finding Order in Diversity
Natural selection and other processes have led to a staggering diversity of organisms.
Biologists have identified and named about 1.5 million species so far.
They estimate that 2–100 million additional species have yet to be discovered.
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Why Classify?
In the discipline of taxonomy, scientists classify organisms and assign each organism a universally accepted name.
When taxonomists classify organisms, they organize them into groups that have biological significance.
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5. Assigning Scientific Names
Common names of organisms vary, so scientists assign one name for each species.
Because 18th century scientists understood Latin and Greek, they used those languages for scientific names.
This practice is still followed in naming new species.
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6. Assigning Scientific Names
Early Efforts at Naming Organisms
The first attempts at standard scientific names described the physical characteristics of a species in great detail.
These names were not standardized because different scientists described different characteristics.
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7. Assigning Scientific Names
Carolus Linneaus developed a naming system called binomial nomenclature.
In binomial nomenclature, each species is assigned a two-part scientific name.
The scientific name is italicized.
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8. Assigning Scientific Names
The first part of the name is the genus to which the organism belongs. A genus is a group of closely related species. The genus name is capitalized.
The second part of the name is unique to each species within the genus. This part of the name often describes an important trait or where the organism lives. The species name is lowercased.
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9. Linnaeus’s System of Classification
Linnaeus not only named species, he also grouped them into categories.
What is Linneaus’s system of classification?
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10. Linnaeus's System of Classification
Linnaeus's seven levels of classification are—from smallest to largest—
species
genus
family
order
class
phylum
kingdom
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11. Linnaeus's System of Classification
 Each level is called a taxon, or taxonomic category.
Species and genus are the two smallest categories.
Grizzly bear
Black bear
Linnaeus’s hierarchical system of classification uses seven taxonomic categories. This illustration shows how a grizzly bear, Ursus arctos, is grouped within each taxonomic category. Only some representative species are illustrated for each category above the species level.
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12. Linnaeus's System of Classification
Genera that share many characteristics are grouped in a larger category, the family.
Grizzly bear
Black bear
Giant panda
Linnaeus’s hierarchical system of classification uses seven taxonomic categories. This illustration shows how a grizzly bear, Ursus arctos, is grouped within each taxonomic category. Only some representative species are illustrated for each category above the species level.
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13. Linnaeus's System of Classification
An order is a broad category composed of similar families.
Grizzly bear
Black bear
Giant panda
Red fox
Linnaeus’s hierarchical system of classification uses seven taxonomic categories. This illustration shows how a grizzly bear, Ursus arctos, is grouped within each taxonomic category. Only some representative species are illustrated for each category above the species level.
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14. Linnaeus's System of Classification
The next larger category, the class, is composed of similar orders.
Grizzly bear
Black bear
Giant panda
Red fox
Abert squirrel
Linnaeus’s hierarchical system of classification uses seven taxonomic categories. This illustration shows how a grizzly bear, Ursus arctos, is grouped within each taxonomic category. Only some representative species are illustrated for each category above the species level.
Class Mammalia
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15. Linnaeus's System of Classification
Several different classes make up a phylum.
Grizzly bear
Black bear
Giant panda
Red fox
Abert squirrel
Coral snake
PHYLUM Chordata
Linnaeus’s hierarchical system of classification uses seven taxonomic categories. This illustration shows how a grizzly bear, Ursus arctos, is grouped within each taxonomic category. Only some representative species are illustrated for each category above the species level.
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16. Linnaeus's System of Classification
The kingdom is the largest and most inclusive of Linnaeus's taxonomic categories.
Grizzly bear
Black bear
Giant panda
Red fox
Abert squirrel
Coral snake
Sea star
KINGDOM Animalia
Linnaeus’s hierarchical system of classification uses seven taxonomic categories. This illustration shows how a grizzly bear, Ursus arctos, is grouped within each taxonomic category. Only some representative species are illustrated for each category above the species level.
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17. Linnaeus's System of Classification
Grizzly bear
Black bear
Giant panda
Red fox
Abert squirrel
Coral snake
Sea star
Linnaeus’s hierarchical system of classification uses seven taxonomic categories. This illustration shows how a grizzly bear, Ursus arctos, is grouped within each taxonomic category. Only some representative species are illustrated for each category above the species level.
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18. 18-2 Modern Evolutionary Classification
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19. Evolutionary Classification
Which Similarities Are Most Important?
Linnaeus grouped species into larger taxa mainly according to visible similarities and differences.
How are evolutionary relationships important in classification?
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20. Evolutionary Classification
Phylogeny is the study of evolutionary relationships among organisms.
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21. 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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22. 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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23. 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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24. Evolutionary Classification
Superficial similarities once led barnacles and limpets to be grouped together.
Appendages
Conical Shells
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.
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25. Evolutionary Classification
However, barnacles and crabs share an evolutionary ancestor that is more recent than the ancestor that barnacles and limpets share.
Barnacles and crabs are classified as crustaceans, and limpets are mollusks.
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26. Classification Using Cladograms
Many biologists now use a method called cladistic analysis.
Cladistic analysis identifies and considers only new characteristics that arise as lineages evolve.
Characteristics that appear in recent parts of a lineage but not in its older members are called derived characters.
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27. Classification Using Cladograms
Derived characters can be used to construct a cladogram, a diagram that shows the evolutionary relationships among a group of organisms.
Cladograms help scientists understand how one lineage branched from another in the course of evolution.
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28. 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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29. Similarities in DNA and RNA
How can DNA and RNA help scientists determine evolutionary relationships?
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30. 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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31. Similarities in DNA and RNA
DNA Evidence
DNA evidence shows evolutionary relationships of species.
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.
The more two species have diverged from each other, the less similar their DNA will be.
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32. Molecular Clocks
Molecular Clocks
Comparisons of DNA are used to mark the passage of evolutionary time.
A molecular clock uses DNA comparisons to estimate the length of time that two species have been evolving independently.
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33. Molecular Clock Molecular Clocks A gene in an ancestral species
2 mutations
2 mutations
new mutation
new mutation
new mutation
By comparing the DNA sequences of two or more species, biologists estimate how long the species have been separated.
Species
Species
Species A B C
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34. A molecular clock relies on mutations to mark time.
Molecular Clocks
A molecular clock relies on mutations to mark time.
Simple mutations in DNA structure occur often.
Neutral mutations accumulate in different species at about the same rate.
Comparing sequences in two species shows how dissimilar the genes are, and shows when they shared a common ancestor.
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35. 18-3 Kingdoms and Domains
Photo credit: ©Gary Randall/Visuals Unlimited
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36. The Tree of Life Evolves
Systems of classification adapt to new discoveries.
Linnaeus classified organisms into two kingdoms—animals and plants.
The only known differences among living things were the fundamental traits that separated animals from plants.
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37. The Tree of Life Evolves
Five Kingdoms
Scientists realized there were enough differences among organisms to make 5 kingdoms:
Monera
Protista
Fungi
Plantae
Animalia
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38. The Tree of Life Evolves
Six Kingdoms
Recently, biologists recognized that Monera were composed of two distinct groups: Eubacteria and Archaebacteria.
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39. The Tree of Life Evolves
The six-kingdom system of classification includes:
Eubacteria
Archaebacteria
Protista
Fungi
Plantae
Animalia
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40. The Tree of Life Evolves
Changing Number of Kingdoms
Introduced
Names of Kingdoms
1700’s
Plantae
Animalia
Late 1800’s
Protista
Plantae
Animalia
1950’s
Monera
Protista
Fungi
Plantae
Animalia
This diagram shows some of the ways organisms have been classified into kingdoms over the years. The six-kingdom system includes the following kingdoms: Eubacteria, Archaebacteria, Protista, Fungi, Plantae, and Animalia.
Archae-bacteria
1990’s
Eubacteria
Protista
Fungi
Plantae
Animalia
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41. The Three-Domain System
Molecular analyses have given rise to a new taxonomic category that is now recognized by many scientists.
The domain is a more inclusive category than any other—larger than a kingdom.
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42. The Three-Domain System
The three domains are:
Eukarya, which is composed of protists, fungi, plants, and animals.
Bacteria, which corresponds to the kingdom Eubacteria.
Archaea, which corresponds to the kingdom Archaebacteria.
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43. The Three-Domain System
Modern classification is a rapidly changing science.
As new information is gained about organisms in the domains Bacteria and Archaea, they may be subdivided into additional kingdoms.
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44. Domain Bacteria
Domain Bacteria
Members of the domain Bacteria are unicellular prokaryotes.
Their cells have thick, rigid cell walls that surround a cell membrane.
Their cell walls contain peptidoglycan.
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45. The domain Bacteria corresponds to the kingdom Eubacteria.
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46. Domain Archaea
Domain Archaea
Members of the domain Archaea are unicellular prokaryotes.
They live in extreme environments.
Their cell walls lack peptidoglycan, and their cell membranes contain unusual lipids not found in any other organism.
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47. The domain Archaea corresponds to the kingdom Archaebacteria.
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48. Domain Eukarya
Domain Eukarya
The domain Eukarya consists of organisms that have a nucleus.
This domain is organized into four kingdoms:
Protista
Fungi
Plantae
Animalia
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49. Domain Eukarya
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50. Its members display the greatest variety.
Domain Eukarya
Protista 
The kingdom Protista is composed of eukaryotic organisms that cannot be classified as animals, plants, or fungi.
Its members display the greatest variety.
They can be unicellular or multicellular; photosynthetic or heterotrophic; and can share characteristics with plants, fungi, or animals.
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51. Members of the kingdom Fungi are heterotrophs.
Domain Eukarya
Fungi 
Members of the kingdom Fungi are heterotrophs.
Most fungi feed on dead or decaying organic matter by secreting digestive enzymes into it and absorbing small food molecules into their bodies.
They can be either multicellular (mushrooms) or unicellular (yeasts).
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52. Plants are nonmotile—they cannot move from place to place.
Domain Eukarya
Plantae 
Members of the kingdom Plantae are multicellular, photosynthetic autotrophs.
Plants are nonmotile—they cannot move from place to place.
Plants have cell walls that contain cellulose.
The plant kingdom includes cone-bearing and flowering plants as well as mosses and ferns.
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