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Classification of Life

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Why Classify Organisms?

• To aid them in their study of the large diversity of organisms, biologists classify organisms by naming them and by grouping them in a logical manner.

• Taxonomy is the discipline scientists use to classify and name organisms.

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Binomial Nomenclature

• From the time of Aristotle until the 1700s, all living things were classified into two kingdoms, the plant and animal kingdom.

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• In the 1700s, Carolus Linnaeus, a Swedish botanist, developed a classification system based on

Carolus Linnaeus1707 - 1778

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anatomical similarities and differences.

• Linnaeus developed binomial nomenclature, a two-word naming system.

• The first word is the genus. The genus is always capitalized.

• The second word is the species. The species is written in lowercase.

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• Both the genus and the species are italicized. For example

Homo sapiens (human)

Felix domestica (house cat)

Panthera leo (lion)

Ajuga reptans (ajuga)

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• A taxon is a group or level of organization. Linnaeus used seven taxons:

KingdomPhylumClassOrderFamilyGenus

Species

Incr

easi

ng

sim

ilari

ties

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Problem with Linnaean System

• Due to convergent evolution, organisms that are quite different from each other evolve similar body structures.

• These similarities made it hard for taxonomist to classify many organisms accurately.

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CONVERGENT EVOLUTION

The process by which

unrelated organisms come to

resemble each other.

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• Scientist now understand that organisms share certain traits because of similarities in their evolutionary history.

• Biologist now group organisms into categories based on evolutionary descent rather than on physical similarities.

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• This evolutionary classification uses cladograms to show what characteristics organisms have in common.

• Scientists use cladograms to understand how things are similar and different.

• See page 452 and handout provided.

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A Cladogram

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Similarities in DNA and RNA

• The similarities of genes in DNA and RNA are now used to help determine classification.

• See handout provided.

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Activity: The Six-Kingdom Classification System

• The current classification system now has six kingdoms and three domains.

• Refer to the classification handout provided.

• Record a brief description of each domain and each kingdom on the handout.

• See page 458 in your textbook for descriptions.

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ACTIVITIES

“Amino Acid Activity”

“Using and Constructing a Classification Key”

“How to Read a Cladogram”

“Investigating Common Functions Among Diverse Organisms”

“Introduction to Viruses” – How do viruses fit into the classification scheme?