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How Biologists Classify Life

The figure here represents biologists’ current understanding of the tree of life — the phylogenetic tree that shows relationships among all organisms on Earth. Each of the main branches on the family tree represents a unique type of life on Earth, which biologists place into categories they call domains. Within each domain, biologists create smaller categories that represent groups of related organisms.

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The categories that biologists use for grouping related organisms are called taxa (singular: taxon).

Biologists organize all these categories into a taxonomic hierarchy, a naming system that ranks organisms by their evolutionary relationships. Within this hierarchy, living things are organized from the largest, most-inclusive group (domains) down to the smallest, least-inclusive group (called species).

From largest to smallest groups, the taxonomic hierarchy is

  • Domain: Domains group organisms by fundamental characteristics like cell structure and chemistry. The three domains are Bacteria, Archaea, and Eukarya.

  • Kingdom: Kingdoms classify organisms based on developmental characteristics and nutritional strategy. The most familiar kingdoms are those in domain Eukarya: Animalia, Plantae, Fungi, and Protista. (Kingdoms aren’t well defined for the other domains.)

  • Phylum: Phyla separate organisms based on key characteristics that define the major groups within the kingdom.

  • Class: Classes categorize organisms based on key characteristics that define the major groups within the phylum.

  • Order: Orders group organisms based on key characteristics that define the major groups within the class.

  • Family: Families classify organisms based on key characteristics that define the major groups within the order.

  • Genus: Genera separate organisms based on key characteristics that define the major groups within the family. Genera names are typically capitalized and italicized.

  • Species: Species categorize eukaryotic organisms based on whether they can successfully reproduce with each other and create viable offspring. (For prokaryotes, which reproduce asexually, species are defined by a set of common characteristics such as metabolic pathways and genetic similarity.) Species names are lower case and italicized.

You can think of all the categories of the taxonomic hierarchy as a set of nesting dolls like the Russian matryoshka dolls, where you open a big doll to find a smaller doll inside, which opens to reveal an even smaller doll, and so on.

The only difference is that when you open the big domain doll, you find several kingdom dolls inside, and you find several phylum dolls inside the kingdom dolls, and so on.

To remember the taxonomic hierarchy, try memorizing a sentence like this one: Dumb Kids Playing Chase On Freeways Get Squished. The first letter of each word in the sentence represents the first letter of a category in the taxonomic hierarchy, in order. If you don’t like this sentence, search the Internet for “taxonomic hierarchy mnemonic” and you’ll find many more.

Here are some practice questions on the classification of life.

Questions 1 through 8 give the complete taxonomic identification for a ball python in order from most-inclusive category to least-inclusive category. Use the following terms to identify each category.

a. Domain

b. Family

c. Species

d. Genus

e. Order

f. Class

g. Kingdom

h. Phylum

  1. Eukarya

  2. Animalia

  3. Chordata

  4. Reptilia

  5. Squamata

  6. Boidae

  7. Python

  8. regius

The following are the answers to the practice questions.

  1. The answer is a. Domain.

  2. The answer is g. Kingdom.

  3. The answer is h. Phylum.

  4. The answer is f. Class.

  5. The answer is e. Order.

  6. The answer is b. Family.

  7. The answer is d. Genus.

  8. The answer is c. Species.

    (The correct format for writing the complete species name would be to write either Python regius or P. regius. The species name, or specific epithet, is never written alone.)

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