Classification of Living Things

Grade 9 · Science · Study Guide

Understand how biologists organize the diversity of life using taxonomy, binomial nomenclature, and the evolutionary relationships revealed by phylogeny.

Taxonomy is the science of naming, describing, and classifying organisms. The modern classification system was developed by Carl Linnaeus in the 18th century and has since been refined using evolutionary data and DNA analysis. Organisms are grouped into a hierarchy of taxa from broadest to most specific: Domain, Kingdom, Phylum, Class, Order, Family, Genus, and Species. A helpful mnemonic is: Dear King Philip Came Over For Good Soup.

There are three domains of life: Bacteria (prokaryotes without membrane-bound nuclei), Archaea (prokaryotes that often live in extreme environments), and Eukarya (organisms with membrane-bound nuclei). Eukarya is divided into four kingdoms: Protista (mostly unicellular), Fungi, Plantae, and Animalia.

Binomial nomenclature is the two-part naming system using genus and species (e.g., Homo sapiens for humans). Names are always italicized and the genus is capitalized while the species is lowercase.

Phylogenetic trees (cladograms) show evolutionary relationships among organisms based on shared derived characteristics. Organisms that share more recent common ancestors are more closely related. Modern taxonomy increasingly relies on molecular data — comparing DNA and protein sequences — to determine true evolutionary relationships, sometimes revising groupings that were originally based on physical appearance alone.

Key terms

Taxonomy
The science of classifying and naming organisms based on shared characteristics and evolutionary relationships.
Binomial Nomenclature
The two-part scientific naming system using genus (capitalized) and species (lowercase), always italicized.
Domain
The broadest taxonomic rank; the three domains are Bacteria, Archaea, and Eukarya.
Phylogenetic Tree
A branching diagram showing the inferred evolutionary relationships among organisms based on shared characteristics.
Prokaryote
An organism whose cells lack a membrane-bound nucleus; includes Bacteria and Archaea.
Eukaryote
An organism with cells that contain a membrane-bound nucleus; includes protists, fungi, plants, and animals.

Flashcards

Domain Bacteria
The most diverse domain; prokaryotes with peptidoglycan cell walls; includes most known bacteria.
Domain Archaea
Prokaryotes that often inhabit extreme environments (hot springs, salty lakes); different membrane lipids from bacteria.
Domain Eukarya
Organisms with a true nucleus and membrane-bound organelles; includes Protista, Fungi, Plantae, and Animalia.
Species
The most specific taxonomic rank; members can interbreed to produce fertile offspring under natural conditions.
Cladogram
A branching diagram (type of phylogenetic tree) showing proposed evolutionary relationships based on shared derived characters.
Kingdom Animalia
Multicellular, heterotrophic eukaryotes without cell walls; includes invertebrates and vertebrates.
Kingdom Fungi
Eukaryotes with chitin cell walls that obtain nutrients by absorption (decomposers); includes mushrooms, molds, and yeast.
Kingdom Plantae
Multicellular, autotrophic eukaryotes with cell walls of cellulose; includes mosses, ferns, and flowering plants.

Questions

  1. In the taxonomic hierarchy, which level is more specific than Order but broader than Species?

    1. Kingdom
    2. Phylum
    3. Family
    4. Domain
    Show answer

    Answer: Family

    The hierarchy from broadest to most specific is: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species. Family falls between Order and Genus, making it more specific than Order but broader than Species.

  2. The scientific name Homo sapiens uses the two-part system known as ___ nomenclature.

    Show answer

    Answer: binomial

    Binomial nomenclature, developed by Carl Linnaeus, uses a two-part Latin name: the genus (capitalized) followed by the species (lowercase), e.g., Homo sapiens.

  3. Fungi are classified in the Kingdom Plantae because they are stationary and do not move.

    Show answer

    Answer: False

    Fungi belong to their own kingdom, Kingdom Fungi. Although they share some superficial features with plants (immobility, cell walls), fungi are heterotrophs with chitin walls and are more closely related to animals than plants.

  4. Which of the three domains contains organisms with cells that have a membrane-bound nucleus?

    1. Bacteria
    2. Archaea
    3. Eukarya
    4. Prokaryota
    Show answer

    Answer: Eukarya

    Domain Eukarya includes all organisms whose cells have a membrane-bound nucleus. Bacteria and Archaea are both prokaryotes — their genetic material floats freely in the cytoplasm without a nuclear envelope.

  5. Two organisms classified in the same genus are always members of the same species.

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    Answer: False

    Members of the same genus are closely related but can be different species. For example, lions (Panthera leo) and tigers (Panthera tigris) are both in genus Panthera but are different species that do not naturally interbreed.

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