Organizing Living Things

Biologists classify living organisms using a taxonomic hierarchy.

A hierarchy is a system in which groups are arranged from broad categories to increasingly specific categories.

The main taxonomic levels are:

Domain → Kingdom → Phylum → Class → Order → Family → Genus → Species

Each level is called a taxonomic rank.

As we move down the hierarchy, groups generally contain fewer organisms, but those organisms share increasingly specific characteristics and closer evolutionary relationships.

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The Taxonomic Levels

The eight major levels commonly taught are:

• Domain – the broadest level.
• Kingdom – divides domains into major groups.
• Phylum – groups organisms with important similarities in body organization.
• Class – divides a phylum into more specific groups.
• Order – divides a class into related groups.
• Family – contains closely related genera.
• Genus – contains very closely related species.
• Species – the most specific major level.

A useful way to remember the order is:

Dear King Philip Came Over For Good Soup

The first letter of each word represents:

D K P C O F G S

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Domain

Domain is the broadest major taxonomic rank.

The three domains of cellular life are:

• Bacteria
• Archaea
• Eukarya

Domains are distinguished using fundamental cellular and molecular characteristics.

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Domain Bacteria

Bacteria are prokaryotic organisms.

They generally:

• Are unicellular.
• Lack a membrane-bound nucleus.
• Lack membrane-bound organelles.
• Reproduce mainly through binary fission.
• Have characteristic bacterial cell structures.

Bacteria are extremely diverse and occur in almost every environment on Earth.

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Domain Archaea

Archaea are also prokaryotic organisms.

Like bacteria, they:

• Are generally unicellular.
• Lack a membrane-bound nucleus.

However, molecular and biochemical evidence shows that Archaea and Bacteria represent distinct evolutionary lineages.

Differences occur in their:

• DNA and RNA-related machinery.
• Cell membranes.
• Cell-wall chemistry.
• Metabolism.

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Domain Eukarya

Domain Eukarya contains organisms with eukaryotic cells.

Eukaryotic cells contain:

• A membrane-bound nucleus.
• Membrane-bound organelles.
• More complex internal organization than prokaryotic cells.

Eukarya includes:

• Animals.
• Plants.
• Fungi.
• Diverse protists and other eukaryotic groups.

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Kingdom

A kingdom is a major classification group below domain.

For example, within Domain Eukarya are familiar kingdoms such as:

• Animalia.
• Plantae.
• Fungi.

Different classification systems may treat some other eukaryotic groups differently, so the exact number of kingdoms can vary.

Kingdom is therefore a broad classification level, but it is more specific than domain.

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Phylum

A phylum contains organisms sharing important aspects of body organization and evolutionary history.

For example:

Phylum Chordata

includes animals such as:

• Fish.
• Amphibians.
• Reptiles.
• Birds.
• Mammals.

Chordates share important characteristics during at least some stage of development, including a notochord.

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Class

A class is a taxonomic level below phylum.

For example:

Phylum Chordata

contains:

Class Mammalia

Mammals share important characteristics, including:

• Hair or fur during at least part of their lives.
• Mammary glands.
• Three middle-ear bones.
• A shared evolutionary ancestry.

Humans, dogs, whales, bats, and elephants all belong to Mammalia.

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Order

A class is divided into orders.

For example, Class Mammalia contains many orders.

These include groups containing:

• Primates.
• Carnivorans.
• Rodents.
• Bats.
• Whales and dolphins.

Humans belong to:

Order Primates

Primates include humans, apes, monkeys, lemurs, and related species.

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Family

An order contains one or more families.

Families contain groups of closely related genera.

Humans belong to:

Family Hominidae

Hominidae includes the great apes:

• Humans.
• Chimpanzees.
• Bonobos.
• Gorillas.
• Orangutans.

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Genus

A genus contains one or more very closely related species.

Humans belong to:

Genus Homo

The genus name forms the first part of an organism's scientific name.

For example:

Homo sapiens

Here:

Homo = genus

sapiens = specific epithet

Together, the two words form the species' scientific name.

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Species

Species is the most specific of the major taxonomic ranks.

For many sexually reproducing organisms, a species can often be described as a group of organisms capable of reproducing with one another and producing fertile offspring under natural conditions.

However, this definition does not work perfectly for every organism.

Scientists may also use:

• DNA.
• Anatomy.
• Ecology.
• Behavior.
• Evolutionary history.

when determining species boundaries.

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From Broad to Specific

The hierarchy can be visualized as a series of increasingly specific groups:

Domain

↓

Kingdom

↓

Phylum

↓

Class

↓

Order

↓

Family

↓

Genus

↓

Species

At the top, groups contain enormous numbers of organisms.

At the bottom, groups contain organisms that share much more specific characteristics.

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A Nested System

Taxonomic groups are nested inside one another.

This means that every organism belonging to a species also belongs to:

• A genus.
• A family.
• An order.
• A class.
• A phylum.
• A kingdom.
• A domain.

For example:

Homo sapiens

is inside:

Homo

which is inside:

Hominidae

which is inside:

Primates

and so on.

This is similar to organizing files inside folders and subfolders.

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Worked Example: Human Classification

The complete major classification of humans is:

Domain: Eukarya

Kingdom: Animalia

Phylum: Chordata

Class: Mammalia

Order: Primates

Family: Hominidae

Genus: Homo

Species: Homo sapiens

Notice how each level becomes increasingly specific.

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Worked Example: Domestic Cat

A domestic cat can be classified as:

Domain: Eukarya

Kingdom: Animalia

Phylum: Chordata

Class: Mammalia

Order: Carnivora

Family: Felidae

Genus: Felis

Species: Felis catus

Humans and cats share several higher classification levels.

Both belong to:

• Eukarya.
• Animalia.
• Chordata.
• Mammalia.

They separate at the level of order.

Humans belong to Primates.

Cats belong to Carnivora.

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Comparing Humans and Cats

The classification shows that humans and cats share some important biological characteristics.

Both are:

• Eukaryotes.
• Animals.
• Chordates.
• Mammals.

However, they belong to different:

• Orders.
• Families.
• Genera.
• Species.

Taxonomy therefore allows us to compare organisms systematically.

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Worked Example: Domestic Dog

A domestic dog can be classified as:

Domain: Eukarya

Kingdom: Animalia

Phylum: Chordata

Class: Mammalia

Order: Carnivora

Family: Canidae

Genus: Canis

Species: Canis lupus

The domestic dog is commonly treated as a subspecies of the gray wolf:

Canis lupus familiaris

Cats and dogs therefore share the same classification through Order Carnivora, but belong to different families.

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Comparing Dogs and Cats

Dog:

Carnivora → Canidae → Canis

Cat:

Carnivora → Felidae → Felis

Because dogs and cats share the same order but belong to different families, they are more closely grouped taxonomically than either is with humans at those lower ranks.

Taxonomic comparisons can therefore provide information about evolutionary relationships.

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Worked Example: Lion and Domestic Cat

Consider a lion and a domestic cat.

Lion:

Family: Felidae

Genus: Panthera

Species: Panthera leo

Domestic cat:

Family: Felidae

Genus: Felis

Species: Felis catus

Both belong to Family Felidae.

They therefore share a lower and more specific taxonomic rank than a cat and a dog do.

This reflects their closer evolutionary relationship.

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Comparing Organisms Using Taxonomy

A useful general rule is:

The more specific taxonomic levels two organisms share, the closer their evolutionary relationship is generally expected to be.

For example:

Organisms sharing only a domain may be very distantly related.

Organisms sharing a kingdom have more in common.

Organisms sharing a family are much more closely related.

Organisms sharing a genus are usually very closely related.

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Worked Comparison

Suppose three organisms have these classifications:

Species A

Animalia → Chordata → Mammalia → Carnivora → Felidae → Panthera

Species B

Animalia → Chordata → Mammalia → Carnivora → Felidae → Felis

Species C

Animalia → Chordata → Mammalia → Primates → Hominidae → Homo

Species A and B share:

• Kingdom.
• Phylum.
• Class.
• Order.
• Family.

Species A and C share:

• Kingdom.
• Phylum.
• Class.

Therefore, A and B are classified as more closely related than A and C.

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Scientific Names and Taxonomy

Scientific names contain two parts:

Genus + specific epithet

For example:

Panthera leo

The genus is:

Panthera

The specific epithet is:

leo

Together they form the scientific name of the species.

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Rules for Writing Scientific Names

Scientific names follow standard conventions.

For example:

Homo sapiens

The rules are:

• The genus begins with a capital letter.
• The specific epithet begins with a lowercase letter.
• Both words are normally written in italics when typed.
• When handwritten, each word is traditionally underlined separately.

Correct:

Homo sapiens

Incorrect:

Homo Sapiens

Incorrect:

homo sapiens

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Organisms in the Same Genus

Species within the same genus are generally very closely related.

For example:

Panthera leo – lion

Panthera tigris – tiger

Panthera pardus – leopard

Panthera onca – jaguar

All four species belong to the genus Panthera.

Their shared genus reflects their close evolutionary relationship.

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Organisms in the Same Family

A family can contain several genera.

For example:

Family Felidae

contains genera including:

• Panthera.
• Felis.
• Lynx.
• Puma.

Therefore, lions and domestic cats belong to the same family but different genera.

The family level represents a broader relationship than the genus level.

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Broad Groups Contain Greater Diversity

A domain contains organisms that may differ enormously.

For example, Domain Eukarya contains:

• Animals.
• Plants.
• Fungi.
• Many other eukaryotes.

These organisms share important cellular characteristics but differ greatly in many other ways.

By contrast, organisms within the same genus usually share many characteristics.

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Taxonomic Hierarchy and Evolution

Modern taxonomy attempts to organize organisms according to their evolutionary relationships.

If two organisms share a relatively recent common ancestor, they should generally be grouped together at more specific taxonomic levels.

For example:

Lion and tiger

share the genus Panthera.

Lion and domestic cat

share the family Felidae.

Lion and human

share the class Mammalia but separate at the order level.

The hierarchy therefore reflects different degrees of evolutionary relationship.

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Classification Is Based on Evidence

Scientists determine relationships using evidence including:

• DNA sequences.
• Protein sequences.
• Anatomy.
• Embryology.
• Fossils.
• Reproductive characteristics.

Modern classifications increasingly rely on phylogenetic evidence showing patterns of common ancestry.

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DNA Can Change Classification

Historically, organisms were classified mainly using visible characteristics.

Modern DNA analysis sometimes reveals that organisms previously thought to be closely related are actually more distantly related.

The opposite can also occur.

Two organisms that look quite different may have strong molecular evidence indicating close evolutionary relationships.

As evidence improves, scientists may revise classifications.

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Taxonomic Ranks and Phylogenetic Trees

Taxonomic hierarchies and phylogenetic trees are related, but they are not exactly the same thing.

A taxonomic hierarchy places organisms into named categories such as:

Class → Order → Family → Genus

A phylogenetic tree represents hypotheses about branching evolutionary relationships.

Modern taxonomy attempts to make these classification groups consistent with evolutionary history.

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Using Taxonomy to Organize Information

Taxonomic systems allow scientists to organize enormous amounts of biological information.

Suppose a scientist studies a newly discovered species.

Instead of treating it as completely unrelated to existing knowledge, the scientist can identify its classification.

If it belongs to Family Felidae, scientists already know that it shares important characteristics and evolutionary history with other members of that family.

Classification therefore helps organize and connect biological knowledge.

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Classification and Databases

Modern biological databases contain information about millions of organisms.

Taxonomic classification allows this information to be organized according to:

• Species.
• Genus.
• Family.
• Higher taxonomic groups.

Scientists can search these databases to investigate:

• DNA sequences.
• Geographic distributions.
• Evolutionary relationships.
• Ecological characteristics.
• Conservation status.

Taxonomy therefore remains essential even in modern molecular biology.

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Subdivisions of the Main Ranks

The eight major ranks are useful for learning classification, but real taxonomy can include additional levels.

Examples include:

• Subphylum.
• Subclass.
• Suborder.
• Subfamily.
• Tribe.
• Subspecies.

For example, domestic dogs are commonly classified as a subspecies:

Canis lupus familiaris

These additional ranks allow scientists to describe relationships more precisely.

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Species and Subspecies

A subspecies is a population or group within a species that has recognizable differences from other populations but remains part of the same species.

Subspecies are sometimes used when populations have:

• Geographic separation.
• Consistent physical differences.
• Genetic differences.

Subspecies classification can change as scientists obtain additional evidence.

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Worked Example: Which Organisms Are Most Closely Related?

Suppose:

Organism A and Organism B share the same genus.

Organism A and Organism C share the same family, but not the same genus.

Organism A and Organism D share only the same class.

Which organism is most closely related to A?

Organism B

Why?

A and B share the most specific taxonomic level: genus.

The general pattern is:

More specific shared classification → generally closer evolutionary relationship

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Worked Example: Finding Where Organisms Separate

Consider:

Lion:

Animalia → Chordata → Mammalia → Carnivora → Felidae → Panthera

Tiger:

Animalia → Chordata → Mammalia → Carnivora → Felidae → Panthera

Domestic cat:

Animalia → Chordata → Mammalia → Carnivora → Felidae → Felis

Human:

Animalia → Chordata → Mammalia → Primates → Hominidae → Homo

Lion and tiger remain together through the genus level.

Lion and domestic cat separate at genus.

Lion and human separate at order.

Therefore, the hierarchy indicates increasingly distant evolutionary relationships.

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Classification as Nested Groups

Imagine a set of containers.

A species fits inside a genus.

The genus fits inside a family.

The family fits inside an order.

The order fits inside a class.

The class fits inside a phylum.

The phylum fits inside a kingdom.

The kingdom fits inside a domain.

This nested organization is one of the central features of biological classification.

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Common Mistakes

Putting the Taxonomic Levels in the Wrong Order

Remember:

Domain → Kingdom → Phylum → Class → Order → Family → Genus → Species

Thinking Kingdom Is the Broadest Level

In the commonly taught modern hierarchy, domain is broader than kingdom.

Thinking Genus and Species Are the Same Thing

A genus can contain multiple species.

Writing the Species Name With Two Capital Letters

Correct:

Homo sapiens

Not:

Homo Sapiens

Thinking Organisms in the Same Family Must Be in the Same Genus

A family contains multiple genera.

Thinking Organisms in the Same Class Must Be Very Closely Related

A class can contain many different orders, families, genera, and species.

Assuming Classification Is Based Only on Appearance

Modern taxonomy also uses DNA, proteins, anatomy, development, fossils, and other evidence.

Thinking Taxonomic Groups Never Change

Classification can be revised when new evolutionary evidence becomes available.

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Check Your Understanding

1. What is a taxonomic hierarchy?

2. List the eight major taxonomic ranks from broadest to most specific.

3. What is the broadest major taxonomic rank?

4. What is the most specific major taxonomic rank?

5. What are the three domains?

6. Which domain contains animals, plants, and fungi?

7. What happens to the number of organisms in a group as you move toward species?

8. What generally happens to the similarity of organisms as you move toward species?

9. What is a genus?

10. What is a species?

11. What two parts form a scientific species name?

12. Write Homo sapiens correctly.

13. Which indicates a closer relationship: sharing a family or sharing a genus?

14. Lions and domestic cats both belong to Felidae but have different genera. What taxonomic level do they share?

15. Lions and tigers both belong to Panthera. What does this suggest about their evolutionary relationship?

16. Why are taxonomic groups described as nested?

17. How can DNA evidence affect classification?

18. Why might scientists revise the classification of an organism?

19. Explain how taxonomy can be used to compare two organisms.

20. Explain how the taxonomic hierarchy reflects evolutionary relationships.

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Key Terms

• Taxonomy – science of naming and classifying organisms.
• Taxonomic hierarchy – organization of organisms into nested classification levels.
• Taxonomic rank – a level within the classification hierarchy.
• Domain – broadest major taxonomic rank.
• Kingdom – major taxonomic rank below domain.
• Phylum – rank below kingdom.
• Class – rank below phylum.
• Order – rank below class.
• Family – rank below order containing related genera.
• Genus – group of closely related species.
• Species – most specific major taxonomic rank.
• Binomial nomenclature – system of naming species using a genus and specific epithet.
• Subspecies – recognizable population within a species.
• Phylogeny – evolutionary history and relationships among organisms.
• Common ancestor – ancestral population shared by evolutionary lineages.

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Key Takeaways

• Biological classification uses a hierarchical system.
• The eight major ranks are Domain, Kingdom, Phylum, Class, Order, Family, Genus, and Species.
• Domain is the broadest major level.
• Species is the most specific major level.
• Taxonomic groups are nested inside larger groups.
• As classification becomes more specific, organisms generally share more characteristics.
• Organisms sharing more specific taxonomic levels generally have closer evolutionary relationships.
• The three domains are Bacteria, Archaea, and Eukarya.
• Scientific species names use genus + specific epithet.
• The genus begins with a capital letter and the specific epithet begins with a lowercase letter.
• A genus can contain several closely related species.
• A family can contain several related genera.
• Taxonomy allows scientists to compare organisms systematically.
• Modern taxonomy uses DNA and other evidence to reconstruct evolutionary relationships.
• Classification can change when new scientific evidence becomes available.
• The taxonomic hierarchy provides an organized way to understand both the diversity and evolutionary relationships of living things.