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Plant and Animal Classification

(A Beginner’s Guide)

Plants and animals are classified using the Linnaean taxonomy. It is a hierarchy system that organizes living things into groups based upon common features.

The system operates on a 8-step cascading hierarchy that branches off from the top category (’domain’), which separates all living things into three buckets, down to ‘species’, which indicates a specific animal.

The eight levels (or ‘taxa’) of the hierarchy are shown below:

  1. Domain
  2. Kingdom
  3. Phylum/Division
  4. Class
  5. Order
  6. Family
  7. Genus
  8. Species

This taxonomy was first proposed by Carl Lannaeus in his 1735 book Systema Naturea. Despite several modern adaptations, the Linnaean taxonomy remains the foundation for our current understanding of the biological relationships between living things.

Biological Taxonomic Classification Explained

1. Domain

Plural: Domains

The domain is the top taxa in the biological classification system and sits above the kingdom taxa.

There are three domains of life. They include Bacteria, Archaea, and Eukarya.

Bacteria and Archaea are buckets that hold two broad types of microorganisms (often single-celled) whose cells do not have membranes around the nucleus.

Eukarya is the bucket that holds all life with a complex cell that has a membrane, nucleus, and organelles. Animals and plants both fit into the Eukarya bucket.

Linnaeus didn’t actually come up with the ‘domain’ taxa. It was added to the Linnaean taxonomy by Carl Woese, Otto Kandler and Mark Wheelis in 1990.

2. Kingdom

Plural: Kingdoms

The kingdom is the second taxa in the biological classification system. It sits below domain and above phylum. There are six kingdoms.

Below the domain level, living organisms branch out into six kingdoms. These kingdoms reveal the basic different types of living things: animals, plants, fungi, protozoa (single-celled organisms), Chromista, and Bacteria.

Linnaeus only proposed two kingdoms (plants vs animals). This was replaced by Whittaker’s five kingdom model in 1968 and then Cavalier-Smith’s six kingdom model in 1998.

Here are the six kingdoms:

  1. The Animal Kingdom (Animalia): This kingdom contains all multi-cellular organisms that (with some exceptions) can move around, breathe, eat, and reproduce sexually. There are over 1.5 million named animal species in the world today. This includes everything from humans to ants to jellyfish.
  2. The Plant Kingdom (Plantae): The plant kingdom includes most multi-cellular organisms that generate energy through sunlight via a process called photosynthesis. They generally cannot move and more produce seeds as part of their reproduction. Examples include sunflowers, ferns, and pine trees.
  3. Fungi: Fungi used to be considered part of the plant kingdom but are now considered their own kingdom. They’re unique because they don’t get energy from the sun via photosynthesis, but actually digest other organic matter (they eat!). Examples include mushrooms, molds, and yeasts.
  4. Protista: Protista are a class of organisms that don’t fit well within any other category, so they sit into this ‘leftover’ category that was named by its founder the “primitive forms” of life. They’re single-celled organisms. An example is algae.
  5. Eubacteria – Eubacteria and archaebacteria (discussed next) are two separate types of bacteria. Eubacteria are single-celled organisms that live all around us. There are natural eubacteria in our food (especially yoghurt) and our intestines.
  6. Archaea – Archaea are the oldest living single-celled organisms and believed to be the original lifeforms. They were originally only believed to be found in extreme environments like hot springs, but scientists have now discovered them all over the place. We call the archaea who live in extreme environments “extremophiles” and study them to try to find out how life began.

3. Phylum / Division

Plural: Phyla / Divisions

Phylum and division are the third taxa in the biological classification system. They sits below kingdom and above class. We use ‘phylum’ for animals and ‘divison’ for plants. There are 7 animal phyla and 12 plant divisions.

3.1 Animals Example

There are 7 known animal phyla. Some recognizable ones are:

  1. Porifera – sponge-like animals found in the ocean (like spongebob squarepants!)
  2. Cnidaria – Marine animals like jellyfish, sea anemone, and coral. The world register of marine species[1] lists over 11,000 varieties.
  3. Mollusca – Soft invertebrates such as snails, octopi, oysters, and squid. There are over 85,000 known species[2] of mollusca.
  4. Arthropoda – These are invertebrate animals that hold themselves up with exoskeletons rather than bones. Examples include spiders, crabs, lobsters, centipedes, and scorpions.
  5. Chordata – These are vertebrates, meaning animals with skeletons. Most large animals fit into this category, including humans, dogs, alligators, and bears.

3.2 Plants Example

There are 12 known plant divisions. Some recognizable ones are:

  1. Bryophyta – includes mosses and liverworts
  2. Filicinophyta – includes ferns such as the lady fern, silver fern, and horsetail
  3. Coniferophyta – includes cone bearing trees such as the spruce, larch, pine, and fir trees
  4. Angiospermophyta – includes flowers such as the rose, sunflower, and lupin

Note: Botanists are increasingly tuning to ‘clades’ rather than domains to categorize plants. Domains focus on common features, while clades focus on common ancestry. Unfortunately, there is no clear taxonomy of clades at this time.

4. Class

Plural: Classes

Class is the fourth taxa in the biological classification system. It sits below phylum and above order.

The ‘class’ taxa is where things get interesting as we can start to see distinct clusters of recognizable lifeforms at this level. We’ll focus specifically on chordata (vertebrate animals) and Angiospermophyta (flowering plants) here.

4.1 Animals Example

The phylum Chordata (commonly known as vertebrates) splits off into seven classes. Some of the most interesting classes within this phylum are:

  1. Amphibians – Cold-blooded vertebrates that live both in and out of water. Examples include frogs, toads, salamanders, newts.
  2. Reptiles – Cold-blooded vertebrates that (usually) live on land, including lizards, snakes, turtles, and crocodiles.
  3. Birds (aves) – Warm-blooded, feathered animals that lay hard eggs and (usually) are able to fly.
  4. Mammals – Warm-blooded animals that usually have fur or hair and produce milk to feed their young. This includes humans as well as cows and dogs.

4.2 Plants Example

The phylum Angiospermophyta (which are flowering plants) splits off into two classes: monocots and dicots.

  1. Monocots – Monocots start out with a singular-part seed, have a branching root system, and parallel veins on their leaves. But the easiest way to identify them is by looking at the petals on their flower. If the flower has petals in multiples of three (3 or 6 petals), they’re a monocot! Examples include irises and trilliums.
  2. Dicots – Dicots start out with a two-chambered seed, have a tap root system, and leaves that branch out like a maple leaf. Identify them by looking at their flowers which do not have petals in multiples of 3. Examples include roses, sunflowers, and violets.

As with domains, classes are increasingly less common in categorizing plants. These are being replaced with clades (groupings of plants with common ancestors). You may therefore see domains and classes either skipped entirely, or replaced with a list of ‘clades’ when categorizing plants.

5. Order

Plural: Orders

Order is the fifth taxa in the biological classification system. It sits below class and above family.

We break down classes into orders. Let’s take a look again at one example from animals and one from plants.

5.1 Animals Example

The animals class known as mammals can be broken down into over 25 different orders. Here are some recognizable ones:

  • Carnivora – Animals that mostly eat meat. This includes bears, dogs, tigers, hyenas, raccoons, and walruses.
  • Rodentia– Rodents account for 40% of all mammals and are identifiable for the way they gnaw when they eat. Examples include squirrels, rats, mice, and beavers.
  • Chiropptera Better known as bats, these are the only mammals that can fly!
  • Primates – Primates have advanced hands and feet (usually with opposable thumbs) and large brains, making them the most advanced of all animals. Humans, monkeys, gorillas, and orangutans are all primates.

5.2 Plants Example

The plants class known as dicots (one of the two classes of flowering plants) can be further broken down into dozens of different orders. Here are some recognizable ones:

  • Rosales – You may recognize the root latin term for ‘rose’ in this name. This order includes roses, but also many other recognizable flowering plants, like strawberries, raspberries, pears, apricots, and elms.
  • Geraniales – You may think of ‘geraniums’ when you read this order, and indeed geraniums are a genus of the geraniales order. Bridal wreaths are another common plant in this order. Many Geraniales plants are used to make perfumes.
  • Sapindales – Genera and species of spindales include maple leaves, cashews, mangoes, and citrus fruits.
  • Asterales – Common asterales you might find in your garden include daisies, sunflowers, and marigolds.

6. Family

Plural: Families

Family is the sixth taxa in the biological classification system. It sits below order and above genus.

Below orders are families. Families are groups of animals that are usually observably related to the casual observer. Here are some examples from the animal and plant kingdoms.

6.1 Animals Example

These are examples of animal families within the carnivora order:

  • Canidae – The canine family fits includes dogs, wolves, foxes, and coyotes.
  • Felidae – The feline family includes cats, lynx, hyenas, tigers, lions, and jaguars.
  • Ursidae – The Ursidae are bears and under this family exist multiple species including the grizzly bear, black bear, polar bear, and giant panda.
  • Procyonidae – The Procyonidae family includes ringtails, raccoons, and olingos. There are many Procyonidae species in South America.

6.2 Plants Example

These are examples of plant families within the Rosales order:

  • Urticaceae – This family within the Rosales order houses all 2,625 known species of nettles.
  • Moraceae – The Moraceae family houses over 1,100 known species of plants including figs and mulberries.
  • Rosaceae – The Rosaceae family, commonly known as the rose family, does house all known roses, but also houses many common fruits including apricots, apples, pears, cherries, and peaches.

7. Genus

Plural: Genera

Genus is the seventh taxa in the biological classification system. It sits below family and above species.

At the genus level, we’re getting specific enough that a species’ latin binomial name will contain the genus as the first word.

For example, the wolf’s scientific binomial name is Canis lupis. Here, we can instantly tell that it belongs to the genus Canis, while its species name is lupis. Always write the genus with a capital letter and italicize the full scientific name.

7.1 Animals Example

The following are genera from the family Felidae:

  • Felis The Felis genus houses closely related species including the domestic cat (Felis catus), European wildcat (Felis silvestris), African wildcat (Felis lybica), and Sand cat (Felis margarita).
  • Leopardus The Leopardus genus houses closely related species including the ocelot (Leopardus pardalis), Andean mountain cat (Leopardus jacobita), and Southern tiger cat (Leopardus guttulus).
  • Lynx The lynx genus houses closely related species including the Eurasian lynx (Lynx lynx), bobcat (Lynx rufus), Canadian lynx (Lynx canadensis), and Iberian lynx (lynx pardinus).
  • Puma The puma genus houses just one species: the cougar (Puma concolor).
  • Panthera Includes tigers (Panthera tigris), lions (Panthera leo), and jaguars (Panthera onca).

7.2 Plants Example

The following are genera from the family Rosaceae:

  • Malus Includes the trees of all species of apple, including the common orchard apple (Malus domesticus) and sweet crabapple (Malus coronaria)
  • Rosa Includes all species of roses, including the common rose (Rosa rosa) and a white Chinese species (Rosa laevigatae)
  • Fragaria This genus contains the plants of all 20 species of strawberries but no other berries.
  • Prunus This genus includes numerous edible fruits including plum trees (such as Prunus americana), cherry tree (such as Prunus avium), peach trees (such as Prunus persica), and apricot trees (such as Prunus armeniaca).
  • Rubus Includes many delicious berry plants including the raspberry plant (e.g. Rubus idaeus), blackberry plant (e.g. Rubus laciniatus), and dewberry plant (e.g. Rubus aboriginum).

8.  Species

Plural: Species

Species is the last taxa in the biological classification system. It sits below genus.

The species is the exact animal or plant. This is the lowest level on the biological taxonomy.

We can generally identify a species because all animals within the species share a fundamentally similar DNA sequence and are able to breed with one another.

However, there are some instances of interbreeding at the higher taxa of genus. Different species within the Canis genus, for example, can interbreed, which leads to hybrid animals such as Coywolves (a cross between a coyote and a wolf).

8.1 Animals Example

The following bears belong to the genus Ursus, which is a step under the family Ursidae (Bear).

  • Polar Bear – The polar bear’s species is U. maritimus and its binomial name is Ursus maritimus.
  • Black Bear – The American black bear’s species is U. maritimus and its binomial name is Ursus maritimus.
  • Brown Bear – The brown bear’s species is U. arctos and its binomial name is Ursus arctos because it belongs to the Ursus genus. The North American grizzly bear (Ursus arctos horribilis) belongs to this species as they share genes and can interbreed.

Note that the giant panda (Ailuropoda melanoleuca), sun bear (Helarctos malayanus) and spectacled bear (Tremarctos ornatus) are not in the above list because they belong to the same bear family (Ursidae) but different genera (Ailuropoda, Helarctos and Tremarctos respectively).

8.2 Plants Example

The following plants belong to the genus Prunus:

  • Plum Trees – There are several species of plum tree. The common American plum tree is P. americana and its binomial name is Prunus americana.
  • Cherry Trees – There are several species of cherry trees, such as the common P. avium, with the binomial name Prunus avium.
  • Peach Trees – There are also several species of peach trees, such as the species P. persica, with the binomial name Prunus persica.
  • Apricot Trees – The common American apricot tree is P. armeniaca, with the binomial name Prunus armeniaca.

Animal Classification Examples

1. American Black Bears

KingdomAnimalia
PhylumChordata
ClassMammalia
OrderCarnivora
FamilyUrsidae
GenusUrsus
SpeciesUrsus americanus

2. Humans

KingdomAnimalia
PhylumChordata
ClassMammalia
OrderPrimates
FamilyHominidae
GenusHomo
SpeciesHomo sapiens

3. Grey Wolves

KingdomAnimalia
PhylumChordata
ClassMammalia
OrderCarnivora
FamilyCanidae
GenusCanis
SpeciesCanis lupus

4. Brown Recluse Spider

KingdomAnimalia
PhylumArthropoda
ClassArachnida
OrderAraneae
FamilySicariidae
GenusLoxosceles
SpeciesLoxosceles reclusa

Plant Classification Examples

1. Sunflowers

Kingdom Plantae
Division Angiospermae*
ClassTracheophyta*
OrderAsterales
FamilyAsteraceae
GenusHelianthus
SpeciesHelianthus annuus

2. Scotch Pine Trees

KingdomPlantae
DivisionPinophyta*
ClassPinopsida*
OrderPinales
FamilyPinaceae
GenusPinus
SpeciesPinus sylvestris

3. Domestic Apple

KingdomPlantae
Division Angiospermae*
Class Dicotyledonae*
Order Rosales
Family Rosaceae
GenusMalus
SpeciesMalus domestica

4. Western Rose

KingdomPlantae
DivisionAngiospermae*
ClassDicotyledonae*
OrderRosales
FamilyRosaceae
GenusRosa
SpeciesRosa Stellata

*As our understanding of plants evolves, classifications at the level of domain and class are becoming less valuable for describing plant groupings. It may be more useful to examine clades (groups of plants with common ancestors which do not fit neatly onto the Linnaean taxonomy) at the higher taxa for plants.

Conclusion

The Linnaean taxonomy enables scientists to categorize animals based on genetic similarities and relationships. This allows us to gain a better understanding of evolutionary theory, the movement and spread of animals over time, and relationships between current animals and extinct species. Knowledge of this taxonomy also helps laypeople develop a deeper understanding of animals and their relationships to one another when their binomial names are used in news and research reports.

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