Scientific Classification Systems

Two kingdoms? Three, five, six? Domains vs super groups? What happened to Carl Linnaeus easy-to-understand scientific classification scheme?




Details more visible below.

As scientific knowledge expanded, so did the scientific classification systems that listed the known organisms.

Here is a breakdown to explain the differences that occurred over time.

Carl Linnaeus and Scientific Classification

1753 - Two Kingdoms

Our taxonomy system dates to the work of the Swedish physician and scientist, Carl Linnaeus (1707-1778). He spent decades cataloging plants and animals. His first work, Systema Natura (1735), was only twelve pages but laid the foundation of his later work.

His 1753 Species Plantarum was two-volumes. In the decades between those two works, Linnaeus had produced other publications with the goal of adding all known living-organisms to his taxonomy.

Original Taxonomy

Linnaeus' work divided living organisms into one of two kingdoms: plants or animals.

This was simple and non-controversial. Plants or animal.

Who could argue?

And, as all biology students can tell you, every organism was further cataloged into these taxonomy groups:

1. Kingdom
2. Phylum
3. Class
4. Order
5. Family
6. Genus
7. Species

This system worked well. We are still using it today - although with the modifications listed below.

Microscopes and Taxonomy

It is true that the microscope was invented prior to Linnaeus birth. The term "microscope" was first introduced in 1625, though previously hand-held lens that we might call a magnifying class were developed earlier.

As Galileo was looking up with the telescope, others were peering downward through the microscope.

Eventually, humanity beheld tiny, one-celled organisms under the microscope that Linnaeus had not listed. These were considered either very tiny plants or very tiny animals.

Protozoa and Taxonomy

Linnaeus taxonomy continued to expand as explorers and scientists found new species. Only a generation after Linnaeus, in 1818, protozoa were included as an animal phylum.

Protozoa, like all animals, could move. But future scientisits would discover that the protozoan's digestion of nutrients was more like a plant.

The debate was on: plant or animal - just how should a protozoan be classified?

4 Kingdoms

1866: Ernest Haeckel

In 1866 Ernest Haeckel published a proposal in General Morphology of Organisms that protozoa and monera (bacteria) should be classified as distinct kingdoms. Not only had the non-majestic, single-celled creatures graduated to their own kingdom, but they were divided by the presence of a nucleus and other organelles. Haeckel's kingdoms were:

1. Plant
2. Animal
3. Protozoa (single cell, with nucleus)
4. Monera ie bacteria (single cell, no nucleus)

While Haeckel's theories were accepted in the scientific community, the general population continued to learn the two system classification.

5 Kingdoms

1969 - Robert Whittaker

By the 1970's, elementary students were proudly telling their parents and grand-parents that the old taxonomy of plants vs animals was out-dated.

Not only had single-celled organisms been granted the right to their own kingdoms, but fungus was added as another kingdom. These unique organisms could be single-celled or multi-cellular and had diverse methods of reproduction.

The kingdoms five accepted kingdoms were:

1. Plant
2. Animal
3. Protozoa (single cell, with nucleus)
4. Monera ie bacteria (single cell, no nucleus)
5. Fungus (single or multi-cellular, cell wall, diverse reproduction)

(For more information on Whittaker's taxonomy, see "Kingdom Classification According to Whittaker" at https://bio.libretexts.org/Courses/Prince_Georges_Community_College/PGCC_Microbiology/04:_Microscopy_Staining_and_Classification/4.03:_Classification_and_Identification/4.3.01:_Kingdom_Classification_According_to_Whittaker)

6 Kingdoms

1977 - Carl Woose

While the single-celled monera were considered simple cells, classifying them turned out to be incredibly complex. Carl Woose divided monera into two distinct groups. Note the growing complexity:

1. Plant (multi-cellular eukaryo = complex cell with nucleus)
2. Animal (multi-cellular eukaryo = complex cell with nucleus)
3. Protozoa (single cell eukaryo)
4. Fungus (single or multi-cellular, eukaryo cell wall, diverse reproduction)
5. Eubacteria (single celled, pro-karyo=simple cell with no nucleus, a polysaccharide double membrane around entire cell)
6. Archaebacteria (single celled, pro-karyo=simple cell with no nucleus, a polysaccharide membrane around entire cell)

(For more information on Woose's taxonomy, see:
Pace, Sapp, Goldenfeld. Phylogeny and beyond: Scientific, historical, and conceptual significance of the first tree of life Proc Natl Acad Sci U S A. 2012 Jan 20;109(4):1011–1018. doi: 10.1073/pnas.1109716109. PMCID: PMC3268332 PMID: 22308526)

Wow, this is starting to get a little complicated. Gone are the days when a student could confidently place any species into one of the two kingdoms (plant or animal.) Now one has to wrack their brain around cellular organelles and what chemicals constitute their membranes.

But, at least we have this whole taxonomy thing nailed. Right?

Wrong.

3 Domains: The Modern System

1990 - Carl Woose

What's bigger than a kingdom? In the new classification system, the answer is: DOMAIN.

The level of "kingdom" lost it's place as king of the taxonomy mountain. "Domain" is now top dog. Here are the three largest taxonomy groups:

1. Eukaryo - complex cell with a membranes around the nucleus; single or multi-cellular
2. Eubacteria (or simply bacteria) - simple cell without organelles, double-layer membrane around entire cell made of polysaccharides (protein + sugar), single celled
3. Archaeabacteria
- simple cell, single-layer membranes around cell without polysaccharides, single cell, many living in extreme climates

Hey, What Happened to Plants and Animals?

Plants and animals maintained their status as separate kingdoms. But now all those kingdoms were under the "Eukaryo" domain.

Domain: Eukaryo

1. Animal
2. Plant
3. Fungi
4. Protozoa

The astute reader will note the similarity between this and the 5 Kingdom Scientific System. We just keep shuffling those bacteria around, trying to find where they best fit into the taxonomy.

Meanwhile, it became harder to explain taxonomy to the average third grader.

Controversy and Diversity in the Classification Schemes

Change #1 - Domain Added

Changes to the Scientific Classification System

The most obvious change is the addition of "Domain" at the top of the taxonomy hierachy.

For students who had easily cited, "Dear King Philip Cried, "Oh For Goodness Sake," a new neumonic was developed:

Dear
King
Philip
Cried
Oh
For
Goodness
Sake

(Dominating King Philip also works and sounds closer to "domain." But "dominating" isn't as common of a word.)

Change #2 - Supergroups

Changes to the Scientific Classification System

As biologists have investigated the DNA of living creatures, the scientific classification schema has evolved into a far more complex system than the 8 taxonomy levels. Super-groups and sub-groups have joined nomenclature.

What Is A Supergroup?

A supergroup is a set of organisms within a group that have share a common ancestor that other organisms in that group do not share. If that's confusing, let's give a simple example before moving on. Let's say we had a domain in a store called "merchandise for sale" and within that domain there was a kingdom "furniture." Within that kingdom, we had the following phylum:

• Tables
• Dining Room Chairs
• Desks
• Sofas
• Office Chairs
• Recliners
• Cabinets

You notice that dining room chairs, office chairs, sofas, and recliners all have something in common: they are furniture that people sit on. But our classification system has a problem:

• Domain: Merchandise
• Kingdom: Furniture
• Phylum: Dining Room Chairs

The category over phylum is kingdom, and it has already been established as the kingdom of furniture. We need a super-phylum:

• Domain: Merchandisev
• Kingdom: Furniture
• Super Phylum: Furniture to sit on
• Phylum: Dining Room Chairs

Now granted, our example is far less complicated than the current taxonomy. But organisms with similar DNA have been grouped together within a larger group.

Plants and Super-group Archaeplastica

• Domain: Eukaryo
• Super Kingdom: Archaeplastica
• Kingdom: Plants
• Kingdom: Algae

So what other organisms share the super-kingdom with plants? Algae.

Now let's check out the animal category.

Animals and Super-group Opimoda

• Domain: Eukaryo
• Super Kingdom: Opimoda
• Kingdom: Animal
• Kingdom: Fungi

As you can see from the picture, animals and fungi have similar enough cellular structures that they have been joined in a supergroup between the domain of eukaryo and the kingdom of animal. (Really not very ego-satisfying is it?)

Change #3 - Sub-Groups

Changes to the Scientific Classification System

You will be happy to know that sub-groups are easier to understand than super-groups. But they follow the same principle. Here's a famous example from our scientific classification of dogs MatchCard.

• Domain: Eukaryo
• Kingdom: Animal
• Phylum: Chordota (has spinal cord)
• Class: Mammalia (live young)
• Order: Carnivora (teeth of meat-eaters, includes dog and cat like creatures)
• Family: Canidae (dog like: wolves, foxes, coyotes, jackals)
• Genus: Canis (78 chromosomes - includes wolves and coyotes)
• Species: Lupis (wolves and dogs)

Now Canis lupis is a category that was established by Carl Linnaeus himself. Humans had domesticated dogs, but they are genetically related to the wolves.

Now if you have seen any chihuahuas lately, you might observe that there is a slightly noticable difference between a wolf and a chiuaua.

• Domain: Eukaryo
• Kingdom: Animal
• Phylum: Chordota (has spinal cord)
• Class: Mammalia (live young)
• Order: Carnivora (teeth of meat-eaters, includes dog and cat like creatures)
• Family: Canidae (dog like: wolves, foxes, coyotes, jackals)
• Genus: Canis (78 chromosomes - includes wolves and coyotes)
• Species: Lupis (wolves and dogs)
• Sub-species: Canis Lupis Familiaris (Domesticated Dog)
• Sometimes simply called Canis Familiaris - they are the same thing!
• Sub-sub-species: Chihuahua
• Sub-sub-sub-species: Hairless Chihuahua

What we call a "dog breed" is actually a sub-sub-species of Canis lupus familiaris. Most of those sub-species have sub-sub species.

Today, you will find most animals and plants will be categorized within super-groups and sub-groups of the original taxonomy.

Change #4 - Regrouping Due to DNA Analysis

Changes to the Scientific Classification System

We have seen that scientists have added a new layer (domain) and added super-groups and sub-groups to accurately categorize living things - particularly those more recently discovered.

Sometimes, they have to do something else - reassign a long-known species.

Here's a common example: Hawks and Eagles

Since Linnaeus time, hawks, eagles, kites and accipiters (ie. sparrowhawks) had been grouped with the falcons in the falconiforme order. These were the diurnal birds of prey.

DNA analysis has shown that the falcons have a distinctly different genetic heritage and a new order - accipitriformes - was developed.

Learn more about bald eagle taxonomy and unit study.

Kids Taxonomy Worksheet