Scientific Classification Systems

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





As scientific knowledge expanded, so did the classification systems. Here is a breakdown to explain the differences.

Carl Linnaeus and Scientific Classification

1753 - 2 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 works 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 (albeit with a few modifications.)

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 now 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)

6 Kingdoms

1977 - Carl Woose

While the single-celled monera were considered simple cells, classifying them turned out to be incredibly complex, leading Carl Woose to divide 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)

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.

Moreover, there are even more kingdoms added under eukaryo. They have become quite numerous: what with ciliates and microspores and other little critters. It's enough to keep a microbiology department of the local college quite busy.

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

Controversy and Diversity in the Classification Schemes

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.)

1969 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 1977 Proc Natl Acad Sci U S A. 2012 Jan 20;109(4):1011–1018. doi: 10.1073/pnas.1109716109 Phylogeny and beyond: Scientific, historical, and conceptual significance of the first tree of life Norman R Pace a,1, Jan Sapp b, Nigel Goldenfeld c Author information Article notes Copyright and License information PMCID: PMC3268332 PMID: 22308526