Unit 8.1 (Taxonomy and Classifications)

Classifying Life

Life forms can be classified by key characteristics, and genetic similarity to determine how they should be grouped. Over time these classifications have been heavily modified, the classifications in the images below are likely out of date. Organisms can be classified using characteristics like how they reproduce, behavior, their diets, genetic homology, and how they derive nutrition. Organisms that make their nutrients using sunlight are called “Photoautotrophs“. “Heterotrophs” are organisms that gain their nutrition by taking it from another organism. Some organisms have developed the ability to gain nutrients from inorganic molecules found in their environment, they are known as “Chemoautotrophs“.

Levels of Classification

Life can be broken up into various categories and subcategories as laid out in the diagram above. Domains represent the broader categories of life and follow the following order moving from least specific to most specific, Domain > Kingdoms > Phylum > Class > Order > Family > Genus > Species. The classification for humans can be found in the diagram below.

For the purposes of high school biology we will focus exclusively on the taxonomical levels of Domain and Kingdoms.

Species

A species is generally the highest level of classification for groups of organisms, and is typically used to described organisms that are able to produce viable offspring when they mate.

3 Domains

Life can be separated into 3 main domains based upon their key characteristics.

Bacteria / Prokaryota (Eubacteria)

Bacteria are single celled prokaryotes described in more detail in our article here and down below. This domain only contains the bacterial kingdom.

Archaea

Archaea are single celled prokaryotes that are more closely related to eukaryotes than bacteria. This domain only contains the archaea kingdom.

Eukaryote

As described in an earlier Article eukaryotic organisms can be both single celled or multicellular organisms. Eukaryotes always possess a nucleus and membrane bound organelles. This domain consists of the protist, plant, animal and fungi kingdoms.

6 Kingdoms

The 3 domains can be further broken into 6 kingdoms.

Bacteria

These are single celled prokaryotic cells, so they lack a nucleus and membrane bound organelles. They have cell walls to help regulate osmosis, small circular chromosomes. These organisms are ubiquitous (can be found virtually everywhere). These organisms can be photoautotrophs, heterotrophs, and chemoautotrophs.

Archaea

Archaea are single celled prokaryotes so they lack a nucleus and other membrane bound organelles. They are mostly chemoautotrophs but some of them are photoautotrophs and heterotrophs. Some of them are “extremophiles” (organisms that can survive extreme conditions).

Protist

Protist are single celled eukaryotic cells, members of this group possess characteristics of plants, animals, and fungi. Protists are an example of a “transition” group, they show the progression from simple single celled prokaryotes to that of plants, animals, and fungi. There are protists that are photoautotrophs, and heterotrophs. Euglena are an example of a protists that perform photosynthesis and some of them consume other cells for sustenance.

Amoebas are a type of protist that are amorphous and hunt prey like animals. They can extend their cellular membranes to trap and consume prey. These organisms can often be found in bodies of fresh water like lakes, and ponds.

Plant

Plant cells always possess a cell wall and conduct photosynthesis to make food making them exclusively photo autotrophs. They are always multicellular and their cells have chloroplasts. For more information on plants check out our previous articles on them here.

Fungi

Fungi cells have a cell wall made of a carbohydrate called “chitin”. Fungi are heterotrophs and as such are required to take their food from the bodies of other organisms. Fungal cells must digest their foods outside of their cells by releasing digestive enzymes to break down their food source. They then absorb the nutrients into their cells using structures called “Hyphae“. Fungi can be single celled like the yeast used in alcohol fermentation or multicellular like mushrooms. Fungi reproduce by using “spores“. Spores spread through the air similar to pollen and will remain dormant until the conditions are right to grow and develop.

Animalia

Kingdom animalia has very unique cellular characteristics. These cells always lack cell walls and possess “centrioles“, an organelle used to produce spindle fibers and move their chromosomes around the cell during cell division. Animal cells are always heterotrophic and so must consume other organisms for energy.

Cladograms and Phylogenetic Trees

Once organisms have been properly identified and categorized by how genetically closely they are related they can be represented on diagrams display how related various species are. A cladogram is typically a linear branched diagram that shows individual species deviating or descending from a main ancestral branch.

A cladogram has a few basic parts;

Component	Function
Root	The ancestor that is common to all organisms on the cladogram
Node	Represent the separation of a group, creates a new species (speciation event)
Clade	Branch in a cladogram that contains all closely related organisms
In-group	Organisms that are more closely related than organisms contained in the outgroup
Out-group	Organisms that share a common ancestor but is not as closely related as other organisms in the cladogram

There are many different ways to represent a cladogram, here are just a few.