The Five Kingdoms
There are 5 kingdoms in the classification system. Organisms are classified according to their evolutionary relationships (their phylogeny).
Phylogeny is the study of the evolutionary history of organisms, and gives us an insight as to how to group them and their extinct relatives. The base hierarchy in the classification system is the Kindom.
Generally, we can order the Kingdoms by increasing complexity. To help remember the names of the kingdoms, I was taught:
Pretty Polly Finds Parrots Attractive – Prokaryote, Protoctista, Fungi, Plantae, Animalia.
||Unicellular; no membrane bound organelles
||Eukaryotes, Unicellular & Multicellular
||Eukaryotes, Unicellular & Multicellular (Yeast)
||Eukaryotic, Multicellular; Large Vacuoles
||Autotrophic, Aerobic Heterotrophic
||Heterotrophic, Digestive System
||Seeds/Spores, Some asexual while some sexual
||Develop from embryo
Q. What’s a photosynthetic organism?
A. An organism that gets its energy by absorbing light.
Q. What’s a autotrophic organism?
A. An organism which gets it’s energy from light (photosynthesis) or from chemical interaction (chemosynthesis).
Q. What’s a heterotrophic organism?
A. An organism that relies on complex organic matter for food.
Remember that 4 of the 5 kingdoms feature Eukaryotes! Only Prokaryotae contains Prokaryotes (no surprise there!).
Taxonomy (Breaking it down)
We break down organisms into a total of 7 hierarchical classes (including Kingdom above). That’s a lot of possible choices for organisms, and is know as Taxonomy, or Alpha Taxonomy.
The 7 levels are Kingdom, Phylum, Class, Order, Family, Genus and Species. You could remember this as:
King Penguins Climb Over Frozen Grassy Slopes
Here’s an example of two organisms and their taxonomy:
||Large White Butterfly
As you can see, humans are sapiens of the Genus Homo. AKA Homo sapiens (I bet you’ve heard that before!).
The only similarity between these two examples is that they are both in the Animalia kingdom. This means they share a great number of common traits, and so actually tells us a lot about the organisms.
It is also worth bearing in mind that Protoctista is often the ‘Other’ category where organisms who have no clear Kingdom are put. For example, Slime Moulds have fungi characteristics, yet are not quite suitable for classification in the Fungi Kingdom.
Species is the final tier on the taxonomy hierarchy; and is a group of organisms with similar traits. These include:
- Morphology (The outside appearance of an organism, including shape, colour, structure and pattern)
- Physiology (The way in which an organisms works, by looking at it’s biochemical, mechanical and physics functions)
BUT most importantly, we can class two organisms as the same species if they can naturally breed together and produce fertile offspring.
The fertility point is an important one, as there are several organisms that can breed together, but produce a sterile offspring which cannot breed any further – such as a horse and a zebra which can produce a hybrid. This hybrid is sterile, so we know what the horse and the zebra are different species.