You write a postal address from the country down: district, then neighbourhood, then house. The further down you go, the more exact the address and the fewer people share it. Every organism has an address like that too, seven rungs long. By the end of this note you can write the address of a human being, with the reason for every rung.
1. The lower the rank, the more characteristics
Each rank of taxonomy takes the characteristics of the rank above it and adds new ones. Step down once and the conditions grow.
So the higher the rank, the fewer its characteristics and the more organisms belong to it. The lower the rank, the more characteristics and the fewer organisms. Slide up from species to kingdom and watch the two bars run in opposite directions.
two opposite directionsHigher rank, more organisms.
What changes as you go up
The bars show an idea, not counted numbers.
2. Seven ranks, big to small
To place an organism down to its species, the International Code marks seven main ranks. The order is: Kingdom, Phylum (or Division), Class, Order, Family, Genus, Species.
In this order the largest rank is kingdom and the smallest is species. The ranks used in taxonomy are called taxa.
the order of ranksFrom the big set down to the small subset.
Put the seven ranks in order
Start with the largest rank, the one at the top.
Right. Which rank sits just below it?
Not yet. Each rank is a subset of the one before, so think from big to small.
Kingdom, Phylum/Division, Class, Order, Family, Genus, Species. A phylum is a subset of a kingdom, a class of a phylum, an order of a class; family comes after order, genus after family and before species; species is the smallest rank.
3. Sets inside sets
Zoom from kingdom down to species. Each rank adds a condition, and someone drops out.
zoomdrag the handle
The human's address, circle by circle
The circle sizes are drawn for the idea, not in proportion to numbers of organisms.
This arrangement is called a nested hierarchy. The topmost rank is like one big set and the ranks below it are subsets. The subset of a kingdom is a phylum, of a phylum a class, of a class an order, and so on down to species. Tap the circles.
nested hierarchySmall circles inside bigger ones.
Sets and subsets
Kingdom. The biggest set. Its subset is the phylum.
Phylum. A subset of the kingdom. Its subset is the class.
Class. A subset of the phylum. Its subset is the order.
Order. A subset of the class. Its subset is the family.
Family. A subset of the order. Its subset is the genus.
Genus. A subset of the family. Its subset is the species.
Species. The smallest rank, inside all the others.
All seven seen. This system is called a nested hierarchy.
The circle sizes are drawn for the idea, not in proportion to numbers of organisms.
There is a shortcut when writing. To avoid repetition, the characteristics of the previous rank that carry on into the next are often not mentioned again.
4. The address of a human being
Now one complete address: the human being, Homo sapiens. At each rank, try to say the reason yourself first, then press next.
step by stepSay the reason before revealing it.
Homo sapiens step by step
Kingdom → Animalia Reason: cells with a distinct nucleus, multicellular, heterotrophs, and a complex tissue system.
Phylum → Chordata Reason: a notochord is present at some stage of life.
Class → Mammalia Reason: they feed their young on milk and the body is covered with hair.
Order → Primate Reason: hands with five fingers suited to grasping, and vision more developed than the sense of smell.
Family → Hominidae Reason: similarity with the chimpanzee, gorilla and orangutan.
Genus → Homo Reason: the brain is large for the size of the body, and they can walk upright on two feet.
Species → Homo sapiens Reason: a broad, upright forehead, a skull thinner than in other species of Homo, and more intelligent.
Knowing the names of the ranks is not enough. As a student you need to know why a species sits at each rank, because in the traditional system the reasons are not written out beside the names. One trap to avoid: walking upright on two feet is the reason for genus Homo, not for order Primate.
5. Check yourself
Three questions. The first one to begin.
check yourselfpick one answer
Question 1
How many main ranks of classification does the International Code mark?
Five is the number of Whittaker's kingdoms; there are seven ranks.
Six is Cavalier-Smith's number of kingdoms; there are seven ranks.
Right. The International Code marks seven main taxonomic ranks.
Not eight; from Kingdom to Species there are seven ranks.
Now the second.
check yourselfpick one answer
Question 2
Which is a reason for placing humans in genus Homo?
A broad, upright forehead is the reason for the species sapiens, not the genus Homo.
Right. The book puts humans in genus Homo because of the largest brain for body size and walking upright on two feet.
A complex tissue system is the reason for Kingdom Animalia.
Well-developed vision is the reason for Order Primate, not genus Homo.
And the last.
check yourselfpick one answer
Question 3
Which belongs to Class Mammalia?
An ostrich does not feed its young on milk, so it is not Mammalia.
Right. A cat feeds its young on milk and has hair, so it is Mammalia.
A pigeon neither feeds its young on milk nor has hair, so it is not Mammalia.
A crocodile does not feed its young on milk, so it is not Mammalia.
The whole note on seven cards.
cut from the notetap to turn over
Seven cards
QuestionAs you go up the ranks, how do the number of characteristics and of organisms change?