How to Classify Living Things Based on Characteristics

Instructor: Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

How in the world do scientists classify living organisms? There are so many critters on earth, it seems really challenging. This lesson will explore how organisms are classified and will delve into the classification system.

Classification Activity

Image 1
classify

Take a look at image 1. You are a taxonomist, a scientist that classifies, or organizes, critters based on similar characteristics. So how would you organize these critters? Maybe you could organize them into groups based on where they live, or perhaps you would classify them based on how they behave? Or maybe based on how they look, or what they eat? Believe it or not, taxonomists look at all of these things when they decide how to organize organisms. Let's take a closer look at how they do it!

Domains

So far taxonomists have discovered and classified around 1.75 million species. A species is defined as a group of critters that can interbreed and produce healthy offspring. Classifying that many species may sound like a herculean task, but those darned taxonomists are up for a challenge.

Taxonomists start with a broad classification known as a domain. There are three domains, and a critter gets placed into one of them based on certain characteristics. For example, if a critter were a prokaryote, or a simple single-celled organism lacking some of the structures of more complicated cells, and lived in really extreme environments, like the vents of hot springs or in the guts of a cow, it would go into the archaea domain. Take a look at image 2 so you can familiarize yourself with the three domains. You may notice that you belong to the eukaryota domain, along with plants, animals and fungi (eucarya in the image).

Image 2
domain

The Kingdoms within the Eukaryota Domain

Let's take a closer look at the domain you belong to and see how taxonomists further divide it up. Domains are the broadest classification, and the next is kingdom. There are four kingdoms within the eukaryota domain. Scientists determine which kingdom a critter belongs to based on several characteristics. For example, how does the organism obtain food? Is it made up of multiple cells? What sorts of structures are within the cells? All of these characteristics (and more) factor into placing the organism into the correct kingdom. Take a look at the table to see some characteristics that taxonomists use to clump critters into specific kingdoms. Of course, there are more shared characteristics, but this will give you some insight into how they are classified.

Kingdom Name Characteristics Shared Example Organism
Plantae Multicellular (made up of more than one cell), autotroph (make their own food), have a cell wall Ferns, roses and trees
Animalia Multicellular, heterotrophs (cannot make their own food), no cell wall Cat, sponge, and worm
Fungi Multicellular, no cell wall, heterotroph Mushroom and yeast
Protista Unicellular (one celled), some have cell walls others do not, some autotrophs others heterotrophs Amoeba, and paramecium

A couple of notes before we move on. A cell wall has an additional layer compared to a cell membrane. And you should note that sometimes taxonomists use a five or six kingdom system and do not use the domains. In this case, there is a bacteria and archaebacteria kingdom. But we are going to stick with domains for this lesson.

Further Classification

After kingdom, the critters get divided up further into the following:

  • Phylum (plural phyla)
  • Class
  • Order
  • Genus
  • Species

As you go down from kingdom to species, the requirements for placing organisms get more specific. It may be worth taking a moment to come up with a way to remember the names from domain to species. For example: Did King Philip Come Over For Great Spaghetti (Domain, Kingdom, Phylum, Class Order, Genus, Species)? Or you can make up your own.

Let's look at a handful of examples for phylum, which follows kingdoms. In the animal kingdom, there are many different phyla. Here is a sampling:

  • Chordata Phylum: animals that have a backbone, like you
  • Cnidarian Phylum: animals with soft bodies and tentacles, like jellyfish
  • Arthropod Phylum: includes animals with an exoskeleton, or a hard outer skeleton, and jointed legs, like a beetle

Image 3 The Ladybug belongs to the Arthropod Phylum
ladybug

And if we take a look at plants, you also have several different phyla; however, instead of phyla, they are called divisions. Here's a quick look at a couple:

  • Magnoliophyta Division: plants with flowers
  • Coniferophyta Division: plants with cones

So you may be starting to see how taxonomists divide up organism, first into domains, then into kingdoms and then into phyla (or divisions if it's a plant).

Classification Example

We covered domains, kingdoms and phyla so I know you're getting the idea, but let's finish this lesson off with a specific example so you can see how it works. Let's say some taxonomists came upon the critter in image 4, and they needed to figure out how to classify it.

Image 4 Mystery animal
bear

Let's start with the domain and the kingdom. You probably recognize this critter as an animal, so it belongs to the Eukaryota Domain and Animalia Kingdom. What makes it an animal? Well, because it's so large, it is multicellular. And it can't make its own food, like a plant, so it is a heterotroph. And it doesn't look like a fungi, so it must be an animal!

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