Classification of Living Things: Study Notes & Interactives
All of life descended from a single common ancestor
Over 2 million individual species have been identified, but estimates suggest that there may be from 10 to 100 million species living at this time. Scientists also estimate that 99% of all organisms that ever lived on earth are now extinct. All organisms are thought to have descended from a single common ancestor that lived over 3.6 billion years ago (that's the earliest fossil evidence we have found for bacteria). Most species leave no fossil traces, however. And of those that do, most are never fossilized.
If you go far enough back in time, you can say that ALL of life is related. But as organisms evolved over time, some organisms are more closely related than others. For example eukaryotes are more related to one another because they are made up of cells that have organelles such as a nucleus that contains DNA. Bacteria are prokaryotes and do not have organelles, so their DNA is free within the cell.
Scientists classify these organisms based on common physical characteristics. As more species are identified and their relationships to other species are understood, scientists adjust the classification system accordingly. As such, a larger grouping called a "domain" is generally agreed upon today. Not long ago, the largest grouping was the kingdom.
When the classification system we use today was first invented 200 years ago by Carolus Linnaeus (he actually created his name in the same classification form that he invented), organisms were classified according to whether they shared similar physical features. With DNA technology, we can now investigate relationships based on the physical characteristics of their genes. As this new technology comes to greater use, we are finding that some species had to be reclassified into different taxa.
Each level is called "taxa" and the study of classifying organisms is called "taxonomy".
How Organisms are grouped and classified
Life: The largest category of all is all of life, or biota (things that are biotic)
Domain: Scientists now suggest a larger grouping called a "domain". (Previously, the largest grouping was Kingdom) There are 3 domains: Archae, Bacteria and Eukaryota. Eukaryota is further subdivided into 4 kingdoms (Fungi, Plantae, Animalia & Protista). Domains Archae and Bacteria are each composed of one kingdom, the Archaebacteria and Eubacteria, respectively.
Kingdom: Most textbooks now list 6 kingdoms -- animal, plant, protists (algaes, amoebas and such), fungi, bacteria, and archaebacteria (also called extremophiles). The animal, fungi, protist and plant kingdoms all belong to the domain Eukaryota.
Phylum: There are more than 30 phyla in the Animal Kingdom and 9 or 10 in the Plant Kingdom.
Phylum Chordata is the one we're most familiar with -- it includes animals that have a nerve cord. A subphylum of Chordata is Vertebrata, or animals which have a backbone that protects the nerve cord. Vertebrata includes humans, birds, fish, and all other animals with a backbone.
Phylum Arthropoda includes insects, spiders, lobsters, etc. Arthropods have segmented bodies with the segments grouped into two or three distinct sections. They have hard external skeletons, or exoskeletons, that are shed and regenerated as the animals grow.
Class: The various phyla are divided into classes -- Phylum Chordata is divided into several classes, including: amphibians, birds, mammals, reptiles and fish. Phylum Arthropoda includes classes such as Chilopoda (centipedes), Diplopoda (millipedes), and Insecta (insects are characterized by having 6 legs)
Order: Scientific groupings don't follow hard and fast rules. Once we get to the "order" of a living thing, there sometimes begins to be some disagreement about where it belongs. You may find that different sources group creatures in different orders or families. And you may find that a creature has its order or family changed as more information is learned (like the African and North American moles were once thought to be in the same order, but are now classified differently). One order in the class Mammalia is the Primates, which includes lemurs, monkeys, apes and humans.
Family: The family is a way scientists group similar genuses together. This is not the "mom, dad, brother and sister" type of family!
Genus: Two or more species that share unique body structures or other characteristics are considered to be closely related and are placed together in a genus. Sometimes a genus might include only a single species if there is nothing else in the world that has similarities with it. The genus is the first part of the scientific name of a species. The genus is always spelled with a capital letter and is underlined (when handwritten) or italicized (when typed).
species: A species can be defined as a group of individuals that breed together to produce fertile offspring. Individuals of a species cannot breed with other such groups. It is sometimes possible for different species to breed, but the offspring will be sterile. A mule is the sterile offspring of a donkey and a horse, and the mule can never mate and reproduce itself. The species is the second part of the scientific name of a species. The species is always spelled with a lower case letter and is underlined (when handwritten) or italicized (when typed).
Many of the organisms we have found in our site studies belong to the Phylum Arthropoda. The image above shows which classes within the Phylum Arthropoda some of these organisms belong to.
What makes an arthropod?
If you're really interested, you can click here to find out the many different classes and orders in the phylum Arthropoda and what distinguishes them.
This is how humans are classified. The genus name is "Homo", and the species name is "sapiens", which means wise or knowing.
Can you come up with a better mnenomic?
Here are some examples of mnenomic devices that can help you remember the classification order. See if you can come up with one of your own and illustrate it. Now that we have added a new taxa, we might want to rewrite this to say Dead King Phillip Came Over For Good Spaghetti or Dueling Kings Play Checkers on Fat Green Stools.
Click here to try to link to a NOVA website where you can try to classify 3 different organisms.
In class we will use the 'dichotymus' system for figuring out the common names of the fish pictured below. Your homework is to click on the pictures to find the Genus & species of each fish. Then click on two pictures of your choice and find out what their complete classification is. You should find the names of the Domain Kingdom, Phylum, Class, Order, Family, Genus and species.