Taxonomy is a field in which biologists develop a classification system used to demonstrate the relationships of different kinds of organisms. It is hierarchical, meaning that the most general classifications start with three groups, each of which further branch into more detailed groupings of organisms.
Taxonomic classification is highly influenced by theories of evolution and genetic relationships. This is the study of systematics, which concerns genetic diversity among organisms.
Taxonomic classifications from the most general to specific are: domain, kingdom, phylum, class, order, family, genus, and species. Someone's dog, a house pet, would belong to the Domain Eukarya, which consists of all organisms with cells that have nuclei. The dog would be further classified in more detailed groups until it is specifically classified as Canis lupus familiaris (Canus is the genus, lupus is the species, and familiaris is the subspecies), or the domestic dog.
The Three Domains of Life
The most general taxonomic classification is Domain. One of the important characteristics used in domain classification is the presence of a cell nucleus. Those organisms without a cell nucleus are prokaryotes, whereas eukaryotes have a cell nucleus. There are three domains of life:
- Domain Archaea are single-celled prokaryotes with cell walls made up of the L-glycerol isomer. Organisms in the other two domains have the D-glycerol isomer.
- Domain Bacteria are single-celled prokaryotes that are genetically dissimilar to archaea. Bacterial cell walls contain peptidoglycan. There are some bacteria that do not have a cell wall.
- Domain Eukarya consists of all of the eukaryotes.
Kingdoms
Domain Eukarya is further divided into kingdoms:
- Kingdom Animalia consists of multi-celled organisms with cells that do not have cell walls, differentiate to have a specific purpose, and form tissues. These organisms must take in food from the surrounding environment.
- Kingdom Fungi consists of single- and multi-celled organisms. The multi-celled organisms can have tissues. Their cell walls are a tough exoskeleton made of chitin. These organisms must acquire food from the surrounding environment.
- Kingdom Plantae organisms have cellulose in their cell walls. Most of these organisms make their own food, although some trap insects for food.
- Kingdom Protista consists of organisms that do not really fit into the above taxonomic classifications. They can possess any combination of characteristics from any of the other kingdoms.
Five Kingdoms Including the Kingdom Monera
Another taxonomic classification system bypasses domains and groups organisms into five kingdoms. The four eukaryotic kingdoms are used as well as the Kingdom Monera, which consists of all prokaryotes. This is an older system that was common before Archaea were found to be more genetically similar to eukaryotes than bacteria, and that bacteria has peptidoglycan cell walls whereas Archaea do not.
Subdivisions of Taxonomic Classification and Systematics
According to Brooker et al., "Biologists use systematics to develop the methods to construct taxonomic groups. By studying the similarities and differences among species, biologists can gain information about…the evolutionary history of a species or group of species." This becomes more prevalent in the phyla and its subdivisions.
- Phylum is classified using characteristics such as the presence of a fluid-filled body cavity called a coelom, the presence of segments, whether an organism is symmetrical or asymmetrical, and the general body plan.
- Classes are animals that share characteristics that are unique to their class. For example, horses are hairy creatures that produce milk for their young, so they are classified as Mammalia.
- Order further divides classes into more distinct groups. Horses belong to the Order Perissodactyla, which are animals whose feet are hooves with an odd number of toes. Rhinoceroses are also in this order.
- Family further divides orders. Horses belong to Equidae, along with other closely-related animals that are now extinct, such as Orohippus, Hypohippus, and Mesohippus.
- Genus further classifies animals within a family that are more closely related. Horses, donkeys, and zebras are all Equus, which is the only Equidae genus still in existence.
- Species are subdivisions of genus that can interbreed. For example, what we know as the horse can breed with other horses. Their species name is ferus, so they are referred to as Equus ferus.
- Variety, or breed, concerns different types of species. There are different types of horses such as thoroughbreds and quarter horses.
As demonstrated by the horse example, taxonomic classification includes all living organisms, including extinct organisms that are only known to have existed after finding fossilized remains, or that are hypothesized as having existed to explain evolutionary gaps in physical development. Taxonomic classification illustrates relationships between all organisms as well as evolutionary paths. These classifications can and do change as we discover new organisms and unearth unfamiliar fossils.
Source for Taxonomic Classification and Phylogenetic Systematics
Brooker, Robert J., Widmaier, Erip P., Graham, Linda E., and Peter D. (2008). Biology. McGraw-Hill.
Diane Ursu - Diane Ursu joined Suite101 as a contributing writer in August 2009 and became a Feature Writer in January 2010. She is a freelance writer ...
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