The diversity of life on Earth is staggering. The science of identifying, describing, naming, and classifying all of these organisms is called taxonomy. Carolus Linnaeus, an eighteenth-century Swedish botanist, is considered the father of modern taxonomy. He carefully observed and compared different species, grouping them according to the similarities and differences he found. Taxonomists today still use his system of organization, though they classify organisms based on their evolutionary relationships, or phylogeny, rather than on simple physical The classification system used in taxonomy is hierarchical and contains seven levels. The seven levels of taxonomic classification, from broadest to most specific, are:
Kingdom
Phylum
Class
Order
Family
Genus
Species
A good way to remember the sequence of taxonomic categories is to use a mnemonic:
King Philip Came Over From German Shores
Each kingdom contains numerous phyla; each phylum contains numerous classes; each class contains numerous orders; etc. It is more accurate to draw the diagram of the taxonomic categories in a tree structure, with each level of the hierarchy branching into the next:
As one moves through the hierarchy from species to kingdom, the common ancestor of all the species at a certain level dates further back in evolutionary history than the common ancestor of organisms in more specific levels. For example, the common ancestor of humans and chimpanzees (which are both in the order Primates) was alive more recently than the common ancestor of humans and dogs (which are both in the class Mammalia). Much in the same way, members of the same genus are more closely related than members of the same family; members of the same family are more closely related than members of the same order.
Each species is placed into the classification system with a two-part name. The first half of the name is the species’ genus, while the second is the species’ own specific name. The genus name is capitalized, and the species name is lowercase. Humans belong to the genus Homo and the species sapiens, so the name for humans is Homo sapiens.
The Five Kingdoms
Taxonomy splits all living things into five kingdoms: Monera, Protista, Fungi, Plantae, and Animalia. For the SAT II Biology, you should know the basic of the organisms that belong in each of these kingdoms, and you should also be familiar with the names and features of the major phyla within each kingdom.
Kingdom Monera
Monerans are prokaryotic: they are single-celled organisms that lack a nucleus and membrane-bound organelles. Of the four kingdoms, monerans are the simplest, and they generally evolved the earliest. Of all the kingdoms, only monerans are prokaryotic.
Monerans are by a single circular chromosome of DNA, a single cell membrane that controls the transport of substances into and out of the cell, and a process of asexual reproduction called binary fission that involves dividing into two identical clones. Some monerans have a cell wall made of a sugar-protein complex called peptidoglycan, which can be determined by Gram staining. A Gram-positive moneran has a thick peptidoglycan cell wall, while a Gram-negative moneran has a much thinner one. Monerans are broken down into phyla according to their means of procuring food.
