All living things fall into a careful classification scheme that goes from the broadest level of similarity (domain) to the narrowest (species). The classification scheme has seven different levels. They are:
Domain: the broadest category; there are only three domains. This level of classification is a fairly recent change to the classification system. You may not have been taught about domains in school, and you may or may not see them on the ASVAB.
Kingdom: which contains several related phyla.
Phylum: which contains several related classes.
Class: which contains several related orders.
Order: which contains several related families.
Family: which contains several related genera.
Genus: which contains several related species.
Species: which contains organisms so similar that they can only reproduce with one another to create viable fertile offspring.
For example, this is the classification of human beings:
Domain — Eukaryota
Kingdom — Animalia
Phylum — Chordata
Class — Mammalia
Order — Primates
Family — Hominidae
Genus — Homo
Species — Sapiens
The three domains are:
Eukaryota: All living things whose cells have nuclei are in this domain. Almost all multi-celled organisms (including plants, animals, and fungi) are in this domain.
Bacteria and Archaea: Both of these domains contain single-celled organisms whose cells do not have nuclei. Living things in the two domains are distinguishable by metabolic and chemical differences.
The five or six kingdoms are:
| Kingdom | Description |
|---|---|
| Monera (sometimes broken into two kingdoms, one of which belongs to domain Bacteria and one of which belongs to domain Archaea) | Includes bacteria, cyanobacteria (blue green algae), and primitive pathogens. Considered the most primitive kingdom, it represents prokaryotic (as opposed to eukaryotic) life forms—that is, the cells of Moneran organisms do not have distinct nuclei. |
| Protista | Protista are the simplest eukaryotes (that is, their cells have nuclei). Includes protozoa, unicellular and multicellular algae, and slime and water slime molds. Ancestor organisms to plants, animals, and fungi; many can move around by means of flagella. Some are also photosynthetic. |
| Fungi | Includes mushrooms, bread molds, and yeasts. Fungi lack the ability to photosynthesize; they are called decomposers, breaking down and feeding on dead protoplasm (extracellular digestion). |
| Plantae | Have the ability to photosynthesize, so they are called producers. There are four major phyla: Bryophyta, or mosses; Tracheophyta, which have vascular systems; gymnosperms; and angiosperms. |
| Animalia | Produce energy by consuming other organisms, so they are called consumers. Can be either vertebrates (which belong to phylum Chordata) or invertebrates such as mollusks, arthropods, sponges, coelenterates, worms, etc. Human beings belong to the kingdom Animalia. |
There are many, many phyla (the plural of phylum), classes, orders, genera (the plural of genus), and species. If you have additional time before Test Day and you have mastered all of the other subject tests and maximizing your General Science score is crucial to your career goals, you may choose to use Internet sources to learn more about some of the largest phyla, classes, etc. Otherwise, that study is unlikely to be the best use of your time.
Study how an expert test taker would approach a general science question about the classification of living things:
| Question | Analysis |
|---|---|
| Mammals are part of the kingdom | Step 1: The question is asking you to determine which kingdom mammals belong to. |
| Step 2: There is nothing to simplify here. | |
| Step 3: Mammals are animals; they belong to kingdom Animalia. | |
| (A) Animalia (B) Mammalia (C) Protista (D) Monera |
Step 4: Based on the prediction above, you would select answer choice (A). |
Try one on your own.
“Species” is the most specific category in taxonomy and “Kingdom” is the most general (not including domains), so choice (A) is correct. Choice (B) lists the categories from largest to smallest. Choice (C) almost does the same but confuses Order and Class. Choice (D) is incorrect because it switches the places of “Order” and “Family.”