Profpage                                                                               (updated 09/08/06)

Classification of Organisms - It was a necessity for primitive cultures to know their surroundings in order for survival. They had to know what was safe to eat or to use in various ways, as well as what organisms could harm them. When men began to explore new lands, they almost always included one or more naturalists to collect and/or catalog the plants and animals in each area. Over 2000 years ago the Greek philosopher Aristotle categorized living things as plants or animals. They were also classified as land, water, or air dwellers. During the Middle Ages, names were given in Latin to all known creatures. In the mid-1700s, the Swedish biologist, Carolus Linnaeus, shortened the long descriptive terms for each organism to a binomial system, using only the genus (group) and species (individual kind) name. For example, the two oaks shown below belong to the genus Quercus (always italicized or underlined) for oaks, and phellos or rubra (always in lower case italics or underlined), for the willow or red oak. But what is a species? The best definition is that a species is "a group of actually or potentially interbreeding organisms that is reproductively isolated from other such groups." However, there are problems with this definition, because a few organisms only reproduce asexually, or they contain only females that produce more females. In addition, a few species can hybridize, usually producing sterile offspring.

To date, over 1,600,000 species of organisms have been described; at least one million of these are insects, but it is believed that possibly 25 million species of insects may exist, primarily in the tropics.

The hierarchical system of classification normally includes seven levels: Kingdom, Phylum, Class, Order, Family, Genus and species. However, super, sub, and infra- groups may occur. One way to remember these in order is: "kindly pay cash or furnish good security." Every new species discovered must be placed into this system. One of the most exciting things for a biologist is to discover a new species.

Hybridization is one difficulty of distinguishing species, but since most hybrids, such as the mule, are usually sterile, and cannot reproduce with either parental species or among themselves.

The change from two to five kingdoms developed in the 1960s, when it became apparent that some organisms had characteristics of both plants and animals.

Three major Domains of life - the first two are Prokaryotes (before the nucleus), while the Eukaryotes all have a true nucleus in each cell. Below are the major characteristics of each Domain and some characteristics of each Kingdom, and variations in the life cycle of each group.

Over 5000 species of Bacteria have been described, composing at least six groups, and probably thousands more have yet to be identified. The Cyanobacteria used to be known as blue-green algae, since they are photosynthetic. The Archaea are the most recently discovered group; Archaebacteria live at the bottom of the ocean. Eukaryotes are divided into 5 groups of 15 very divergent major phyla of  Protista (over 40,000 species), the Slime Molds, four groups of Fungi (over 72,000 species), nine phyla of Plants (over 263,000 species), and 16 phyla of Animals (over 1,224,500 described species). Ribosomal RNA (rRNA) analyses reveal the evolutionary relationships among the three domains, with the bacteria as the most primitive (dating back to about 4 billion years before the present).

Below are major characteristics of the three Domains (Bacteria, Archaea, Eukarya) and the six Kingdoms of life, as well as their evolutionary relationships, based upon genetic similarities and differences. The Protista represent an artificial grouping of extremely diverse groups that possess more differences between each other than do Fungi, Plants, and Animals.

The Plants and some algae arose from one group of Bacteria, while Animals, Fungi, and other Protists arose from a different group of Bacteria. Chloroplasts and Mitochondria represent two groups of bacteria that made photosynthesis and aerobic cellular respiration possible. These bacteria entered into other cells (see the theory of endosymbiosis below), and possess their own DNA in a circular chromosome, ribosomal RNA (which differs from eukaryotic rRNA), and they divide and grow on their own within eukaryotes. The Archaebacteria have more in common with Eukarya than they do with the Eubacteria. Among Eukaryotes, basal bodies, centrioles, flagella, and cilia arose from an endosymbiotic spirochaete-like bacteria. Some colonial bacteria also exist with fruiting bodies and spores (similar to fungi).

The origin of Eukaryotes possibly occurred in the process of endosymbiosis, shown below. Both mitochondria and chloroplasts have characteristics of bacteria, including their own DNA and bacterial ribosomes.


Although viruses_and_prions are nonliving, they are studied by biologists because they cause many diseases in living organisms.


 For more information on Bacteria, see Kingdom Eubacteria. The table below compares major characteristics of the six kingdoms of organisms.

Continuation of major characteristics of the six kingdoms of life.


Three major types of life cycles exist, and diploidy (possession of two sets of chromosomes) has arisen many times. Diploid multicellularity has arisen numerous times, and is highly advantageous to survival. What are the advantages of two copies of each gene, and of sexual reproduction versus asexual reproduction? Syngamy means a fusion of two cells, and results in a zygote. In this first example, the zygote divides the chromosome number into half (haploid) in the process of meiosis. This system is typical of Fungi..

In gametic meiosis, a reproductive cell produces (usually four) haploid gametes (sex cells) that combine to produce a zygote, which grows into a diploid individual. Humans and most animals reproduce through this method.

Plants and many algae reproduce by sporic meiosis, in which a diploid Sporophyte generation produces spores (asexually), which grow into a haploid Gametophyte generation that reproduces sexually to form a zygote and new sporophyte. Most plants that you would recognize are sporophytes, and the gametophytes grow within flowers or cones; however some plants have separate gametophytes.

Not all organisms reproduce sexually; some reproduce only asexually. Numerous variations in both sexual and asexual reproduction exist.

 For more information on protists, see Kingdom Protista


Kingdom Fungi - The bacteria and Fungi are "decomposers," in that they break down dead bodies of other organisms, so that the chemicals can be recycled through the environment. Although some species cause diseases, many are beneficial, and we could not live without bacteria and fungi.

Characteristics of the different groups of fungi.

Kingdom Plantae - Plants are the "producers" in the ecosystem, in that they produce oxygen, living spaces, and food for animals.

An illustration of the life cycle of a plant.

Key characteristics of the nine phyla of extant (living) vascular plants, and the approximate number of species of each (1).

Key characteristics of the nine phyla of extant (living) vascular plants (2).

 Plants and simple animals can be haploid, diploid, or polyploid, whereas humans cannot. Our brain is so complex that even part of one chromosome missing or duplicated, causes retardation, and often sterility. Some polyploid species of plants are produced artificially to increase food supplies.

Evolutionary changes, often with the help of man, are shown below in wheat (1).

Evolutionary changes, often with the help of man, are shown below in wheat (2).

Kingdom Animalia


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