There are an estimated 50 million species on Earth. Of the 1.5 million species that have been named, about 5% are single-celled organisms (prokaryotes and eukaryotes), and plants and fungi make up about 22%. About 70% of the known species are animals. Most of these are invertebrates, especially insects, of which there are about 1 million known species.The earliest and, until this century, the most dominant activity in biology has been the classification of species. Aristotle was concerned with it. Even Charles Darwin, who started developing his theory of evolution while on the famous round-the-world voyage of the Beagle, was occupied primarily with collecting and categorizing new species of organisms.

Why should environmental engineers and scientists study taxonomy? One reason is so they won't be at a loss to see how any particular organism fits in with others. Does that worm always live where it is, or will it someday metamorphose into an insect and fly away, perhaps carrying biological or chemical contamination with it? At a more fundamental level, classification is a prerequisite to the identification of patterns, which leads to generalizations or hypotheses, and ultimately to tests of hypotheses by experiment or further observations. Thus, classification can be seen as a basis for the scientific method. The generalizations came later to biology than they did to physics and chemistry, perhaps owing to the dazzling variety of life and the inherent complexity of the underlying mechanisms.

Taxonomy is the science of classification of organisms. Taxonomy (except for microorganisms) was a fairly settled area until recently. Now, genetic techniques are reopening old questions and revealing new things. The possibility that humans are causing a new mass extinction lends a new urgency to knowing the organisms that now exist on Earth. This loss in the diversity of life would have several potential consequences. From a utilitarian point of view, destruction of species results in a loss of genetic material that may include useful traits, such as production of chemicals that may have medical uses, or traits that may be useful for invigorating agricultural plants or animals. Ecological relationships may link the survival of one species to many others. Finally, it may be argued from several ethical viewpoints that we have a moral imperative to preserve Earth's biological heritage regardless of actual or potential utility.

Two ways to classify organisms are to group them (1) by structure and function or (2) by closeness of evolutionary descent. The two approaches often produce similar results, but not always. Convergent evolution can make dissimilar evolutionary branches form similar characteristics, whereas divergent evolution does the opposite. Barnacles and limpets both live in shells glued to rocks in the sea, but barnacles are arthropods, like crabs, whereas limpets are mollusks, like clams. The use of evolutionary descent was constrained in the past due to the incompleteness of the fossil record, resulting in ambiguity in classifying existing organisms. Recently, however, the development of genetic engineering techniques has led to quantitative measures of genetic similarity. These have settled old classification questions and even led to increased detail in classification by dividing old groups, forming new ones, and producing new species, phyla, and even kingdoms (see below).

Biologists use the Linnaean system to classify organisms, which consists of a hierarchy of groupings and a naming convention. The lowest level of the hierarchy is the species. Similar species are grouped into a genus. The naming convention, called binomial nomenclature, assigns to each species a two-word name. The first word, which must be capitalized, is the name of the genus, and the second, which is uncapitalized, is the name of the species. For example, modern humans are in the genus Homo and the species sapiens; thus, they're called Homo sapiens. Other species in our genus are extinct, such as Homo habilis. Genus and species names are either italicized or underlined when written, and the genus name may be abbreviated by a capitalized first initial once it has already been written out in full: H. sapiens.

In increasing degrees of generality, the other classifications are family, order, class, phylum or division (in plants and fungi), kingdom, and domain. Each of these may sometimes be subdivided: for example, subkingdom, subphylum, or subspecies. The following phrase is a memory trick for the sequence from kingdom to species:

King Phillip came over from Greece Saturday.

Kingdom Phylum Class Order Family Genus Species

The highest-level category is the domain. It is based on the type of cell comprising the organism, of which there are three: the bacteria, the archaeans, and the eukarya. Each domain is subdivided into kingdoms. Some biologists consider the archeans and the bacteria each to consist of only one kingdom. However, microbiologists note that molecular biology methods have shown that different groups of bacteria, for example, are more distinct from each other genetically than plants are from animals. Therefore, archaea have been divided into three kingdoms, bacteria into 15, and eukarya into four. Although this method of classification has not yet become universal among biologists, in this book we adopt this approach.

One domain is that of the bacteria, also sometimes called eubacteria. Their cell type is the prokaryote, which is a simple type lacking any internal membrane structures. Their size is typically 0.2 to 2.0 micrometers (mm). Based on morphology (physical structure) and biochemical characteristics, the bacteria have classically been organized into 19 groups. Examples are Pseudomonas, a common soil bacterium that is also exploited in wastewater treatment processes; and the Cyanobacteria, or blue-green bacteria, important in the environment because they convert atmospheric nitrogen into a useful nutrient. Based on genetic techniques, new groups have been found and some of the older ones combined; one current classification system now has 15 high-level groupings.

Another prokaryotic domain is that of the archaea, which includes three kingdoms. Archaea includes the methanogenic bacteria that produce methane gas in a wastewater treatment plant's anaerobic digester. Archaeons were once considered to be bacteria. However, unsuccessful attempts to transfer genes from eubacteria to archaebacteria led to the discovery that their membranes, although structurally similar to the other domains, are chemically distinct. Many of the archaea are notable for the harsh chemical and physical environments in which they thrive. Because of this it is thought that archeons may be relics of the earliest forms of life that still exist today.

The third domain is that of the eukarya, which comprises four kingdoms, including the familiar plants and animals. The cells of this domain are called eukaryotic. This is a more

Domain Eukarya EUKARYOTES

Domain Eukarya EUKARYOTES

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