Unit 2 Classification Word Study

1. Learn the meaning of the words:

internal	внутренний
external	внешний
to estimate	исчислять
diversity	многообразие
to distinguish	различать
to warrant	заслуживать
tier	ярус
assumption	предположение
immense	огромный
frequently	часто
to encounter	сталкиваться
fluctuations	колебания
to measure	измерять
to assign	задавать
tally	совпадение
to strive	стремиться

2. Practice the following for pronunciation. Use these words in the sentences of your own:

scheme dynamic process to minimize confusion the naked eye taxon – taxa genera – genus bacterium – bacteria

fungus – fungi phylum – phyla unicellular multicellular phenetics taxonomy fauna and flora

Text a Classification

Classification, in biology, identification, naming, and grouping of organisms into a formal system based on similarities such as internal and external anatomy, physiological functions, genetic makeup, or evolutionary history. With an estimated 10 million to 13 million species on Earth, the diversity of life is immense. Determining an underlying order in the complex web of life is a difficult undertaking that encompasses advanced scientific methods as well as funda­mental philosophical issues about how to view the living world. Among the scientists who work on classification problems are systematists, biologists who study the diversity of organ­isms and their evolutionary relationship. In a related field known as taxonomy, scientists iden­tify new organisms and determine how to place them into an existing classification scheme.

Classification determines methods for organizing the diversity of life on Earth. It is a dy­namic process that reflects the very nature of organisms, which are subject to modification and change over many, many generations in the process of evolution. Since life first appeared on Earth 3.5 billion years ago, many new types of organisms have evolved. Many of these organ­isms have become extinct, while some have developed into the present fauna and flora of the world. Extinction and diversification continue nonstop, and scientists are frequently encoun­tering fluctuations that may affect the way an organism is classified.

In addition to ordering organisms, scientists give a new species a scientific name, typically a two-word name in Latin, to distinguish it from similar organisms. This naming process creates a standard way for scientists around the world to communicate about the same organism. This standard minimizes confusion, particularly when common names are applied to organisms. For instance, the bird Europeans commonly call a robin is a different species of bird from the robin Americans recognize. The confusion ends when the birds are referred to by their scientific names: the European robin is Erithacus rubecula, while the American robin is Turdus migrato­rius.

When classifying organisms, scientists study a wide range of features, including those visi­ble to the naked eye, those detectable only under a microscope, and those that can be deter­mined only by chemical tests. Scientists compare the external shapes and sizes of organisms as well as the anatomy and function of internal organs and organ systems, such as the digestive or reproductive systems. Biochemists study and compare the molecular interactions within an or­ganism that enable it to grow, make and store energy, and reproduce. The early stages of an organism's development, or embryology, as well as an organism's behaviour, or ethology, are also useful in grouping organisms. Even the role an organism plays in its habitat can help place it in a particular group. Scientists use the fossil record to learn how certain animals have changed and evolved through Earth's history, which may provide clues for classification.

More recently, scientists have employed the techniques of molecular biology to compare the units of heredity, or genes, among organisms. Scientists study the fundamental units of deoxy­ribonucleic acid (DNA), the molecule that makes up genes, and organisms that share a similar DNA structure may be more closely related. Called molecular systematics, this approach is a powerful analytical tool. Used in combination with the other features studied in classification, molecular systematics can provide valuable insight into classification problems. For many or­ganisms, molecular systematics studies have supported traditional classification; however, in some cases, the evidence from genetics studies has indicated that organisms should be reclassi­fied. Skunks, for example, traditionally have been classified with badgers, ferrets, and minks in the family Mustelidae. But recent studies of molecular traits indicate that skunks differ signifi­cantly from these animals and may warrant classification in their own family.

Scientists classify organisms using a series of hierarchical categories called taxa (taxon, sin­gular). This hierarchical system moves upward from a base containing a large number of or­ganisms with very specific characteristics. This base taxon is part of a larger taxon, which in turn becomes part of an even larger taxon. Each successive taxon is distinguished by a broader set of characteristics.

The base level in the taxonomic hierarchy is the species. Broadly speaking, a species is a group of closely related organisms that are able to interbreed and produce fertile offspring. On the next tier of the hierarchy, similar species are grouped into a broader taxon called a genus (genera, plural). The remaining tiers within the hierarchy are formed by grouping genera into families, then families into orders, and orders into classes. In the classification of animals, bacteria, protists (unicellular organisms, such as amoebas, with characteristics of both plants and animals), and fungi, classes are grouped into phyla, while plant classes are grouped into divisions. Both phyla and divisions are grouped into kingdoms. Some scientists go on to group kingdoms into domains.

Grouping organisms according to shared characteristics is not a simple task, and scientists often disagree about the best way to classify organisms. Some think that organisms should be grouped according to differences or similarities in the way they look or act. Other scientists argue that classification should be based on characteristics derived from a shared evolution. Conflicting philosophies about classification have resulted in a variety of classification meth­ods, each with their own set of assumptions, techniques, and results.

The classification of insects, birds, and bats illustrates a traditional classification process. In­sects, birds, and bats are all animals – that is, they are multicellular organisms that obtain en­ergy from food. Scientists group these organisms into the Kingdom Animalia.

Birds and bats both have spinal cords, causing scientists to classify both birds and bats in the phylum Chordata. Within the phylum Chordata, key features cause scientists to separate birds and bats. Birds are placed in the class Aves, which includes egg-laying animals, while bats are placed in the class Mammalia, which includes animals that give birth to live young and nurse their young from mammary glands. Insects, which lack spinal cords, are classified in the phy­lum Arthropoda, the taxon that includes animals with jointed legs and a skeleton on the outside of the body. Insects are further divided based on such broad physical features as the presence or absence of wings.

Scientists using the classical approach must judge the relative importance of characteristics. They may decide, for example, that wing structure is more important than the presence of fin­gernails in certain cases of classification. Some critics argue that this interpretation and evaluation is too subjective. To introduce more objectivity into classification, some scientists devised the phenetics approach to classification.

In the phenetics approach, scientists rely on quantitative methods and consider only the ob­servable characteristics of modern organisms. Pheneticists identify a set of characteristics to measure and assign a certain numerical value to each characteristic. The tally is used to determine the extent of similarity between organisms. For example, pheneticists may find that, overall, birds and reptiles have a 77 percent similarity of body structure, or morphology, com­pared to a 55 percent morphological similarity between birds and mammals. From this measurement, pheneticists would suggest a classification that grouped birds and reptiles more closely than birds and mammals.

A third classification method is the cladistic approach, which strives to classify organisms by natural evolutionary relationships, known as phylogeny. Cladists use the fossil record, mo­lecular genetics, and other techniques to create an evolutionary tree called a cladogram. This branched diagram shows the relationship of a group of species based on the fewest number of shared changes that have occurred from generation to generation.