Classification

One of zoology's main purposes is to identify all animals through classification. Classification is accomplished by comparing the characters, or features, of groups of animals. These characters may be of very different nature: morphological characters refer mainly to body structures, whereas histological and cytological characters are those of body tissues and cells, respectively. The number and forms of the chromosomes (karyology) are also considered a distinctive feature, but methods of DNA complementation ("molecular systematics") are being used increasingly. The presence, absence, or even the structure of certain biochemical compounds are used as characters. Certain physiological functions (for example, temperature regulation) are also considered in classification.

The rules and procedures for classification constitute a division of zoology called animal taxonomy. Various opinions about whether characters should be differentially weighed (i.e., some characters should be considered more important than others), and on how they should be weighed have been voiced since the eighteenth century. Carolus Linnaeus (1707–1778), the founder of modern taxonomy, maintained that characters should be weighed according to their functional value, whereas Michel Adanson, a French naturalist, thought they should be arbitrarily selected.

Modern classification has relied mostly on weighed characters; however, in the twentieth century insect taxonomists began using quantitative (numerical and graphic) methods, bringing about new debate on this matter. The tendency now called phenetics, initiated by Russian taxonomist E. S. Smirnov in the 1920s, proposed methods for comparing unweighed characters to determine overall similarity, whereas another tendency, now known as cladism, developed by the German zoologist Willi Hennig in the 1950s, insisted on weighing characters according to their evolutionary importance. In the 1970s and 1980s, discussions took place in the United States between the supporters of both. As a result of this debate, numerical methods (which allow for the use of computers) were perfected; this brought about a certain degree of compromise between the differing quantitative approaches, but not between the underlying philosophies. The use of cladistic criteria, however, seems to have prevailed.

Characters and Conservation

Characters used for classification are generally determined in laboratories and natural history cabinets, but other features can only be studied in the field. Captive animals are not usually reliable when studies of behavior (ethology) or of relations between animal populations and their environment (ecology) are intended. Field studies are an important part of zoological research. Although they do contribute to classification, they also stand by themselves as a valuable source of information for species conservation. The only true way to preserve a species is within its own typical habitat (i.e., the natural conditions in which it lives).

By using an array of characters, many previously undescribed species are discovered every year. In their research, zoologists often go further than mere classification, contributing to a better understanding of biological processes and discovering previously unknown qualities of animals. For example, a collateral result of the study of insect pheromones (hormones that attract the opposite sex) was the use of some compounds in pest control that, unlike traditional insecticides, do not contaminate the environment. Research on sounds produced by certain dangerous or obnoxious animals have also served to devise methods to repel them without damaging the environment.