96 



CLASSIFICATION, PHYLOGENY, AND EVOLUTION: 



animals present a warped view of conditions in 

 nature. Perhaps the best way of contemplating a 

 species is to think of its existence in view of the 

 death of many of its ancestors. For example, the 

 African and Indian elephants are two similar but dif- 

 ferent species that are related through common an- 

 cestry; if time could be ignored and the ancestors 

 and all intermediate types to these two species were 

 living today, one would find it impossible to separate 

 the elephants into two species. For this reason, if one 

 considers the known possibility of two living, geo- 

 graphically isolated groups of organisms that possess 

 similar form and habits, the problem of determining 

 what is and what is not a distinct species can be ap- 

 preciated. Although past life does not exist to create 

 problems in species definition, some very closely re- 

 lated living organisms cause the same sort of dif- 

 ficulty. What then is a species? The common idea, 

 gathered from the less complex kinds of species, is 

 that a species is a group of organisms freely reproduc- 

 ing with one another but reproductively isolated from 

 all other groups of organisms. For many purposes 

 this is a satisfactory way of thinking of a species. 



Categories above the species are strictly for con- 

 venience and may not be accepted by all biologists. 

 For example, within a single family the number of 

 genera recognized by diflferent biologists may range 

 from five to ten. This occurs because individual 

 higher categories are never equivalent when applied 

 to different groups, no matter how hard a biologist 

 might try for uniformity. Uniform higher categories 

 are impossible, laecause evolution does not produce 

 units of distinct and discontinuous size; rather, the 

 consequence of evolution is a continuous variation in 

 the size and complexity of different units. In spite of 

 this, larger categories are valuable when they repre- 

 sent natural groups; hence, the primary criterion for 

 any of these larger groups is common ancestry and 

 the larger the category, the more organisms are in- 

 cluded. Similar species with common ancestry are 

 grouped inw genera (singular genus), genera into fam- 

 ilies (singular/ami(y), families into orders, orders into 

 classes, classes into phyla (singular /(Ay/j/m), and phyla 

 into kingdoms. Depending on the classification 

 scheme, there are two kingdoms, Animalia and 

 Plantae; three kingdoms, those two, plus Protista; or 

 four kingdoms, the preceding three, plus Monera. It 

 should be noted that botanists tend to use the term 

 division instead of phylum. Technically speaking, the 

 term "phylum" should be reserved for animals and 



the term "division" for plants; however, phylum is 

 acceptable and is used here for plants, animals, 

 monerans, and protistans. 



Many categories below the species level may be 

 recognized. However, only subspecies and variety have 

 much use and meaning. The subspecies designation 

 is restricted mostly to animal species. In its usual 

 meaning it refers to a geographic race that has some 

 structural or other difference from the rest of its 

 species. A variety, a plant subunit of the species, can 

 be a structural variant without regard to distribution, 

 a structural variant forming a geographic race, a 

 structural variant sharing the range of other variants 

 of the same species, or a color or habit (form of 

 growth) variant. 



The species and other categories already men- 

 tioned are the basis of a framework within which one 

 can start with a very large group, such as a phylum, 

 and work down through finer groupings, all at- 

 tempting to show relationship and usually assuming 

 common ancestry, until the individual organism is dis- 

 tinguished. In some cases, and to obtain additional 

 groups, the species and higher categories are further 

 subdivided or grouped by the addition of the prefixes 

 sub- and super- (respectively, something less than and 

 something more than the group so designated). Also, 

 infra- may be used as a unit just below sub-. The sub- 

 species is an example of this type of division. Also, 

 such categories as tribe and variety assume particular 

 places in the heirarchy of biological classification. In 

 certain detailed taxonomic studies special categories 

 (e.g., cohort, brigade, legion, and section) are created 

 and defined to fulfill special needs. These latter cate- 

 gories always must be defined, because they do not 

 possess a standardized position in the taxonomic 

 heirarchy. 



An example of the categories generally used to 

 classify modern man are as follows: 

 Kingdom Animalia 

 Subkingdom Eumetazoa 

 Phylum Chordata 



Subphylum Vertebrata 

 Superclass Tetrapoda 

 Class Mammalia 

 Subclass Theria 

 Order Primates 



Suborder Anthropoidea 

 Family Hominidae 

 Genus Homo 

 Species sapiens 



Subspecies sapiens 



