The Groups and Ancestry 



of Organisms 



Phylogeny is the study of the history of life, es- 

 pecially the evolutionary interrelationships and an- 

 cestry of living organisms. On the basis of our present 

 knowledge of phylogeny, vk'e can place living things 

 into convenient categories or groups. In this chapter 

 we will examine the methods by which organisms are 

 classified into laxonomic categories and the method of 

 naming those categories, nomenclature. 



To understand the basis for classification of or- 

 ganisms we must examine the principles of organism 

 complexity, the origin of life, the physical and biological 

 history of the earth, and the changes, or evolution, of 

 life on our planet. 



CLASSIFICATION AND TAXONOMY 



There are over a million species of living organisms 

 in the world today. Needless to say, it is impossible 

 for anyone to know all of them. Yet, for various 

 reasons, man is a great labeler. He appears to have a 

 passionate desire to name things, a characteristic that 

 seems to obscure and all too often to satisfy his ig- 

 norance. Probably since the time of the earliest 

 humans, names have been applied. Perhaps, in order 

 to simplify things and arrange them in his mind, 

 ancient man attempted many schemes of classifica- 

 tion. 



Man's efforts to name organisms led to the science 



of taxonomy, the grouping of organisms. Early 

 schemes were purely matters of convenience, using 

 such things as habits and habitats to group animals 

 together. According to that now-defunct system 

 whales, seals, and porpoises would be grouped with 

 fishes. Today, biologists do their best to determine 

 animal ancestry and relationships and to use that in- 

 formation for purposes of classifying. In modern 

 taxonomy, the whales, seals, and porpoises are classi- 

 fied with mammals. Even today, however, the com- 

 plexity of known phylogeny may necessitate a classi- 

 fication of pure convenience; known evolution may be 

 too intricate to be shown by modern or any relatively 

 simple means of classification. In spite of this pos- 

 sibility, the modern biologists' attempts to use re- 

 lationships to form groups have resulted in the most 

 convenient, especially for learning and remembering, 

 and most informative scheme to date. 



TAXONOMIC CATEGORIES 



The basic unit of taxonomy is the single kind of 

 organism, or species. (The term "species'" is both 

 singular and plural; the word "specie" means coined 

 money.) A species may seem to be a very definite 

 thing. Everyone recognizes dogs, cats, cows, and 

 horses as species because each group is unique and 

 easily distinguished. However, these well-known 



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