CHAPTER /^ 



EVOLUTION, NAMES, AND CLASSIFICATION 



As the swimmer explores the underwater world, he soon becomes aware of the 

 diversity of shapes, forms, and habits of marine life. The bizarre shape of the 

 sargassum fish, the sleek beauty of the tuna, and the plantlike form of gorgonians 

 lead him to wonder what their relationships are, both to the environment and 

 to each other. The adaptation of each of these to their environment depends 

 upon the many small hereditary changes which, if advantageous, allow them 

 to find food more easily, breed more successfully, or live longer. This adaptation 

 involves evolution. 



EVOLUTION 



Figure 16 gives in "tree" form an outline of evolution of fishes for the last 

 400 million years, the period of time that their evolution is known through 

 paleontological studies. No more than the barest summary of the evolutionary 

 process from which the tree took root and grew can be given here. (The 

 excellent books noted in the Bibliography, especially Simpson's Meaning of 

 Evohition, should be consulted for further information.) 



The basic unit in evolution is a 'population, all members of which have free 

 access to all other members without spatial, ecologic, social, or any other barriers. 

 Take, for example, a hypothetical isolated coral reef, over and around which 

 swim several individuals of a certain species of butterfly fish. These individuals 

 compose a single population; all of them could breed freely together if they 

 so chose. This population may be thought of as possessing a number of char- 

 acters, hereditarily controlled by genes, in common. That is, the population 

 represents a common gene pool. Because all members of the population breed 

 tooether and share genetic features in common, thev would not be likelv to 

 diff"er from each other significantlv, perhaps only varying slightly in size or 

 intensity of coloration, etc.— differences due to the possession of slightly different 

 assortments of genes from individual to individual. Butterfly fishes, for example, 

 show variations which are due to different gene assortments. For instance, 

 members of a single population may have eye stripes of varying widths and 



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