70 UNDERWATER GUIDE TO MARINE LIFE 



lengths. However, no single individual can have all widths or all lengths of 

 eye stripe. Instead, there are individuals with long, wide stripes, short, wide 

 stripes, long, narrow stripes, and short, narrow stripes. 



A species may consist of one population, but it is usually composed of several 

 populations which are more or less independent, but do have some gene flow 

 between them. That is, some members of neighboring populations do breed, so 

 that these populations also have genes in common, though the species gene 

 pool is less restricted in scope than is the population gene pool. Within the 

 species, gene flow may be restricted enough between some populations to allow 

 them to differ recognizably from others. In such cases, these are called suhs-pecies. 

 In humans the four races, Caucasoid, Negroid, Mongoloid, and Australoid, could 

 be biologically classified as recognizable subspecies. Each has its own charac- 

 teristics and, until recently, had its own geographical location. With increased 

 gene flow between these races, the result in time will probably be a breakdown 

 of race distinctions. 



Evolution would probably not progress very far, however, if it depended solely 

 on differing assortments of genes that are already present. Somehow new build- 

 ing blocks must be found, and these are produced by mutations. The nature of a 

 mutation is only partially understood. Presumably, it is a structural change in 

 a gene or even a rearrangement of genes on a chromosome which causes that 

 gene to have a different effect than formerly. 



Most mutations are small changes. Drastic changes are rarely tolerated in 

 nature, and in the great majority of cases of large changes due to mutation, 

 the result is early death. Mutations are usually deleterious in effect, since 

 mutations are purely random changes, not dictated by the environment or the 

 needs of the organism. Purely random changes do not usually benefit an or- 

 ganism which has evolved over a period of thousands of years in response to 

 its environment. But, of course, beneficial mutations do occur occasionally. 



Mutations are rather common. Any one gene may mutate once in several 

 thousand individuals. Evolution is the process by which the fate of these rather 

 common, mostly small, sometimes beneficial mutations is determined. The 

 assortment of genes also plays an important role since certain assortments of 

 genes are more or less beneficial than others. The environment determines the 

 fate of genes in evolution. Every gene or combination of genes can be assigned 

 a selective value in its environment. If the gene is beneficial, the selective value 

 is plus; if deleterious, the value is minus; if it has no effect, the value is zero. 

 The environment determines this value since a gene, and an animal possessing 

 genes, does not exist in a vacuum but in a definite place at a definite time. 

 What may be beneficial in one place may not be so beneficial in another. By 

 the process of natural selection, the beneficial genes are selected for and the 

 deleterious ones selected against. Since the genetic change is not usuallv drastic, 

 neither is the selection drastic. It usually expresses itself in a differential breeding 

 rate between the individuals possessing beneficial and deleterious genes. Those 

 individuals possessing beneficial genes find adjustment to their environment 

 easier and produce on an average more offspring than those possessing deleterious 

 genes. This results in time in a marked shift in the whole population toward 

 the beneficial change within an environment which is presumed to be stable. 

 Environmental change usually results in a change in direction of evolution. 



