502 INTRODUCTION TO EVOLUTION 



The term tachytelic has been assigned evolutionary hnes in which the 

 mean rate of evolution is above the average, or horotelic, rate. Apparently 

 there are no evolutionary lines in which rate of change remains perma- 

 nently at the tachytelic level. This exceptionally rapid evolution is excep- 

 tional in that in any given line it is "effective only during certain crucial 

 relatively short evolutionary episodes" (Simpson, 1949b). 



What causes a line which has been evolving at its average, or horotelic, 

 rate to make a sudden evolutionary spurt and become for a time tachytelic 

 in its evolution? We have suggested that the answer is probably not to be 

 found mainly in increased rate of mutation. Rather it seems to lie in the 

 field of what Simpson terms the "organism-environment relationship." We 

 observe that the large evolutionary changes are usually concerned with the 

 adaptation of animals to environment; i.e., the changes are in the nature of 

 adaptive responses. Throughout our discussions, for example, we have re- 

 ferred repeatedly to the emergence of vertebrates from life in the water to 

 life on land. This was certainly one of the largest evolutionary changes in 

 all history. The bodily changes involved were clearly associated with 

 meeting the needs of the new environment. Similarly the changes involved 

 in the evolution of horses, elephants, and many other groups not cited in 

 our brief discussion have been of a nature to adapt animals to conditions of 

 life facing them. Accordingly, it seems that change in environment is a pri- 

 mary factor in the speeding up of evolution. This change in environment 

 may occur in one of two ways: (1 ) The environment in which the species 

 is living may change, e.g., as a result of geologic change, or (2) the species 

 may enter a new environment, not previously available to it, or with the 

 demands of which it was not previously able to cope. The entering of new 

 environmental niches seems to have provided the stimulus for the most 

 radical, and rapid, evolutionary changes. 



Mega- Evolution 



On page 361, we noted Goldschmidt's division of evolution into "mi- 

 croevolution" (that of subspecies) and "macroevolution" (that of spe- 

 cies and genera, and perhaps also of higher categories). Simpson (1953) 

 has proposed the additional term "mega-evolution" for really large-scale 

 evolution, such as that of families, orders, classes, and phyla. It is evolution 

 at these levels that claims the chief attention of students of the fossil record. 

 Unfortunately, on the other hand, most experimental studies of the evolu- 

 tion of living animals must perforce concentrate on differences between 

 subspecies, species, and at times genera. But principles revealed by these 



