Major Patterns of Variation . 263 



Some living genera have had distinctly longer spans than genera 

 that have become extinct. The longest-lived extinct genera lasted 

 275 million years, but extant genera that have survived for 400 mil- 

 lion years are also known. According to Simpson, then, any living 

 pelecypod genus older than about 250 million years represents a 

 bradytelic genus. Its rate of evolution, measured in this fashion, is 

 outside the horotelic distribution, toward the slow end of the scale. 



One can easily think of many groups of plants and animals in 

 which the rate of evolution appears to be very slow. It must be re- 

 membered, however, that bradytely is a statistical effect, detected by 

 special means of analysis. So to label contemporary organisms that 

 have changed little for long periods of time is thus not strictly cor- 

 rect. Organisms of many different kinds, such as opossums, giant 

 sequoias, crocodiles, and club mosses, appear to have evolved very 

 slowly. Whether they can be classified as bradytelic is another mat- 

 ter. Nor is it possible, with the present state of our knowledge 

 of these or of groups such as the pelecypods, to determine why 

 evolution apears to have been at least partly arrested. There is 

 no reason to believe that such organisms are more "primitive," have 

 a low mutation rate, or are depauperate genetically and less 

 variable. 



The method of study using survivorship curves, which makes 

 bradytely apparent, cannot show the presence of rates faster than 

 those of the horotelic distribution. Organisms with the very rapid 

 rates of evolution referred to as tachytely exhibit this phenomenon 

 presumably for relatively short periods of time and then slow to 

 horotelic or bradytelic rates if they do not become extinct. Rapid 

 evolution or tachytely often is associated with a new mode of exist- 

 ence, what Simpson has called a new adaptive zone, and is charac- 

 teristic of quantum evolution. By adaptive zone is meant here not 

 simply the place where the organisms live but the mode of life in 

 such a place as well. It has been suggested that tachytely is one of 

 the various possible reasons for the gaps in the fossil record which 

 often occur at the apparent origination of a new phyletic hne. If 

 evolution took place at an unusually rapid rate and particularly if 

 the ancestors of the new line were distributed in partially isolated, 

 small populations, the chances of their becoming fossilized would 

 be very small. Simpson's view of a possible relationship between 

 horotely, bradytely, and tachytely is shown in Fig. 11.6. Here a 

 bradytelic Hne produces a tachytelic branch which becomes horo- 

 telic. 



