Major Patterns of Variation 261 



scale is most useful for rather long periods of time. For shorter 

 spans of geologic time, it is often necessary to use a relative scale, 

 placing organisms earlier or later in time, according to their relative 

 positions in layers of sediment, the level of their pollen in varves, 

 etc. For the most part, the evolutionist must be content with an ap- 

 proximate age for fossils, even though he may be almost certain of 

 the sequence of appearance of a given series of forms. 



Rates of Evolution 



Rates of phyletic evolution and of splitting vary greatly. Evolution- 

 ary rates are complex functions of many factors in the environment 

 and in the organism and of interactions among these factors. Changes 

 in genetic systems, the disappearance or appearance of predators or 

 parasites, changes in temperature, increasing salinity of the sea, rise 

 in sea level, mountain building, and change in intensity of solar 

 radiation are a random sample of the factors that may act and inter- 

 act to affect the rate of evolution. 



If the rates of evolution for the various members of a major tax- 

 onomic grouping are plotted as a frequency diagram, they form an 

 asymmetrical distribution with the modal rate nearer the maximum 

 rate. Simpson has called this distribution horotelic, and the rates 

 falHng within it are horotelic rates. In most groups it will be found 

 that some organisms apparently evolved at rates either slower or 

 faster than those making up the horotelic distribution. These have 

 been termed bradytelic and tachytelic rates, respectively. 



The land carnivores and the pelecypod mollusks may be compared 

 to illustrate horotelic rates. Simpson has calculated survivorship 

 curves for these groups (Fig. 11.4). The fact that the mean sur- 

 vivorship for genera of the mollusks is 78 million years in compari- 

 son with 8 million years for a genus of mammals strongly suggests 

 that the carnivores have evolved perhaps ten times faster than pelec- 

 ypods (in terms of duration of arbitrarily established genera). If it 

 is postulated that correlation of survivorship and rate of evolution is 

 perfect and positive, then a frequency distribution of rates can be 

 plotted (Fig. 11.5). These distributions are quite similar, even 

 though the absolute rates differ markedly, and form the horotelic dis- 

 tributions for the Pelecypoda and the Carnivora. 



If the pelecypods are now examined in greater detail, with sur- 

 vivorship of exclusively fossil genera compared with genera arising 

 in the past and still existing, the phenomenon of bradytely is re- 

 vealed. Again using Simpson's calculations and graph (Fig. 11.4), 

 it can be seen that actual survival is greater than expected survival. 



