264 I The Process of Evolution 



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Deciles 



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Deciles 



Fig. 1 1.5 I Frequency distributions of evolutionary rates in genera of 

 pelecypod mollusks and land carnivores. Histograms are based on 

 survivorship of extinct genera (see Fig. 11.4) and on the assumption of 

 perfect negative correlation of survivorship and rates of evolution. 

 Ordinate, percent of genera in each class; abscissa, rate of evolution 

 expressed in deciles so that histograms may be compared despite differing 

 absolute rates. Normal curves equal in area to the histograms are drawn 

 for comparison. (From Simpson, 1953, The Major Features of Evolution, 

 Columbia University Press. ) 



MAJOR EVOLUTIONARY PATTERNS 



Adaptive Radiation 



The birds perhaps represent a group that experienced an initial 

 period of rapid evolution. Tliey split rapidly from the archosaurian 

 reptiles in the Mesozoic, apparently perfecting flight through selec- 

 tion for increased flight range and agility in a group of climbing- 

 gliding reptiles. Once true flight was attained, its advantages (for 

 escape, dispersal, food seeking, etc.) led to rapid proliferation of 

 the group. Such proliferation is sometimes called adaptive radiation. 

 Apparently there was sufficient overlap between the flying and glid- 

 ing adaptive zones that the more successful birds replaced the group 



