EVOLUTION AS SEEN IN THE STRUCTURE OF MODERN ANIMALS 29 



radiating branches. Tiius the legs of antelopes and horses are not analo- 

 gous to the legs of moles. 



What does the diagram of adaptive radiation indicate as to the ancestry 

 and evolutionary relationships of the animals included? In the first place 

 we recall that, if the evolutionary interpretation is correct, possession of 

 homologous structures is evidence of common ancestry. All the animals in- 

 cluded in the diagram have modified pentadactyl limbs, hence they must 

 be related to each other. Possession of this common limb pattern does not 

 indicate close relationship, however, since the pattern is shared, not only 

 by all mammals, but also by birds, reptiles, and amphibians — by all verte- 

 brates except fishes, in other words. 



What of the animals grouped together on one of the radiating lines? Are 

 they related to each other? We have just noted that possession of the 

 pentadactyl pattern indicates that they are distantly related, but does their 

 position together on one of these lines indicate that they are closely re- 

 lated? The answer will be evident if we recall that the groupings on the 

 radiating lines are based upon possession of analogous similarities, and 

 that possession by two animals of analogous similarities is not in itself in- 

 dicative of common ancestry. 



Parallel Evolution 



As examples of cursorial adaptation we have mentioned antelopes and 

 horses. These two are placed on the same branch of the diagram because 

 they have limbs serving the same function. But, as just mentioned, posses- 

 sion of analogous similarities does not indicate relationship. Both ante- 

 lopes and horses are believed to have evolved from ancestors having short, 

 pentadactyl limbs ("terrestrial," Fig. 3.4); both have achieved elongated, 

 slender limbs adapted for rapid running. But the antelopes have de- 

 veloped two toes on each foot (after the manner of the sheep shown in 

 Fig. 3.2), while the horses have developed but one toe (Fig. 3.2). Both 

 have achieved the same goal but have done so separately and in differing 

 ways. Thus the horse and antelope form an example of what is termed 

 parallel evolution — two forms independently undergoing similar changes 

 in the courses of theTr'~respective evolutionary histories. Another example 

 of parallel evolution is afforded by the flipperlike forelimbs of seals and 

 those of manatees and sea cows. Seals and sea cows are not closely related, 

 but through parallel evolution they have developed similar forelimbs. 



If forms which have independently developed similar adaptations are 

 far removed from each other in the scale of relationship, their evolution 

 toward the common adaptation is frequently termed convergent evolu- 



