THE GROUPS AND ANCESTRY OF ORGANISMS 



121 



Figure 7.11 Adaptive radiation in the forelimbs of mammals. A, tenrec, a large insectivore with 

 limb structure much like the shrew except for size; B, bat wing; C, mole; D, horse, E, rhinoceros; 

 and F, deer. Each is modified from the primitive form (approximated by the tenrec) by changes of 

 proportion, fusion of parts, or loss of parts. Not drawn to the some scale. {From Edward O. 

 Dodson, Evolution: Process ond Product, rev. ed., Reinhold Publishing Corp., New York, 1960.) 



ful, or lethal, genes. But this does not explain wide- 

 spread extinction, a biogeographic subject discussed 

 later. 



Adaptive Convergence and Parallelism. A general 

 impression is that the more closely allied the organ- 

 isms, the more similar their appearance, and the 

 more distantly related, the less similar their appear- 

 ance. One can well ask, what is meant by related? 

 For present purposes, let it imply time. Using time 

 alone as a basis for affinity does not allow strict as- 

 sociation between structural differentiation and kin- 

 ship. For example, adaptive radiation and ortho- 

 genesis affect degree of relationship, and convergence 

 and parallelism present additional problems. Con- 

 vergence is the tendency for different lines of evolu- 

 tionary development to approach one another. This 

 means that ancestors quite remote and different from 

 one another can give rise to populations that are more 

 similar structurally and functionally and give the im- 

 pression of similar or identical ancestry. Parallelism 

 causes less difficulty. It is the continuation through 



time of a particular amount of differentiation between 

 two or more lines of evolutionary development, and 

 causes the time of common ancestors to appear less 

 remote than it was. 



Both convergence and parallelism are associated 

 easily with environmental conditions. Convergence 

 exists when, for some reason, organisms occupy either 

 the same environments or environments approaching 

 one another. Parallelism takes place when environ- 

 ments maintain an equivalent differentiation 

 throughout a period of geological time. 



TEMPO 



The speed of evolution is highly variable. There 

 are periods of geological time when most life had a 

 relatively high, moderate, or low rate of evolution; 

 most frequently, however, different groups of organ- 

 isms evolve at different rates, from very slowly to very 

 rapidly. This does not mean that the speed of evolu- 

 tion is constant in a line of development or even in a 



