270 The Process of Evolution 



Competition 



The same problems arise with respect to the consequences of two 

 organisms meeting in the same adaptive zone. It is usually thought, 

 or at least stated, that the inevitable result is some sort of "competi- 

 tion." The term competition has been used in so many different ways 

 that its general unqualified use conceals rather than reveals informa- 

 tion (see Chap. 13). Furthermore, with respect to plants, the term 

 usually must have a meaning rather different from its possible mean- 

 ings when applied to animals. By extension, the word competition 

 has been used to refer to situations where major groupings of organ- 

 isms have replaced others in adaptive zones of great breadth. Thus 

 it is often said that placental mammals have eliminated, through 

 competition, the marsupials and monotremes over much of the earth's 

 surface or that mammals as a group have completely exterminated 

 the mammal-like reptiles. The angiosperms appear to have replaced 

 the gymnosperms in most parts of the world also; this too is described 

 as the result of competition somehow extended to the level of higher 

 taxonomic categories. In retrospect, one can look at any one of these 

 situations and say that, by definition, the surviving organisms were 

 "better adapted" than those which became extinct. Extinction of 

 one group, however, is not the only possible outcome of "competitive 

 occupation" of the same adaptive zone. One or both of the occupants 

 may become more specialized, thus restricting or eliminating the 

 interaction between the types. Cockroaches, at one time the domi- 

 nant group of insects, have changed from what was once probably 

 a generalized herbivorous or omnivorous zone and now make their 

 living mainly as scavengers. Their former zone seems to have been 

 highly "fragmented" among a great many insect specialists, particu- 

 larly of the orders Hemiptera, Homoptera, Orthoptera, Coleoptera, 

 Lepidoptera, Diptera, and Hymenoptera— mostly relative new- 

 comers on the evolutionary scene. 



Convergence 



Sometimes organisms that are not closely related enter similar adap- 

 tive zones and, as a result of selection, come to bear a superficial 

 resemblance to each other. Classic examples are those of whales and 

 fish and desert succulent plants of various families. In the aquatic 

 environment, the selective advantage of genotypes producing pheno- 

 types with a certain type of streamlining is obvious. Desert plants 

 (Fig. 11.8) often resemble each other in having a thick cuticle and 



