Major Patterns of Variation | 271 



low surface-volume ratio, both of which reduce water loss, and are 

 often armed with spines which tend to discourage desert animals 

 from eating them. The three plants of Fig. 11.8 belong to three sepa- 

 rate famihes, Euphorbiaceae, Asclepiadaceae (Old World), and 

 Cactaceae (New World). The spines in all three kinds are derived 

 from diflFerent structures. 



Euphorbiaceae 

 Africa 



Asclepiadaceae 

 Africa 



Cactaceae 

 New World 



Fig. 1 1.8 I Convergence in desert plants. Left, Euphorbia, Euphorbiaceae, 

 Africa; center, Huernia, Asclepiadaceae, Africa; right, Cereits, Cactaceae, 

 Latin America. 



More complex are the various patterns of mimicry in which selec- 

 tion favors genotypes that are least likely to be destroyed by preda- 

 tors. An example of mimetic convergence is shown in Fig. 11.9. In 

 this case Alcidis agathijrsus (moth) and Ideopsis daos (butterfly) 

 are presumably distasteful to predators, whereas Papilio laglaizei 

 (butterfly) and Cyclosia hestinioides (moth) are thought to be 

 palatable. Selection has apparently favored any genotypes within 

 populations of the latter two that produced phenotypes resembling 

 in any degree the protected forms. 



Convergence among relatively closely related forms is sometimes 

 called parallelism. For example, several lines of mammal-like reptiles 

 probably independently acquired characteristics by which we define 

 mammals. There is a continuum between convergence in the strict 

 sense and parallelism, and the difference between the two is un- 

 important. 



