666 



ECOLOGY AND EVOLUTION 



categories, such as families, while others 

 may distinguish species. Certain adapta- 

 tions have convergently and independently 

 evolved in numerous phylogenetic branches. 

 Some adaptations are highly successful and 

 common, while others are comparatively 

 rare. 



Radiate evolution, of course, is found in 

 physiological sequences as well as in mor- 

 phological characters. The physiological 

 adaptations of both plants and animals to 

 desert conditions are an illustration. The 

 diflFerential toleration of closely related spe- 

 cies of ants to different degrees of humidity 

 (p. 652) probably represents radiate evolu- 

 tion. 



Behavior radiation is sometimes apparent. 

 Wheeler (1930) discusses the possible 

 phylogenetic stages in the caenogenetic 

 evolution of the pit-making behavior of ant 

 lions (Myrmeleonidae). The ancestral 

 Hemerobiid-like forms had predatory lar- 

 vae that walked about and fed on the 

 juices of their prey. The maxilla enclosed 

 in a groove on the ventral side of the man- 

 dible formed a sucking tube. The next stage 

 in evolution is represented by lethargic 

 larvae (Palpares) that bury themselves in 

 sand and detritus. The third stage is seen 

 in larvae that make pitfalls, but move both 

 forwards and backwards (Myrmocaelnrus) . 

 Finally, in Myrmeleon, we find a sedentary, 

 pit-making ant lion with exclusive retro- 

 grade locomotion. 



When different stocks radiating into 

 similar habitats become adapted to the 

 same ecological factor or combination of 

 factors, the environmental influence upon 

 evolution through selection becomes clearer. 

 If the compared structures are homologous, 

 the evolution is termed parallel; if the com- 

 pared structures are analogous, the evolu- 

 tion is called convergent (Simpson, 1937; 

 Richardson, 1942; see p. 631, Fig. 234). 

 It may be assumed that parallel evolution 

 is the result of one or a few genetic 

 changes, sometimes even of homologous 

 genes or parallel mutations, while conver- 

 gent evolution is genetically more complex, 

 and commonly involves selection of differ- 

 ent genetic characters in two or more spe- 

 cies within the same environment. Parallel 

 evolution may be somewhat adaptive, or 

 may be neutral or nonadaptive (Gates, 

 1936). Convergent ecological types may 

 sometimes be found in similar but geo- 



graphically separated environments, thus 

 giving rise to the concept of ecological 

 equivalence (pp. 470, 471, 492, 493). Con- 

 vergence may occur in unrelated organisms 

 that compete in the same general habitat. 

 The factor or factors to which the conver- 

 gent types have become adapted deter- 

 mine the degree of their competition. 



A notable example of convergence of 

 ecologically equivalent organisms in geo- 

 graphically separated similar habitats is 

 found in the adaptive similarities of the 

 marsupials of Australia to various types of 

 placental mammals in the rest of the world. 

 It is generally conceded that Australia be- 

 came separated from the Indomalayan con- 

 tinental mass during the Mesozoic before 

 the rise of placental mammals, so that only 

 monotremes and marsupials were present in 

 the original Australian mammalian fauna. 

 There are only a few species of placental 

 land mammals, in addition to bats, indige- 

 nous to Australia. These are the dingo dog, 

 probably brought in by Pleistocene man, 

 and a few rodents which could have been 

 transported by natural rafts in relatively 

 recent times (Raven, 1935). Marsupials 

 (phalangers) reached the Celebes, but did 

 not become established in Borneo, pos- 

 sibly because of more severe placental com- 

 petition (Mayr, 1944). Thus, the main 

 marsupial fauna of Australia and Tasmania 

 evolved during the Tertiary without compe- 

 tition from the more efficient placental 

 mammals, and remarkable ecological equiv- 

 alents were independently evolved. The 

 Tasmanian wolf (Thylacinus cynocephaliis) 

 is a marsupial convergent with the canine 

 carnivores; the banded anteater (Myrmeco- 

 biiis fasciatus) is a marsupial convergent 

 with the South American anteaters; the 

 marsupial mole (Notoryctes typhlops) is 

 astonishingly convergent with the golden 

 moles of South Africa; the locomotion of 

 the kangaroo is duplicated by various 

 jumping rodents; and the flying phalangers 

 are convergent with the flying squirrels and 

 colugo (the so-called flying lemur). 



Convergent evolution may be seen in the 

 adaptations to specialized food (Fig. 164). 

 Those mammals that have become adjusted 

 to a diet of ants and termites have cylindri- 

 cal tongues and a reduction of the teeth. 

 Convergent evolution of this feeding ap- 

 paratus in five orders of mammals is 

 found in the spiny anteater (Monotremata), 



