Major Patterns of Variation 265 



from which they arose. It is interesting to note that the "gliding 

 zone" still remains largely unoccupied (except for a few fish, 

 lizards, snakes, and mammals), although the mammals have suc- 

 cessfully entered the aerial zone (or a specialized part thereof), in 

 part at least, by specializing in night flying and sonar hunting. 



An outstanding example of adaptive radiation in animals is the 

 tremendous diversity of the marsupials in Australia. In the absence 

 of placental mammals, marsupials developed grazing forms (some 

 kangaroos), burrowing forms (marsupial moles), forms resembling 

 tree and flying squirrels (phalangers), rabbit-like forms (hare 

 wallabies), wolf -like carnivores (Tasmanian wolves), badger-like 

 carnivores (Tasmanian devil), ant-eating carnivores (banded ant- 

 eater), and arboreal forms with no obvious placental equivalent 

 (koalas, tree kangaroos). Unfortunately for the fields of comparative 

 psychology, linguistics, and sociology, no marsupial equivalents of 

 the primates appeared. The poisonous snakes of the family Elapidae 

 (cobras and their relatives) similarly show adaptive radiation in 

 Australia. They have produced, among other types, forms super- 

 ficially resembhng the vipers or pit vipers ( Viperidae or Crotalidae). 

 There are, of course, many other examples of adaptive radiation. 

 The Galapagos finches and African lake cichlids are good examples 



Fig. 1 1.6 I Diagram to show a representation of horotely, bradytely, and 



tachytelv. See text. ( From Simpson, 1953, The Major Features of 

 Evolution, Columbia University Press.) 



1 1 



O <J^ 



'^ c 



-o a> 



S E 



8 ^ 



a> =3 



> cr 



u-i <x> 

 a. >- 



TO 1^ 



■a o 

 < 



Shifting 

 adaptive 

 zone, with 

 horotelic 

 populations 



ion 



Stable 

 adaptive 

 zone, with 

 bradytelic 

 populations 



Time 



