Major Patterns of Variation 267 



than land ones. Other organisms in the same general habitat may 

 not be bradytelic. Similarly, the "cause" of an instance of tachytely 

 is usually impossible to determine. Entry into a new adaptive zone 

 clearly is involved. It is almost impossible to avoid visualizing adap- 

 tive zones and thinking of discontinuities (nonadaptive zones) be- 

 tween them even though they represent an abstract space whose 

 many dimensions are all the factors of the environment, including 

 other organisms. In the past, as today, the situation must be exceed- 

 ingly intricate, involving complexly (and often cyclically) shifting 

 gradients. 



In some way, it would appear that certain lineages gain "access" 

 to a new adaptive zone that is constantly changing. This new zone 

 must be "contiguous" in a multidimensional sense. The organisms 

 must be physically near it, happen to be provided (by the mech- 

 anisms of population genetics ) with genotypes that will survive and 

 function in it, and be able to exist with the organisms already present 

 or possibly to replace them. Having successfully "entered" an adap- 

 tive zone, the group may then diversify to occupy subzones of the 

 original zone or spread further into contiguous zones. The amount 

 of proliferation of adaptive subtypes seems to be primarily a func- 

 tion of the extent ( diversity ) of the adaptive zone. The Strepsiptera, 

 curious, highly modified insects which are endoparasitic in bees, 

 wasps, and other insects, seem to have entered a rather narrow adap- 

 tive zone. The birds apparently have found a relatively wide one. It 

 is important also to remember that what today appears to be a rela- 

 tively unstable (and relatively unoccupied) intermediate zone may 

 at one time have been quite stable. The successful entry of reptiles 

 into the completely aerial zone quite possibly "unstabilized" the 

 gliding zone. 



As has been emphasized in Chap. 5, there is a strong tendency for 

 biologists to fragment the environment to serve their own analytical 

 purposes. While the concept adaptive zone refers to a way of life 

 rather than a physical area, it nevertheless represents an artificial 

 fragment of the space-time continuum in which the organisms are 

 found. In the final analysis, it is the total situation that evolves, even 

 though we may choose to separate organic evolution from environ- 

 mental change. The results of our study depend to a very large ex- 

 tent on the choice of artificial distinctions. Science progresses by 

 comparing the results of study of similar situations from different 

 points of view until a relatively unvarying answer is obtained. It 

 seems clear that the study of the evolution of organisms together 

 with changes in "their habitat" has not reached the point where 

 answers are unvarying. 



