Species and Species Change 135 



shrink again, and the birds will become relatively rare (Mills, 

 1951). Following the same reasoning some of the rare, "less suc- 

 cessful" species of today might be adapted to conditions which 

 tomorrow would cover a much more extensive area with a conse- 

 quent enlargement of the species range. 



THE SPECIES AS AN ADAPTIVE UNIT 



A comparison of living and fossilized biotas demonstrates that, in 

 their passage through time, phylogenetic lines have changed in all 

 manner of characters, manifested morphologically, physiologically, 

 or ecologically. The genetic composition therefore changed to a 

 comparable extent through natural selection. The theoretical action 

 of natural selection, outlined in Chapter 4, would seem to be a 

 straightforward, albeit complex, mechanism. When the known or 

 inferred natural products of evolution are examined in relation to 

 this theory, however, many paradoxes appear. The explanation of 

 many changes in phylogenetic lines is therefore still on a specula- 

 tive basis. A working hypothesis combines the concepts of genetic 

 homeostasis with ideas arising from phylogenetic analysis. 



Genetic Composition 



The genetic variation demonstrated experimentally in controlled 

 populations appears to extend throughout the species' natural range. 

 The variation may be conspicuous, as in the American butterfly 

 Glaticopsijche xerces (Fig. 56) (Downey and Lange, 1956), or 

 inconspicuous and demonstrable only by careful experiment and 

 observation. In species with a low vagility or other attributes which 

 result in little inter-population mixing, the genetic variation may 

 be segregated geographically; this is true especially of many birds 

 (Pitelka, 1951) and small mammals having well-marked subspecies 

 (Hoffmeister, 1951; Hall and Kelson, 1959). In other instances the 

 selection exercised by local environments has produced the same 

 result in the face of moderate vagility; this has been demonstrated 

 best in plants (Clausen, 1951). In the great bulk of vagile or- 

 ganisms, such as many insects, the characteristics and genetic com- 

 position tend to be either similar over the whole range, or to 

 grade from one edge to the other. 



Little information is available concerning local short-term changes 

 in genetic composition over an entire species range. The observed 

 changes concerning industrial melanism in moths, cited in the 



