Natural Selection 109 



If the same centrifugal selection pressure is maintained in the 

 same direction over a long period of geologic time (what is termed 

 straight-line selection), changes continue in the same direction and 

 form trends. Simpson (1953) stated that in paleontological data 

 long-term trends are extremely common in some characters. As an 

 example he cites the sea urchin Micraster from the English chalk 

 of the late Cretaceous. A continuous series of overlapping fossil 

 populations show trends for increasing relative breadth and height, 

 anterior migration of the mouth, and development of a deep anterior 

 notch at the margin. The evolution of horse teeth is another exam- 

 ple. In phylogenies based on living species similar trends are com- 

 mon. Good examples are the series of changes in the claspers of the 

 caddisfly genus Helicopsyche (Fig. 37) and in the genitahc parts 

 of mosquitoes (Fig. 38). 



Trends in unrelated lineages may follow parallel courses in what 

 appear to be adaptive changes in each line to identical environ- 

 mental conditions. In cave species of fish, salamanders, crustaceans, 

 and insects, for example, eyes are usually reduced in size or absent; 

 in burrowing forms such as the gopher, mole, and molecricket 

 some sort of digging foot is usually developed; and in large swim- 

 ming animals almost the same sort of paddle-like appendages, 

 streamlined body, and large tail are evolved. In many desert plants 

 spines evolve and the leaves become thick and relatively impervious 

 to desiccation. 



These trends usually involve only a few radical structural 

 changes; the great bulk of the characteristics of the species vary 

 around a constant pattern. In animal phyla in particular the great 

 base of "conservative" characters provide the information for deduc- 

 ing the relationships of the groups. Thus the skull and skeletal parts 

 can be homologized within broad limits through the entire series 

 of vertebrate animals and in considerable detail through any one 

 class such as the Reptilia, Aves, or Mammalia. Although it seems 

 that this circumstance may be more easily recognized in the animals 

 than in the plants, in the latter, detailed histological studies dem- 

 onstrate remarkable evidence of relationships in stem and leaf 

 structure and other parts (Bailey and Nast, 1943; Prat, 1936). 



RATE OF CHANGE 



Rate of change has two distinct aspects: the occurrence of ad- 

 vantageous mutations, and the intensity of the selection pressures 

 influencing their subsequent incorporation into the gene pool. 



