It is obvious to anyone observing the variation of living things 

 in nature that organisms do not vary continuously. Variants of 

 one type of organism may be arranged in a continuum, but there 

 are gaps in the variation from continuum to continuum. Plants and 

 animals, viewed by our usual techniques of studying organisms, 

 seem to be aggregated into discrete or nearly discrete clusters 

 usually called species. Certainly the living world may be structured 

 by the scientist in many ways diflFerent from this customary taxo- 

 nomic one; some of these may be of considerable interest to the 

 evolutionist. In the last chapter of this book some of the problems 

 involved in perceiving and describing structure and pattern in 

 nature are discussed. Nevertheless, it is possible to recognize taxo- 

 nomic units and to classify them; this has led biologists to attempt 

 to understand the origin of such units in nature. This generally 

 has been studied from the point of view of how a single sup- 

 posedly interbreeding population can differentiate into discrete 

 clusters. The processes presumed to be involved make up what is 

 frequently referred to as speciafion. • 



Elucidating the mechanisms of speciation often has been regarded 

 as the central problem of evolution. Darwin's classic work was en- 

 titled The Origin of Species . . . , and many monographs in both 

 botany and zoologv in recent years have emphasized the so-called 

 species level of recognizable biological difference. This emphasis 

 may have had the effect of obscuring some exceedingly important 

 and interesting problems usually thought of as falling within the 

 province of ecology (for example, the nature and evolution of com- 

 munities of plants and animals). However, in discussing here the 

 question of how recognizable aggregates of similar organisms arise 

 in nature, we for the moment shall accept the commonly used 

 analyses and designations. 



One usually gains the impression from even a casual study of 

 living things that there is a spectrum of degree of similarity among 

 organisms. Some forms appear to be very distinct from all others; 

 some appear to intergrade almost imperceptibly with others that are 

 closely similar. In approaching the problem of how populations 

 become differentiated, it will be useful to consider the nature and 

 size of the gaps in variation between clusters of similar organisms. 

 In this chapter, examples from the spectrum of variation will be 

 discussed, examples in which the degree of differentiation is rela- 

 tively small. To put it another way, we shall examine situations that 

 seem to be close to branch points in the evolutionary tree— organ- | 211 



