212 I The Process of Evolution 



isms that seem to be on the verge of fragmenting into multiple en- 

 tities, and multiple entities that appear to be of rather recent origin. 

 A series of examples is presented first, to give the reader some 

 "feel" for the types of patterns that occur. The probable causes of 

 these patterns are then discussed, illuminated with further brief 

 examples. The very distinct forms will be dealt with in Chap. 11. 

 In what follows, the term character will denote any trait that 

 varies in the overall group under discussion. Thus the presence or 

 absence of plastids is a character when one considers all organisms. 

 For any given organism one can determine whether or not it 

 possesses plastids. Their presence or absence is not a character in 

 roses for they are uniformly present. Femur length is a character in 

 man because it varies within the group and can be measured for 

 any individual. Femur length is also a character when adult mice 

 and adult men are compared, but the ranges of observed variation 

 in this character are not overlapping. Such discontinuities in variation 

 ( in single characters or in constellations of characters ) are here re- 

 ferred to as gaps. 



EXAMPLES OF tD I F F E R E N T I AT I N 



Continuous Geographic Variation 



In many instances, variation is sufficiently continuous so that no 

 dividing lines between segregates are obvious. Variation in some 

 characters may occur in gradients. These gradients in single char- 

 acters are called dines, and the variation is then called clinal. 



Color, Pattern, and Size Variation in Animals. Geographic vari- 

 ation in color, pattern, and size is one of the most widely studied of 

 all biological phenomena. This variation is often of the sort already 

 described in the previous chapter ( Biston, Cepaea, Natrix ) in which 

 populations diflFer primarily in the frequency of diflFerent types of 

 individuals present. Another example is the North American tiger 

 swallowtail butterfly, Papilio gluucus, some populations of which are 

 composed of yellow and black striped (tiger) males and females, and 

 other populations of tiger males, tiger females, and uniformly dark- 

 brown females. In southern Canada and the extreme northern 

 United States the populations of P. glaucus are composed only of 

 tiger individuals. In south central Florida the proportion of dark- 

 brown females is very low (6 to 8 percent), and in southern Florida 

 dark females may be completely absent. In most of the southern 

 United States, however, populations show high frequencies (up to 

 50 percent) of dark-brown females. 



