The Differentiation of Populations 229 



Therefore they are allopatric, although in one case they both Hve on 

 the same island. The extremes of differentiation within the genus 

 can be seen (Fig. 10.8) by comparing the skulls of the broadly 

 svmpatric Gcospiza magnirostris (the large ground finch) with 

 Ceiihidca oUvacca ( the warbler finch ) . 



Darwin's finches show a pattern of differentiation opposite to that 

 found in most groups of birds. The most closely related forms of 

 these finches differ primarily in the size and shape of the beak, 

 whereas closely related forms of other birds are usually differenti- 

 ated most strongly by plumage color. The very closely related Asi- 

 atic nuthatches, Sitfa tephronota and S. neummjer, are clearly 

 differentiated by plumage pattern as well as bill length but only 

 where they are sympatric (Fig. 10.9). Where their ranges are sep- 

 arate they are almost indistinguishable. This exaggeration of differ- 

 ences in an area of sympatrv is an example of character displacement, 

 a phenomenon common in birds. Other instances have been de- 

 scribed in ants, beetles, crabs, fishes, and frogs. Two kinds of 

 termites have been reported to swarm at the same time of day where 

 they are allopatric and at a different time of day where their ranges 

 overlap. Possible causes of character displacement will be discussed 

 in the second half of this chapter on pages 242-244. 



Host Preference in Parasitic Organisms 



Differentiation in host preference is widespread among parasitic 

 organisms. This phenomenon has been studied in such diverse or- 

 ganisms as cuckoos, human lice, and nematodes. Cuckoos have 

 developed an unusual form of differentiation. They show what has 

 been called "brood parasitism"; that is, they lay their eggs in the 

 nests of other kinds of birds. In most cases the cuckoo egg is incu- 

 bated by the foster parents, and the voracious cuckoo hatchling 

 crowds its pseudosiblings out of the nest, eventually monopolizing 

 the food brought by its adopted parents. Usually the egg laid by the 

 cuckoo bears a remarkable resemblance to that of the host bird 

 (Fig. 10.10). Cuckoo species seem to be subdivided into groups, 

 each of which tends to lay its eggs in the nests of only one kind of 

 host bird. These subdivisions, called gentes, are not geographically 

 isolated from each other, individuals of one being found in close 

 proximity to individuals of one or more of the others. 



There are two distinct forms of human Hce (Pediculus humanis), 

 head lice and body lice, which differ strongly in their "ecology" but 

 are nearly identical morphologically. Head lice, as the name implies, 

 are found primarily in the relatively fine hair of the head. Their eggs 

 are glued to hair. Body lice, on the other hand, hve in the clothes, 



