ECOLOGY AND ISOLATION 



615 



rect, this is a case of speciation correlated 

 with initial habitat isolation. 



Where the parasite has not speciated 

 with the host— the instances are numerous 

 —it may be presumed that the parasite is 

 evolving more slowly than the hosts, and 

 consequently a single parasite infects a 

 phylogenetically related group of host spe- 

 cies; or the parasite may be able to pass 

 from one host to another fairly readily, as 

 fleas do. 



Among the flukes, ectoparasitic Mono- 

 genea are more likely to be strictly confined 

 to particular hosts than are the endopara- 

 sitic Digenea (Baylis, 1938). Nitzschia 

 and Diclibothrium are found only on stur- 

 geons, Discocotyle only on fishes of the 

 family Salmonidae, and Dactylogyrus only 

 on Cyprinidae. The species are often con- 

 fined to a single host species or a group of 

 closely related host species. A number of 

 groups of cestodes show similar specific cor- 

 relation with taxonomic groups of hosts. 

 Physiologic races or species of nematodes 

 that are morphologically indistinguishable 

 seem to be confined to certain hosts (As- 

 caris lumhricoides of man and pig; Ancy- 

 lostoma caninum of dog and cat; Hymeno- 

 lepis nana (of man and rodents). 



Specificity of hosts is more marked in 

 bird than in mammalian cestodes, among 

 which the same genus (not the same 

 species) may occur in several host orders 

 (Baer, 1933). Among the nematodes, 

 the species with a direct life cycle show 

 a greater degree of host specificity than 

 those with an indirect fife cycle. Bay- 

 lis (1938) suggests in explanation that the 

 larvae of the forms with direct life cycles 

 are younger and less resistant at the time of 

 penetration and may be less tolerant of 

 hosts to which they are imperfectly 

 adapted. Forms with an indirect life his- 

 tory usually show greater specificity for 

 the intermediate host than for their final 

 host, possibly for the same reason (p. 702). 



Host specificity is notably more highly 

 developed in the cestodes, which have no 

 free-living stage and are closely inbreeding, 

 than in the trematodes (Baylis, 1938). A 

 high degree of specificity usually denotes 

 evolutionary specialization, and parasites 

 inhabiting more than two hosts in a 

 single stage are usually the more primi- 

 tive. Variation of characters of a phys- 

 iologic nature would allow for host selec- 



tion of races. Preadaptation to invasion of 

 new hosts may be provided by adaptation 

 to related host species (p. 643). However, 

 numerous parasites show no correlation 

 between their own phylogeny and that oi 

 their hosts, and only in special cases may 

 one assume parallel evolution of host-para- 

 site pairs. The cestode genus Tetrabothrius 

 occurs in whales, seals, and fish-eating 

 birds of several orders, indicating transfer 

 from one host to another through fish feed- 

 ing. 



Host specificity is striking among certain 

 species of termitophilous beetles (pp. 718- 

 721; Seevers, 1937; Emerson, 1935). The 

 socially adapted symphiles are far more 

 likely to be confined to the nests of one 

 host species than are the more generalized 

 synoeketes such as the species of the 

 staphylinid genus Perinthus (Fig. 255). 

 The specialized termitophilous fauna can be 

 used as a means of recognizing closely re- 

 lated host species living in the same locaHty 

 and in the same ecologic niche. Reproduc- 

 tive isolation of the host termites is here 

 thought to be associated with brother-sister 

 matings over a period long enough to allow 

 the divergence of the species, though it is 

 possible that some unknown ancient geo- 

 graphic isolation occurred. In a few in- 

 stances, two related species of termitophiles 

 may be found in the same host nest, al- 

 though the original isolating mechanism is 

 unknown (the staphylinids Spirachtha mi- 

 rabilis and S. schiodtei in the nest of the 

 termite Constrictotermes cavifrons; Mann, 

 1923; Fig. 259). 



In India two races of the large hawk- 

 cuckoo (Hierococcyx sparverioides) lay 

 eggs of different color. Each hawk-cuckoo, 

 though inhabiting the same area, is strictly 

 parasitic on host birds with eggs the color 

 of its own race (Baker, 1942; see p. 670). 

 It would appear that such racial divergence 

 is associated with habitat isolation. No 

 knowledge is available of the reproductive 

 isolation of these races, but the evidence 

 points to a genetic egg coloring, and the 

 dimorphism correlated with the associated 

 host eggs could only be maintained by 

 partial reproductive isolation and selection. 



Host selection through conditioning has 

 been referred to as the "Hopkins' host-se- 

 lection principle." Thorpe (1940) cites the 

 experimental transfer of the ichneumonid, 

 Nemeritis canescens, to an unusual host 



