EVOLUTION OF INTERSPECIES INTEGRATION AND ECOSYSTEM 



709 



disease." We believe that further analysis 

 assists in clarifying the problem (p. 260). 



Variations in pathogenicity resulting 

 from individual immunity acquired through 

 individual exposure, not being correlated 

 witli genetic differences, obviously should 

 be distinguished from evolutionary varia- 

 tion. 



In judging particular cases, one has to 

 have some evidence of relatively ancient 

 and relatively recent associations and ef- 

 fects. The phylogeny of some organisms, 

 together with their ecological associations, 

 is too poorly known to form any judgment. 



Ball cites instances in which parasites 

 are artificially introduced into new hosts 

 without gross pathological symptoms. Non- 

 pathogenic intestinal flagellates of man and 

 certain mammals (Chilomastix from guinea 

 pigs. Trichomonas parva from rats, and T. 

 hominis from man), introduced into a new 

 host such as the chicken, carry on their 

 normal hfe cycles without producing any 

 disease. A species of malarial parasite first 

 isolated from a wood thrush has been trans- 

 mitted to a canary, a duck, and a chick, 

 with resulting low pathogenicity in the 

 new hosts. A strain isolated from the Eng- 

 lish sparrow killed 60 per cent of the ca- 

 naries into which it was introduced, but 

 produced no symptoms in ducks, even 

 though it completed its life cycle. Three 

 species of malarial parasites found in 

 nature in the monkey, Silenus (Macacus) 

 irus, produce no symptoms, or only slight 

 ones in their normal host. When introduced 

 into another monkey, Silenus {Macacus) 

 rhesus, one species produced severe disease 

 in the new host, and the other caused only 

 a mild infection. The organism producing 

 severe disease in the new monkey host, 

 however, produced only a mild infection 

 when transmitted to another new host- 

 namely, man. 



Lack of pathogenicity in a new host may 

 sometimes be explained as a lack of evolu- 

 tionary adaptation to the new host on the 

 part of the parasite, rather than adaptation 

 of the host toward a toleration of the para- 

 site. Equivalent low pathogenicity, there- 

 fore, might indicate a recent association 

 with no evolution of adaptation on the one 

 hand, or a long period of mutual evolution 

 toward toleration on the other hand. 



Pathogenicity of a parasite in its individ- 

 ual host might also become greater dur- 



ing its evolutionary history without dis 

 operation, provided the proportion of in- 

 fection in the total population of hosts were 

 not great. We must remember that the 

 evolution of balance in such an instance 

 may be between species populations, and 

 not necessarily between individuals. After 

 all, the majority of highly adapted preda- 

 tors are severely "pathogenic" to their in- 

 dividual prey, but if this mortahty does not 

 destroy all the individuals of the species, it 

 may set up a selection pressure that pro- 

 duces an evolution of the exploited species 

 toward several adjustments. These adjust- 

 ments, not necessarily all developed in any 

 single instance, include {a) individual de- 

 fensive adaptation, {h) reduction of mor- 

 tality to a tolerable rate, (c) increase in 

 reproductive potential to balance the mor- 

 tality, and {d) a community association 

 that makes it more difficult for the predator 

 to find the prey, or that provides other 

 available food that may relieve some of the 

 predator pressure. 



Nash (1944) points out that selective 

 processes that have increased the repro- 

 ductive potential of an animal such as the 

 human tapeworm {Taenia saginata) to a 

 degree that one billion fertilized eggs may 

 be produced by a single individual during 

 its life time, nevertheless have not increased 

 the individual's chances of survival. This 

 is another indication that selection acts 

 upon populations rather than upon individ- 

 uals alone (p. 701). 



Chance efi^ects such as those discussed 

 under Preadaptation (p. 642) require 

 mention. If a parasite introduced into two 

 new hosts with which it has had no evolu- 

 tionary history is pathogenic in one host 

 and harmless in the other, but survives in 

 both, one may allow for the possibility that 

 the variation in adjustment is the result of 

 chance. Or it may depend upon the degree 

 of similarity of all ecologic factors in both 

 the new and the older environment to 

 which it had become adapted through a 

 long evolutionary association. The ecologic 

 factors may involve unknown subtle aspects 

 of the physiology, behavior, population 

 biology, or community relations of the host 

 or parasite. Similar physiological character- 

 istics of taxonomically related organisms 

 often play a role in determining whether a 

 newly introduced parasite will successfully 

 transfer from one host to another. 



