ECOLOGY AND ISOLATION 



619 



Webb (1947) finds that the mating of 

 snails of the subfamily Polygyrinae is a dif- 

 ficult performance, wfiile the Triodopsinae 

 are able to transfer semen from one individ- 

 ual to another with much greater ease. 

 He thinks the greater speciation of the 

 polygyrin species in contrast with the trio- 

 dopsin species east of the Mississippi may 

 be the result of these mechanical factors. 



Mechanical isolation may play a role in 

 the evolution of specialized insect-pol- 

 Unated plants, such as orchids and the Leg- 

 uminosae (Dobzhansky, 1941, p. 269; see 

 also pp. 250 and 715). 



Although some instances in which me- 

 chanical isolation proves to be the initial 

 mechanism of speciation may be discovered 

 ultimately, its role seems to be a minor one. 



PSYCHOLOGICAL ISOLATION 



Reproductive isolation through the lack 

 of sexual attraction between two closely 

 related species is a possibiUty, even though 

 they Uve in the same environment and geo- 

 graphic region and there is no mechanical, 

 physiological, or genetic incompatibility. 

 This form of sexual isolation resulting from 

 behavioristic or psychological interference 

 with mating has been termed ethological 

 isolation (Mayr, 1942). Behavior difFer- 

 ences associated with the general habitat 

 rather than with mating are included under 

 Habitat Isolation (p. 610). In some ex- 

 amples, such as those discussed under 

 Physiological Isolation (p. 622), the individ- 

 uals avoid mating because of physiological 

 barriers. In other instances, the barriers 

 seem to be mainly psychological. The form 

 of intraspecies sex attraction and interspe- 

 cies sex isolation may be visual, auditory, 

 tactile, chemical, or a combination of var- 

 ious stimuli (Kahn, Celestin, and Offen- 

 hauser, 1945; Mayr, 1946a). In territorial 

 passerine birds, there tends to be an in- 

 verse relation between the development of 

 auditory distinctiveness and visual distinc- 

 tiveness' (Huxley, 1938). 



We are here dealing with hereditary be- 

 havior. Conditioned behavior, which may 

 also have an evolutionary effect (Sutton, 

 1931), has already been discussed (p. 

 604). The fact that physiologic and psy- 

 chologic distinctions intergrade and are of- 

 ten diflScult to separate indicates that they 

 are closely related and that a dualistic 



" Orioles and cardinals are exceptions. 



philosophy separating psyche from body is 

 unwarranted. 



Dobzhansky and Koller (1938) have 

 shown that in cultures containing a mix- 

 ture of females of Drosop]iila pseudoobs- 

 ciira and D. miranda and males of one of 

 these species, the males more often fertihze 

 the females of their own species. This tend- 

 ency toward homogamic matings was also 

 observed in mixed cultures of D. azteca 

 and D. athahasca, and to a less degree in 

 mixed cultures of the Olympic and Whit- 

 ney races of D. miranda. Patterson, Mc- 

 Danald, and Stone (1947) say that sexual 

 preferences resulting in a lack of cross 

 breeding are universal between species 

 groups of Drosophila, and complete sexual 

 isolation may occur between species within 

 the same group. Mayr (1946a) has ana 

 lyzed the mechanisms of sexual attraction 

 between two closely related species of 

 Drosophila (D. pseudoobscura and D. per- 

 similis). 



Thus species may be sexually isolated 

 within the same territory. Other types of 

 isolation, such as that afforded by topog- 

 raphy or habitat, may precede the devel- 

 opment of psychological isolation (p. 610). 

 It is probably seldom possible for psycho- 

 logical isolation to initiate the separation of 

 races, but it may augment other types of 

 isolation. 



Two races of salmon, the "steelhead" 

 and "rainbow trout," belonging to the same 

 species (Salmo gairdneri) , spawn on the 

 same grounds and at the same time in the 

 upper portion of the Cowichan River and 

 Cowichan Lake in British Columbia. The 

 steelhead trout migrates to and from the 

 sea, while the rainbow trout resides in 

 fresh water, migrating only from the lake 

 to the river and back. Scale counts indicate 

 that there are two populations with differ- 

 ent but overlapping hereditary characters. 

 Data on interbreeding between these races 

 are wanting, but there appears to be in- 

 cipient speciation without geographic or 

 habitat isolation, and one may assume at 

 least a partial psychologic or genetic isola- 

 tion associated with the difference in migra- 

 tory behavior (Neave, 1944). 



The population of the sockeye salmon 

 {Oncorhijnchus nerka) found in Cultus 

 Lake, British Columbia, is divided into re- 

 sidual and migrating components. The re- 

 sidual population is the progeny of the mi- 



