618 



ECOLOGY AND EVOLUTION 



tat areas may possibly have some effect 

 upon the divergence of species. The races 

 of the periodic cicada, Magicicada septen- 

 decim, may be partially isolated in this 

 way. According to Davidson (Emerson, 

 1943), the races of the pink salmon (On- 

 corhynchus gorhuscha) that breed in the 

 same streams in alternate years are some- 

 what differentiated. It is rare to find Hfe 

 cycles exhibiting periodicities that inhibit 

 gene exchange between populations. Over- 

 laps in the periodicities tend to swamp the 

 differences, and in these cases there is little 

 selection to build genetic distinctions. An- 

 nual isolation should be considered a 

 minor mechanism of reproductive isolation 

 and operable only in a few special in- 

 stances. 



Isolation through differences in breed- 

 ing activity related to diel rhythms {diel 

 isolation) has not been much discussed, 

 although it is obvious that a population 

 active at night would not be likely to inter- 

 breed with a population active only in the 

 daytime (pp. 544-562, 611). 



Closely related species of fishes, the 

 black and white crappies (Pomoxis nigro- 

 macidatus and P. annularis), similar in 

 food habits and general behavior, are 

 found together in the rivers of Illinois, 

 Indiana, and Ohio (Johnson, 1945), and a 

 few interspecific hybrids have been found 

 in nature. The black crappie shows noc- 

 turnal activity, and the white crappie is 

 diurnal, thus suggesting that these species 

 are reproductively isolated through differ- 

 ent activity cycles. In this same family 

 (Centrarchidae), the species that hybrid- 

 ize freely in regions of geographic overlap 

 have similar or broadly overlapping activity 

 periods (Emerson, 1943). 



Diel isolation seems to be a minor isolat- 

 ing mechanism even among species in 

 which it might be expected to occur. Gen- 

 era of may flies and stone flies exhibit dif- 

 ferences in diurnal and nocturnal mating 

 behavior, but closely related species within 

 the same genus commonly are not distin- 

 guished by divergence in such diel 

 rhythms. 



Although cyclic isolation is not of great 

 importance in the evolution of many spe- 

 cies, nevertheless a temporal separation in 

 the same region and in the same habitat 

 may result in speciation. 



MECHANICAL ISOLATION 



Mechanical isolation, a term used by 

 other authors, refers to structural malad- 

 justment of the copulatory apparatus. It 

 may be presumed that a lack of fit between 

 the genital organs of the males of one spe- 

 cies and the females of another would re- 

 productively isolate the two populations. 

 Species of various animals, particularly of 

 many groups of insects and spiders, are 

 easily distinguished by the structure of the 

 copulatory organs. Convincing evidence, 

 however, for the importance of mechanical 

 isolation in evolutionary divergence is sur- 

 prisingly scanty. 



Dobzhansky (1941, p. 267) recently 

 summarized some data on this type of iso- 

 lation. Durfour suggested the "lock-and- 

 key" theory before the days of Darwin, 

 and K. Jordan (1905) amplified the con- 

 cept, which assumed that the genitalia in 

 copulation fitted each other as a key fits a 

 lock. Any substantial variation of the geni- 

 talia would, according to this theory, pro- 

 duce reproductive isolation (Pope, 1941). 

 A number of groups are known in which 

 the males are distinct in each species, 

 while no differentiation of the females can 

 be detected. Interspecies copulation is 

 known to occur (Sengiin, 1944), and no 

 mechanical barriers exist. It would thus 

 seem that speciation associated with other 

 types of isolation ultimately affects com- 

 plex organs such as the genitalia so as to 

 produce distinguishable taxonomic charac- 

 ters in some groups, but that these mechan- 

 ical differences result from the process of 

 speciation, instead of being the cause of a 

 primary isolation (Shull, 1946). The adap- 

 tational aspects of sexual adjustment are 

 discussed later (p. 688). 



Diver (1936) cites two closely related 

 snails, Cepaea hortensis and C. nemoralis 

 (Helicidae), with a parallel range of varia- 

 bility, a wide overlap of ecologic and geo- 

 graphic distribution, and occurrence in 

 mixed colonies. The genital organs differ, 

 and the two species seem to be mechani- 

 cally and psychologically isolated in nature, 

 although they can with difficulty be made 

 to produce viable but sterile offspring in 

 the laboratory (Diver, 1940). Overlapping 

 of the two species also occurs in Pleisto- 

 cene deposits, indicating long-continued 

 reproductive isolation. 



