ECOLOGY AND ISOLATION 



621 



ance of cross breeding experiments would 

 be crucial. No cases of either natural or ex- 

 perimental hybrids are known. Nor is it 

 known how the males recognize the fe- 

 males of their own species." The males 

 never wave at the females of a diflFerent 

 species, except rarely for a few seconds, 

 even though taxonomic distinctions of fe- 

 males are often microscopic and difficult 

 to detect. The speciation of Uca empha- 

 sizes sexual display as an isolating mech- 

 anism. 



There may be selection favoring psycho- 

 logical isolation in sympatric species while 

 no such selection pressure would operate 

 on groups sharply separated by topography 

 or habitat (p. 605). There is also a possi- 

 bility that once separated forms might 

 merge if no psychological or other isolating 

 mechanism persisted. 



Dice (1940) describes an interesting 

 case of geographic and ecologic overlap 

 between two species of mice (Peromijscus 

 leucopus and P. gossypinus) with practi- 

 cally no hybridization in nature, though in 

 the laboratory these two species interbreed 

 readily and produce viable offspring. The 

 two populations were probably separated 

 geographically during the period in which 

 the psychological differences arose. 



George B. Saundersf has studied the 

 eastern and western meadowlarks (Stiirn- 

 ella magna magna and S. neglecta), which 

 broadly overlap in both geographic and 

 ecologic distribution, although the western 

 meadowlark is essentially a prairie form, 

 while the eastern species is a meadow and 

 field form. During the last glaciation these 

 species were probably topographically iso- 

 lated. In the prairie habitats west of Chica- 

 go, both species are now found nesting in 

 the same field. In nature, hybrids occur 

 rarely, if at all, but in the laboratory the 

 species interbreed and produce viable oflF- 

 spring. 



Lack (1945, 1947) suggests that specia- 

 tion in the Galapagos finches (Geospiza) 

 occurred through topographic isolation of 

 small populations and that hybridization in 

 later geographically overlapping species is 

 prevented by psychological and sexual iso- 

 lation. The shape of the bill is important in 

 courtship and species recognition in these 

 birds. 



* Personal communication from Miss Crane, 

 f Personal communication. 



The elaborate courtship procedures of 

 many animals indicate that sexual recogni- 

 tion and stimulation are the result of adap- 

 tive evolution. Mayr (1942, p. 254) points 

 out that sexually dimorphic species often 

 use these characters "to facilitate the meet- 

 ing and recognition of conspecific individ- 

 uals and to prevent hybridization between 

 different species." Referring particularly to 

 the hummingbirds (TrochiUdae), grouse 

 (Tetraonidae) and manakins (Pipridae), 

 he says (p. 261): 



"It is not accidental that we find in [the] 

 families not only the highest development of 

 sexual dimorphism, but also the greatest differ- 

 ence between the males of closely related 

 species. These differences are so striking that 

 even the geographic races are so different in 

 many species as to be considered to be 

 generically distinct by earlier authors. There 

 is necessity for highly specific recognition marks 

 in those species in which copulation is not 

 preceded by pair formation or lengthy engage- 

 ment periods. On the other hand, sexual 

 dimorphism tends to deteriorate on small 

 islands on which selective species recognition 

 is unnecessary, since no other species of the 

 genus is present." 



Fulton (1933) reported three kinds of 

 crickets (Nemobius jasciatus fasciatus, N. 

 f. socius, and N. f. tin7nihis) that are strik- 

 ingly similar in morphologic characters and 

 are not seasonally isolated. These supposed 

 subspecies overlap geographically, but in 

 any one region tend to be restricted ecolog- 

 ically: socius to moist meadows and 

 marsh borders, tinnuhis to shaded wood- 

 lands, and fasciatus to intermediate habi- 

 tats. In certain transitional areas such as 

 the borders of woodlands, two forms may 

 intermingle, but as there is no indication 

 of hybridization in nature, these forms ap- 

 pear to have reached species status. The 

 chirps of the three forms are distinct and 

 under natural conditions probably serve to 

 keep the populations from interbreeding. 

 Under laboratory conditions Fulton was 

 able to raise hybrids of fasciatus and tin- 

 nulus with intermediate songs. These hy- 

 brids were fertile, thus indicating no ge- 

 netic isolation. One might assume a com- 

 bination of habitat and psychological isola- 

 tion in the initiation of speciation of these 

 crickets. 



Sexual dimorphism may often be favored 

 by natural selection because of its benefit 

 to the species as a means of bringing the 



