620 



ECOLOGY AND EVOLUTION 



grating population, and no reproductive iso- 

 lation or speciation is indicated (Ricker, 

 1938; see also p. 617). 



Among frogs and toads, the call notes 

 and sexual behavior reproductively isolate 

 closely related interfertile species, even 

 when these species occupy similar geo- 

 graphic and ecologic regions. The toads 

 (Bufo) and the spadefoot toads (Scaphio- 

 pus) aflFord examples (Bragg, 1945, 

 1945a). 



The common leopard frog {Rana pi- 

 piens) is a close relative of the pickerel 

 frog {R. palustris), and the two species 

 overlap broadly in both their geographic 

 and ecologic ranges. They may be found 

 breeding in the same pond at the same 

 time, although this is unusual. Rana palus- 

 tris secretes a mucus that has been thought 

 to be poisonous to R. pipiens, and would 

 thus prevent the male of one species 

 from clasping the female of the other. 

 Moore (1946), however, states that these 

 two species will clasp each other under lab- 

 oratory conditions without detectable harm 

 to either. Moore (1941, 1946a) artificially 

 hybridized this pair of species and raised 

 adult frogs. They are probably psycholog- 

 ically isolated through their different 

 call notes and breeding behavior. Partial 

 ecological and seasonal isolation between 

 them is usual. It is conceivable that 

 these two species might have initially di- 

 verged in their defensive adaptations, inas- 

 much as the primary function of the more 

 poisonous mucus of R. palustris seems to 

 be to repel enemies such as snakes. The 

 ability of the pickerel frog to exist in small 

 populations in apparent competition with 

 the larger populations of the leopard frog 

 may thus be explained. This example en- 

 ables us to eliminate a number of possible 

 factors in reproductive isolation, but sev- 

 eral others are still involved, and it is diffi- 

 cult to be sure of their separate evolution- 

 ary effects, if indeed the factors acted inde- 

 pendently during speciation. 



A number of species of fiddler crabs of 

 the genus Uca may be found in the same 

 locality. Crane (1941) found fifteen spe- 

 cies, twelve actively courting, in a beach 

 area not more than 600 feet square at 

 La Boca, Panama Canal Zone. This num- 

 ber of species of the same genus occupying 

 the same breeding area is extremely rare. 

 A phylogenetic study of the species along 



the Pacific coast of Central and South 

 America indicated some geographic and 

 some habitat isolation, but in general there 

 seems to be a considerable degree of spe- 

 ciation without clear evidence that pri- 

 mary isolation was caused by either geo- 

 graphic or gross habitat differences. Crane 

 found the courtship behavior and colora- 

 tion to be the striking differential. "Each 

 species proved to have a definite, individ- 

 ual display, differing so markedly from 

 that of every other species observed, that 

 closely related species could be recognized 

 at a distance merely by the form of the dis- 

 play." Furthermore, related species have 

 fundamental similarities of display, and 

 series of species, showing progressive spe- 

 ciaUzation of structure, in general show 

 similar progression in the nature of their 

 display. Miss Crane states*" that morpholog- 

 ic differentiation of closely related species 

 is no greater in geographically or ecologi- 

 cally separated regions than in closely re- 

 lated species occupying the same habitat. 

 It is noteworthy that the differentiation in 

 behavior and often in coloration of the 

 male is greater if the species are found to- 

 gether than if they are found in different 

 habitats or regions. 



The display pattern consists of various 

 combinations and modifications of cheHped 

 waving, elevation of body, position of 

 chelae, motion of minor cheHpeds, motion 

 around the burrow, revolution (dancing), 

 color exhibition, and other special featvures, 

 including display ground. Long periods of 

 courtship, often lasting through several 

 days, are the rule among these fiddler 

 crabs. Color differences in the males of dif- 

 ferent species are produced by four types 

 of monochromatic chromatophores, each 

 with a different pigment, together with a 

 blue pigment not within chromatophores 

 (Crane, 1944). The physiology of chro- 

 matophores in Uca pugilator is discussed by 

 Brown and Sandeen (1948). 



Courtship patterns govern species recog- 

 nition in these animals and, possibly in 

 conjunction with minor habitat and geo- 

 graphic separations, gradually have pro- 

 duced reproductive isolation between popu- 

 lations, followed by speciation. It would be 

 interesting to know whether genetic isola- 

 tion is also involved. The existence of hy- 

 brids between Uca species or the perform- 



* Personal communication. 



