ECOLOGY AND ISOLATION 



isolation is the initial factor in the diver- 

 gence of these two populations, with httle 

 effect of selection associated with environ- 

 mental differences (also see Dalquest and 

 Scheffer, 1944). 



Organisms with great vagihty often fail 

 to speciate in the same regions where sed- 

 entary types exliibit endemism. An inter- 

 esting case of topographic isolation is found 

 in the steamer ducks confined to southern 

 South America and the Falkland Islands. 

 These birds race over the surface of the 

 water, using their wings as "side-wheels." 

 One species, the flying steamer duck 

 {Tachyeres patachonicus) , has a range 

 equal to the combined ranges of the other 

 two species, which are flightless. The Mag- 

 ellanic flightless steamer duck (T. pten- 

 eres) is found only in the region of the 

 Straits of Magellan, while the other, the 

 Falkland flightless steamer duck (T. 

 brachijpterus) , is found only in the Falk- 

 land Islands. The interesting fact is that 

 the populations of flying ducks of this 

 genus iu the Magellanic and Falkland re- 

 gions remained one species, probably be- 

 cause interbreeding occurred, while the 

 flightless forms evolved into separate spe- 

 cies in the two regions. The two flightless 

 species were probably independently de- 

 rived from the flying species, but what fac- 

 tors brought about reproductive isolation 

 between the flying species and its flight- 

 less offshoots are unknown (Murphy, 

 1936). Possibly a habitat isolation was in- 

 volved, because the flightless species avoid 

 coasts having a marked tidal rise and fall 

 on which the flying species is usually found. 

 Although the two species often swim to- 

 gether, the nesting sites probably are differ- 

 ent. The flying species nests around lakes 

 or streams at a distance from salt water, 

 while the flightless species nests on small 

 islets or points near salt water. Confused 

 field identification of these species has re- 

 sulted in a lack of complete information 

 concerning the supposed differences in 

 nesting sites. 



Doutt (1942) reports an isolated land- 

 locked race of seals (Phoca) in a fresh- 

 water lake east of Hudson Bay in Canada. 

 This form is described as a subspecies of 

 the marine species (P. vitiilina) , but prob- 

 ably deserv^es full species rank because of 

 complete topographic isolation. The period 

 of isolation is estimated to be 3000 to 



609 



8000 years, with 300 to 1600 generations 

 (Simpson, 1944, p. 19). 



Worthington (1937, 1940) gives inter- 

 esting instances of topographic isolation 

 caused by falls in African rivers. The Ripon 

 Falls separate Lake Victoria from Lake 

 Kioga by a somewhat broken drop of about 

 15 feet. This would be a barrier to upward 

 dispersion of small fishes, but not to power- 

 ful swimmers. There are a few species and 

 subspecies differences in the fish faunas of 

 the two lakes, but, in general, the genera 

 and species are similar. In contrast, the 

 Murchison Falls separate Lake Kioga and 

 Lake Albert by a vertical drop of 130 feet 

 that effectively separates the fish faunas 

 above and below the falls. Many genera of 

 fishes characteristic of the Nile do not occur 

 in the river above the falls. Only diree 

 species in a fauna of fifty species below and 

 120 species above are the same. 



Interconnected swamps that, because of 

 their oxygen-deficient waters, form an 

 ecologic barrier to most fishes, divide the 

 Zambezi and Congo drainage systems. The 

 two faunas have distinct species and even 

 distinct genera, except for the air-breath- 

 ing Spirobranchus, Protopterus, and Cla- 

 rias, the same species of which occur on 

 both sides of the barrier. 



Worthington (1937) described another 

 noteworthy case of partial topographic iso- 

 lation among east African fishes. A small 

 crater lake Hes within Central Island in the 

 middle of Lake Rudolph, separated from 

 the main lake by a low ridge of water-worn 

 boulders. The populations of the fish spe- 

 cies, Tilapia nilotica (Cichhdae), in the 

 crater lake and in the main lake are sub- 

 specifically distinguishable. 



Blind cave carabid beetles (Pseudanoph- 

 thalmus) have speciated in different caves 

 of the eastern United States in spite of the 

 extreme uniformity of their habitat (Valen- 

 tine, 1943, 1945). After a long period of 

 topographic isolation, species that later 

 come to occupy the same cave remain re- 

 productively isolated, but exhibit greater 

 taxonomic divergence than related species 

 from different caves. 



Perhaps the most frequently cited ex- 

 ample of the effect of topographic isolation 

 is that of the land snails of the islands of 

 the South Seas (Gulick, 1905; Pilsbry, 

 1894-1935; Crampton, 1916, 1925, 1932)! 

 These snails evidently have a high muta 



