Increase in Number of Species 167 



populations having an irregular but moderately frequent arrival of 

 additional members from the parent population would not evolve 

 into distinctive species. The continuous arrival of new immigrants 

 might be considered a sort of link which would preserve enough 

 genetic continuity between colony and parent range to nullify the 

 differential effects of partial isolation. Yet the fact remains that new 

 species have evolved or subspecies differentiated on islands a com- 

 paratively short distance from continents. 



The natural attainment of genetic isolation by insular species 

 under these conditions is demonstrated by the phenomenon called 

 double colonization. One example concerns the hornbill bird species 

 Acanthiza pusilla and A. ewingi. The latter is an endemic Tasmanian 

 species which evidently arose from an early colonization by the 

 Australian A. pusilla. Tasmania can be reached from Australia by 

 two successive island-to-island jumps each of only 50 miles. A. pusilla 

 occurs both on the Australian mainland and Tasmania, but on 

 Tasmania it does not interbreed with A. ewingi (Mayr, 1942). Mayr 

 concluded that colonists of A. pusilla dispersed to and colonized 

 Tasmania and there evolved into a distinct species A, ewingi which 

 is now genetically isolated from its original Australian parent. J. A. 

 Moore ( 1954 ) reported a similar case of insular speciation involving 

 two frogs Crinia signifera and C. tasmanica. The former occurs both 

 on the Australian mainland and Tasmania, and individuals from one 

 area cross successfully with mates from the other area. C. tasmanica 

 occurs only on Tasmania. Although a close relative and an obvious 

 derivative of C. signifera, C. tasmanica will not cross with it and 

 apparently evolved from an old colonization of Tasmania by C. 

 signifera. Mayr (1942) gives additional examples of double invasion 

 in many parts of the world. 



The use of the word "double" might in itself imply that in a 

 "double colonization" only two dispersals from continent to island 

 did indeed occur. The short distance from Australia to Tasmania 

 makes this circumstance highly unlikely, nor is it a necessary 

 postulate to explain the case. It might be argued that if other 

 invasions had occurred, either more than two species would be 

 present on the island, or the island species would never have de- 

 veloped. Other information, however, offers a more plausible ex- 

 planation of insular species evolution based on a premise of prob- 

 able multiple colonization. 



Let us assume a hypothetical example involving a bird such as 

 the hornbill of Australia which might occasionally reach an island 



