166 Increase in Number of Species 



The distinctive color patterns occurring in certain fish during the 

 breeding season (for example, the red-bellied and the black-nosed 

 daces ) , the "booming" pouches of the males in the American prairie 

 chicken group, the sound producing organs, sexual sensory devel- 

 opments, and perhaps the curious development of genitalic struc- 

 tures in many arthropods, and the structural differences in the flowers 

 of closely related orchids in which each species is adapted for pol- 

 lination by a distinctive insect, all may be correlated with the de- 

 velopment of barriers to cross-mating. 



In summary, the processes leading to species fission by geographic 

 isolation begin with ecological changes bringing about range divi- 

 sion and the formation of allopatric species (species whose ranges 

 do not overlap). These may subsequently expand their ranges or 

 reverse ecological changes may occur in the environment causing 

 the once-fragmented ranges to move together. After this congrega- 

 tion of species, sexual isolation evolves which is the final step in the 

 evolution of mature daughter species. 



COLONIZATION 



The idea basic to colonization is that individuals of a species dis- 

 perse across one of the peripheral areas unsuited to the species, 

 reach a suitable ecological area as yet unpopulated by the species, 

 and colonize this latter area. This colony, cut off from all but infre- 

 quent gene exchange with its parent population, then develops in 

 virtual isolation and evolves into a different species. Except for the 

 mode of establishing the disjunct population, colonization presum- 

 ably effects an increase in number of species in the same manner 

 as does a division of the range. 



The occurrence of this process is proven by the distinctive species 

 inhabiting oceanic islands far from and never connected with the 

 mainland, which nevertheless support a varied biota that must have 

 originated on the mainland. The biota of the Galapagos Islands, 

 the Hawaiian Islands, and many others must have originated in this 

 manner (Mayr, 1942). 



In addition to these striking cases of long-distance colonizations, 

 short-distance examples are numerous. The list includes many West 

 Indies insect species closely related to mainland species (Ross, 

 1959c), the unusual insular subspecies of Peromijscus maniculatus 

 on the islands of British Columbia (McCabe and Cowan, 1945; 

 Cowan and Guiguet, 1956), and several birds and frogs on Tasmania 

 (Mayr, 1942; J. A. Moore, 1954). One might consider that colonial 



