NATURAL SELECTION: II 485 



tors to be considered a different species. Mesohippus evolving into 

 Miohippus may be taken as an example. Species A has become species B. 

 Typically, species A will have disappeared in the process, leaving species 

 B in its place. Similarly, as time goes on species B may evolve into, and be 

 replaced by, species C, and so on. This phyletic pattern of evolution along 

 a time axis is abundantly evident in the sequences of fossil forms studied 

 in earlier chapters. 



If, however, instead of tracing a historical sequence through time in this 

 manner we concentrate attention on one period in the earth's history, we 

 see evidences of the second pattern of evolution. This consists of the more 

 nearly "simultaneous" (in terms of geologic time) production of groups of 

 species, the type of evolution to which Mayr (1949) has insisted that the 

 term speciation be restricted. In this second pattern of evolution species A 

 gives rise to two or more species, perhaps to a whole cluster of them, usu- 

 ally, if not always, originating in separate territories. These daughter spe- 

 cies may or may not replace species A. This type of evolution occurs when 

 a species ranges widely over a large territory various portions of which are 

 separated from other portions by distance and perhaps also by other geo- 

 graphic-environmental isolating mechanisms. Under such conditions the 

 original species becomes divided into subpopulations. As mentioned in our 

 discussion of isolation, each subpopulation is free to undergo independent 

 evolution, through action of genetic drift and natural selection. The result 

 will be differing populations of animals inhabiting isolated territories. 



Because of the effectiveness of the barriers producing isolation, animals 

 inhabiting oceanic islands afford particularly instructive examples of this 

 phenomenon. One such example is diagramed in Fig. 21.1, which rep- 

 resents the varieties or races of the golden whistler found on various of the 

 Solomon Islands. The varied colorations and patterns of plumage which 

 have developed in these isolated populations are evident. We may well con- 

 clude that we are seeing the results of genetic drift plus, perhaps, some 

 measure of natural selection. Here is an example of the formation of varie- 

 ties or geographic races comparable to the subspecies of Peromysciis dis- 

 cussed earlier (pp. 320-325), and, on reduced scale, to the races of man 

 (pp. 250-255). 



Most biologists believe that the same process carried one step further 

 leads to the formation of distinct species. As we have noted before, the 

 "step further" is the development of reproductive isolation. If, while iso- 

 lated, two subpopulations accumulate sufficient genetic differences so that 

 they will no longer exchange genes if and when they come into contact, 

 those subpopulations have become separate species. 



