NATURAL SELECTION: II 489 



We have emphasized three factors in the process of speciation, operative 

 in the order named: (I) spatial isolation; (2) development of genetic 

 diversity sufficient to insure reproductive isolation if and when the species 

 come into contact (this genetic diversity may or may not include visible 

 structural differences); (3) divergence in characteristics if the species, al- 

 ready reproductively isolated, come into competition for food, nest sites, or 

 other essentials of life. The spatial isolation considered primary is tanta- 

 mount to the geographic-environmental isolation we discussed earlier. In 

 some cases the main Isolating factor may be an environmental one (e.g., 

 differences in food habits), but enough "space" must also be involved to 

 insure that the two populations do not come into contact and interbreed 

 (prior to the development of reproductive isolating mechanisms). 



Lack (1947, 1949) presented as an example of the necessity for initial 

 spatial separation the case of the finch inhabiting Cocos Island (Fig. 13.2, 

 p. 288 ) . This finch is so unlike the other Darwin's finches that it is placed in 

 a separate genus, suggesting that it has been on the island for a long time. 

 Yet this genus contains but one species, which is not even divided into 

 subspecies. Lack wrote, "Cocos resembles the Galapagos in providing 

 varied habitats and in having a great paucity of other land-birds, but it 

 differs in one essential respect: it is a single island, not an archipelago. 

 Hence there has been no opportunity for the geographical isolation of 

 populations and hence no evolution of new species or of an adaptive radia- 

 tion." The one species present has presumably undergone progressive evo- 

 lution of the replacing or phyletic type described on page 484. Adaptive 

 radiations of birds occur on archipelagos but not on single oceanic islands; 

 such radiations formerly occurred on continents, which are large enough 

 so that means of geographic isolation other than stretches of ocean are 

 operative. Obviously for other animals than such accomplished travelers 

 as birds smaller barriers and distances will suffice to produce the needed 

 isolation. The point is that some effective means of spatial isolation seems 

 essential as an initial step in speciation, and indeed in almost all evolu- 

 tionary change. 



Effect of Population Size 



Our discussion of speciation has emphasized the breaking up of large 

 populations into relatively small, isolated subpopulations. Students of the 

 subject are now generally agreed that optimal conditions for evolutionary 

 change are provided by such conditions (see Wright, 1949). Our preceding 

 discussion will have made evident that a large population not divided in 



