The Differentiation of Populations 247 



of ecological isolation enjoyed by the gens. Where gentes occur 

 together in the same general area, there is no sign that this degree 

 of isolation is sufficient to produce enough genetic differentiation to 

 result in reproductive incompatibility. There is every indication that 

 such gentes are not "incipient species" and that divergence at the 

 species level occurs in geographic isolation. Needless to say, the 

 exact status of the cuckoo gentes is deserving of much additional 

 study. 



Allopatric Speciation 



Differentiation of physically isolated populations, when this differ- 

 entiation goes to the point that reunion of the populations does not 

 occur if contact is reestablished, is known as allopatric speciation. 

 An abundance of evidence suggests that allopatric speciation is the 

 fundamental cause of organic diversity. It is the splitting mech- 

 anism, v/hich, coupled with extinction, is necessary to explain the 

 large numbers of relatively distinct kinds of plants, animals, and 

 microorganisms that populate the earth. 



Little is known about the time required for populations to differ- 

 entiate. In any given case, many variables would affect the required 

 time span, including population sizes, magnitude of selection pres- 

 sures, degree of isolation, and the genetic system of the organism. 

 In most cases speciation seems to be a much more drawn-out process 

 than could be conveniently observed by the evolutionist. However, 

 much recent work (e.g., that on industrial melanic moths and on 

 Cepaea and Nafrix) indicates that selection pressures in nature may 

 be generally higher than once thought; if this is the case, then spe- 

 ciation may also occur more rapidly than has been assumed in the 

 past. At any rate, the evidence on which is based the view that 

 allopatric speciation is the primary splitting mechanism in evolution 

 is not direct observation but the presence of patterns of variation 

 that seemingly represent every conceivable stage in the postulated 

 process. Some of these have been discussed in this chapter; the bio- 

 logical literature is replete with others. 



Sympatric Speciation 



Can distinct new kinds of organisms arise in the absence of physical 

 isolation of populations? The answer is certainly "yes" in the case of 

 allopolyploidy (discussed in Chap. 9) and for organisms in which 

 sexual processes are absent. Each individual of completely asexual 

 organisms (such as some rotifers) is a genetic isolate, and species are 



