630 



generation. Asexual species offer interesting 

 material for an evolutionary comparison 

 with sexual species, particularly when the 

 two kinds of species have evolved together 

 in strict ecological association. 



We draw the following conclusions from 

 our brief consideration of taxonomic cate- 

 gories. The concept of clines (except cHnes 

 of ecophenes) emphasizes the operational 

 aspects of evolution and, in numerous in- 

 stances, may be a measure of recent or con- 

 temporary evolutionary events in contrast 

 with ancient influences that are therefore 

 more diflBcult to evaluate or analyze. 



Isolating mechanisms, biotic as well as 

 physical, are essential for evolutionary di- 

 vergence leading to infraspecific and spe- 

 cific distinction. Various infraspecific terms 

 may be used to emphasize the role of differ- 

 ent isolating factors. After permanent re- 

 productive isolation is attained, further 

 nonadaptive divergence may take place by 

 haphazard genetic changes, while adaptive 

 divergence is guided by selection pressure. 



Asexual species are of importance in the 

 community relations and may serve for cer- 

 tain types of evolutionary analysis. 



SUMMARY 



The various factors that contribute to 

 partial or complete reproductive isolation 

 between populations have a profound effect 

 upon evolutionary divergence. Many of 

 these factors are environmental and there- 

 fore ecologic in the broad sense. 



Some factors, such as topographic isola- 

 tion, are so universal that they influence 



ECOLOGY AND EVOLUTION 



nearly aU speciation to some degree. Others, 

 such as annual isolation, are of minor im- 

 portance, and only a few illustrations of 

 their effect are known. The rare mecha- 

 nisms are interesting because they offer a 

 control over other isolating factors in some 

 instances. 



Certain types of isolation are not neces- 

 sarily permanent if a junction of temporar- 

 ily separated populations occurs. Other 

 types of isolation (intrinsic) result in per- 

 manent reproductive separation, even 

 though close contact between populations 

 may later exist. 



Some factors separate genetically identi- 

 cal or similar populations. Other factors, 

 such as habitat isolation, operate on geneti- 

 cally different populations in most cases. 



Selection pressures may be manifest after 

 isolation, or they may assist in the isolating 

 process. There is some evidence that isolat- 

 ing mechanisms may be adaptive in them- 

 selves, particularly in psychological isola- 

 tion, thus indicating that there may be a 

 survival value in preventing gene flow be- 

 tween differentiating populations. 



In the majority of cases, more than one 

 mechanism may contribute at the same 

 time to the isolation of populations. Also, 

 there may be such close sequence in the 

 operation of isolating factors that the 

 separate effects may be diflBcult to detect. 



The influence of density effects may be 

 considerable, but exact data are difficult 

 to obtain. Critical information on all as- 

 pects of isolation is meager, and active in- 

 vestigations should be encouraged. 



33. ADAPTATION 



GENERAL CONSIDERATIONS 



Adaptation is a universal biological phe- 

 nomenon characteristic of all Hving organ- 

 isms. Evolutionary adaptation in its strict 

 sense refers to hereditary adjustment or fit- 

 ness among the parts of a Hving system 

 and between the whole system and its en- 

 vironment. (See pages 639, 640, 656, for 

 discussion of teleological implications. Also 

 see page 73, for a summary of the con- 

 cept of the fitness of the environment.) 



We are not here discussing at length the 

 adaptability of the individual to meet 



changing conditions, such as the adjust- 

 ment of the pupil of the eye to variations 

 in Ught intensity, the greater development 

 of leaves and branches on the sunny side of 

 a tree at a forest margin, the acchmatiza- 

 tion of an animal to seasonal change, or the 

 conditioned behavior of many animals. 

 These adjustments represent nongenetic 

 physiologic response in contrast wdth ge- 

 netic adaptation (Plunkett, 1944). 



At the same time we should recognize 

 that the capacity for individual adaptability 

 may rest upon genetic factors (see p. 3). 

 Mammals of temperate regions that com- 



