18 CONCEPTS AND DEFINITIONS 



lated from other such groups." Simpson (1943) gave the defini- 

 tion "a genetic species is a group of organisms so constituted and 

 so situated in nature that a hereditary character of any one of 

 these organisms may be transmitted to a descendent of any 

 other," and Dobzhansky ( 1950 ) defined the species as "the 

 largest and most inclusive . . . reproductive community of sexual 

 and cross-fertilizing individuals which share in a common gene 

 pool." 



It might be useful to mention some qualifications which are 

 often included in species definitions but needlessly so. Anything 

 that is equally true for categories above and below species rank 

 should be omitted, since there is no sense burdening a species 

 definition with features which do not help discrimination be- 

 tween species and infraspecific populations. 



1. Species characters are adaptive. This component of Wal- 

 lace's ( 1889 ) species definition was correctly rejected by Jordan 

 (1896). Adaptiveness is not diagnostic for species characters 

 and not even necessarily true. Not every detail of the phenotype 

 needs to be adaptive as long as the phenotype as a whole is 

 adaptive and as long as the genotype itself is the result of selec- 

 tion. 



2. Species are evolved and evolving. Again this is true for 

 the entire organic world from the individual to the highest cate- 

 gories and adds nothing to the species definition. 



3. Species differ genetically. This is only the morphological 

 species concept expressed in genetic terms. It does not permit 

 discriminating species from infraspecific populations or from 

 individuals. 



4. Species differ ecologically. This qualification is unneces- 

 sary and misleading for the same reasons as the genetic one. 

 Ecological differences exist for all ecotypes within species and 

 in general for all geographical isolates. Conspecific populations 

 are sometimes more different ecologically than are good species. 



A yardstick, such as the biological species concept, is not auto- 

 matic. To apply it properly requires skill and experience. This 

 is particularly true in the recognition of situations where it cannot 

 be applied, lor one reason or another, and where the worker has 



