Speciation is the process of evolutionary diflferen- 

 iatioii between populations, which may result in one 

 older species becoming split into two or more new 

 ones. Speciation usually entails divergence of the 

 new species into different niches. An understanding 

 of the basic principles of speciation is therefore a 

 l)rere(|uisitf to an inulerstniuiiiig of animal distribu- 

 tion. 



TAXONOMY 



19 



Ecological Processes 



and Community 



Dynamics: 



Speciation 



.\s the term is used by taxonomists, a popu- 

 lation is a local aggregation of individuals that differs 

 slightly, but characteristically, from other local aggre- 

 gations of the same species. 



Geneticists define a population as a reproductive 

 community sharing a common, characteristic gene 

 pool. I'^very local population is different from every 

 other one : but they are not easily distinguishable from 

 one another, and therefore are given no formal no- 

 menclature. What a population includes may vary 

 from the entirety of a species to but a few individuals, 

 according as the rate and extent to which individuals 

 interdisperse between localities to make a common 

 gene pool. 



Among bisexual forms, a species is a group of 

 populations capable of successfully interbreeding, re- 

 productively isolated under natural conditions from 

 other such populations. Species are usually morpho- 

 logically distinct, but the distinguishing character- 

 istics are sometimes barely discernible. Hybridiza- 

 tion may occasionally occur between clearly defined 

 species under captive or unnatural conditions, but 

 does not occur with any significant frequency under 

 natural conditions. 



Fossils found separated in different geological 

 strata, or living populations separated in space, are 

 considered capable of successful interbreeding, and 

 therefore to be of the same species, if essentially sim- 

 ilar structures, functions, and behaviors can be ad- 

 duced for them. It is unfortunate that species cannot 

 presently be recognized by entirely objective means, 

 but even if this were readily possible there would be 

 diflficulties, inasmuch as populations may be at vari- 

 ous stages in the differentiation of complete repro- 

 ductive isolation. Species are distinguished by the 

 familiar bionomial nomenclature standardized by in- 

 ternational rules (Mayr ct al. 1953). 



A subspecies is a geographically defined group 

 of populations which differs in color, size, or some 

 other taxonomic characteristic from other popula- 

 tions within the same .species but nonetheless inter- 

 breeds with them freely, regularly, and successfully 

 where their ranges come into contact. Subspecies are 

 commonly distinguished by a trinomial nomenclature, 

 although the desirability of this has been questioned 



257 



