138 Species and Species Change 



INTERBREEDING CHARACTERISTICS 



The species was defined as an assemblage of interbreeding popula- 

 tions. In species with small compact ranges or with a high vagility 

 it is likely that members of any population will interbreed effec- 

 tively with any other. In species with large ranges and low vagility 

 members of adjoining populations interbreed effectively, but indi- 

 viduals from widely separated localities frequently reproduce much 

 less effectively. Experimenting with the American leopard frog 

 Rana pipiens, Moore (1949) crossed individuals from Vermont, 

 New Jersey, Oklahoma, Florida, and Texas. Matings from two suc- 

 cessive locations produced normal offspring, but matings between 

 frogs from Vermont and Texas or Vermont and Florida produced 

 young which died before completing development. Through the 

 avenue of interbreeding between adjoining populations, however, 

 genetic changes in the populations at either end of the range can 

 theoretically spread through the entire species. Similar conditions 

 occur in the fly Dwsophila pseudoohscura, in which the vigor of 

 the F2 generation decreases when populations from California, 

 Nevada, Utah, and Colorado are intercrossed. In spider mites of 

 the species Tetramjchus tehrius, Boudreaux (1957) found that 

 crosses between colonies from various states exhibited little to mod- 

 erate inviability of eggs, but crosses between European and North 

 American strains exhibited moderate to complete inviability of eggs. 

 A highly graphic expression of the same circumstance is shown by 

 the "ring species." The range of the herring gull Larus orgentafus 

 extends around the Arctic Ocean and the two ends of the range, 

 represented by different subspecies, meet in northwestern Europe. 

 These two forms do not hybridize but live together like two dis- 

 tinct species even though they are connected genetically through 

 a ring of intermediate interbreeding populations. 



Thus, with time and through progressive accumulation of genetic 

 changes, individuals in different populations may come to ha\'e 

 genetic systems too different to produce a physiologically balanced 

 zygote. If a species has a high vagility, presumably the mixing of 

 new characters is sufficiently thorough to prevent such large local 

 divergence. If a species has a low vagility or a narrow mixing 

 radius, local changes can accumulate to form distincti\e local t\ pes. 

 The situation emphasizes that a species forms a genetic s\\stcm 

 which permits genetic traits to diffuse through all parts of it, 

 although distant populations may be inter-sterile. 



AMOUNT OF VARIABILITY 



Considering the tremendous potential for change inherent in the 



