318 INTRODUCTION TO EVOLUTION 



other such groups" has significance only for organisms that reproduce sex- 

 ually. Plant and animal groups in which reproduction is asexual are not 

 "interbreeding populations." In some cases a single individual may give rise 

 to a whole Hne of descendants (a clone or biotype), all just like the original 

 individual except as new mutations may from time to time introduce 

 changes. Since interbreeding is absent, no test of reproductive isolation is 

 possible. Classifying asexual organisms into species must therefore be based 

 upon possession of distinctive characteristics as mentioned above. This is 

 one reason why a species definition suitable for a majority of organisms is 

 nevertheless not suitable for others (many plants and protozoans, for 

 example). 



Another attribute of species refers to the range or territory occupied. Usu- 

 ally two closely allied species do not occupy the same territory, though fre- 

 quently their territories will adjoin. From the preceding discussion we can 

 appreciate the fact that two closely allied species will usually be very similar 

 to each other in appearance and habits. This implies that they will be likely 

 to depend upon the same or similar food supply, seek the same home or 

 nesting sites, and so on. Thus if they occupied the same territory they would 

 usually be in direct competition. Such competition doubtless occurs and is 

 important in promoting evolutionary change, yet in groups of species in a 

 state of approximate equilibrium at a given stage in evolutionary history, 

 direct competition is reduced when territories occupied by allied species are 

 separate. An example may be found in the two species of Peromyscus in- 

 habiting Vermont. Peromyscus leucopus occupies wooded lowlands and the 

 lower portions of the mountain slopes; maniculatus occupies the higher por- 

 tions of these mountains. While the two species both live in the state of 

 Vermont, they actually occupy separate, though adjoining, territories. 



In the case just mentioned, the wooded lowlands difi'er from the higher 

 portions of the mountains not only geographically but also by environmental 

 differences. Sometimes closely allied species may occupy the same territory 

 if their environmental (ecological) requirements difi'er. We noted earlier 

 (p. 294 and Fig. 13.3, p. 295) that three species of ground finches are en- 

 abled to live together on the Galapagos Islands because each species special- 

 izes in eating seeds of a certain size. Thus direct competition is reduced and 

 we have an exception to the general rule that closely allied species do not 

 usually occupy the same territory. 



A corollary of the reproductive isolation mentioned earlier lies in the fact 

 that when the territories occupied by two species do overlap there is gener- 

 ally no interbreeding between the species (recall the two species of Pero- 



