SOME CONSEQUENCES OF EVOLUTIONARY RELATIONSHIP 



385 



From what has already been said in this and the previous chapter about 

 the role of isolation in species formation, it is easy to understand why 

 this should be so. Most new species arise as populations, not from mutant 

 individuals. A species population cannot split so long as interbreeding 



Fig. 24.20. An illustration of Jordan's rule. Three closely allied species of grasshoppers, 

 Melanoplus clypeatus, M. furcatus, and M. symmetricus, occupy respectively the ranges 

 indicated by C, F, and S. All three live in dense shrubbery on seepage slopes bordering 

 streams and swamps. They can fly but seldom do so. They differ chiefly in the male geni- 

 talia, and do not intergrade. Their adjoining ranges are separated by barriers — the Alta- 

 maha River (X) between C and F, and a belt of uplands that at (Z) narrows to a 2-mile 

 sand ridge between F and S. Melanoplus furcatus, which has the widest and most varied 

 range, consists of five recognizably different subpopulations that occupy the numbered 

 areas and are for the most part connected by intergrading populations. Whether some or all 

 of these should be considered subspecies and given names is a matter of opinion. At (F) 

 is an intrarange barrier, a sand ridge along the St. Marys River east of the Okefenokee 

 Swamp, which sharply separates populations 2 and 3, although they intergrade around its 

 northern end. Populations 3 and 4 are the ends of a cline or gradient of change that parallels 

 the St. Johns River. Population 5 is the most isolated and most distinct, and has not yet 

 been shown to intergrade with any other. (Based on unpublished data from field studies by 

 T. H. Hubbell.) 



permits gene changes to spread through it. However, interpose a barrier 

 to interbreeding between one part of the population and another, and 

 the two isolated parts must inevitably become different in the course of 

 time. By far the commonest isolating factor in species formation is 

 geographic; some believe that geographic isolation is the only means by 



