240 I The Process of Evolution 



not yet known whether the differentiated terminal populations will 

 fuse or whether they will differentiate further to produce a typical 

 ring-of-races pattern. Situations where two or more similar kinds of 

 organisms live in close proximity without apparent hybridizing are 

 numerous, but there seem to be no documented cases where rejoin- 

 ing segregates have shown no interest in each other. This does not 

 indicate that such situations never develop, merely that they have 

 not been observed. 



Perhaps the most likely candidate for a situation in which two 

 closely related kinds of animals have become sympatric and have 

 not required selection against hybrids to reinforce isolating mech- 

 anisms (as described below) is in the flycatcher genus Empidonax. 

 N. K. Johnson has recently studied an area in eastern California in 

 which two forms, Empidonax wrightii (gray flycatcher) and E. 

 oberholseri (dusky flycatcher), are sympatric. It is possible that 

 this sympatry is of very recent origin, the result of habitat changes 

 brought on by logging operations in the middle of the last century. 

 Empidonax wrightii breeds in sagebrush or small trees and tends to 

 forage in open areas, whereas E. oberholseri is a forest-chaparral 

 bird. Where they overlap in an area of mixed clearings and broken 

 forest the birds retain their habitat separation but defend their 

 territories interspecificalhj . The two kinds are so similar that they 

 can be distinguished with assurance only by careful wing measure- 

 ments and wing-tail ratios. Johnson has been able to detect distinct 

 differences in vocalizations, presumably concerned with pairing 

 and pair-bond reinforcement, but the challenge calls and appear- 

 ance are so similar that E. oberholseri territories are defended 

 against E. wrightii and vice versa. In spite of this similarity, several 

 dozen mated pairs collected by Johnson all showed positive assort- 

 ment (no oberholseri-wrightii pairings), indicating that the birds 

 have no trouble in properly choosing mates. Perhaps the recogni- 

 tion signs (whatever they may be) were originally weaker and 

 were reinforced by selection, but if this was the case the process 

 must have been completed very rapidly. 



Limited Gene Exchange. In a similar case of sympatry, pre- 

 sumably permitted by human disturbance of the environment, two 

 kinds of Mexican towhees, Pipilo erijthrophthalmus and P. ocai, 

 have come into contact and are now hybridizing. In this case previ- 

 ous ecological isolation (P. erijthrophthalmus mostly in oaks and 

 brushy undergrowth, P. ocai mostly in coniferous forest and associ- 

 ated undergrowth) seems to have broken down when lumbering 

 and agriculture produced second-growth situations suitable to both 

 forms. Indeed, hybridization at one level or another is widespread 



