624 



ECOLOGY AND EVOLUTION 



swamping of diflEerentiated adaptive char- 

 acters tlirough hybridization would be so 

 deleterious to the species that there might 

 be selective pressure favoring any device 

 that would prevent cross breeding. Hybrid- 

 ization might also interfere with sexual 

 adaptations with resultant negative selec- 

 tion of the hybrid individuals. 



Turrill (1936) cited an interesting exam- 

 ple of hybrid eUmination among plants. In 

 the French Alps the campion, Suene cu- 

 cubaltis, is tall and erect and hves in hay 

 meadows and on the edge of woodlands. 

 Silene alpina has a low growth form and 

 lives on open talus slopes, sometimes in 

 close proximity to S. cucubalus. Neither 

 species invades the territory of the other. 

 A few hybrids are found in intermediate 

 habitats. These species are thus known to 

 cross in nature, and they are readily crossed 

 in the laboratory. Nevertheless, the species 

 remain pure under natural conditions in 

 their respective habitats. Hybrid elimina- 

 tion seems to be responsible for their re- 

 productive isolation. 



Epling (1947) discusses the elimination 

 of hybrids between two closely related 

 heathers (Arctostaphylos mariposa and A. 

 patiiJa), one occupying lower and drier 

 sites near Yosemite Park in California, the 

 other occupying higher, shaded, and cooler 

 sites. Many hybrids occur in overlapping 

 areas, but both species maintain them- 

 selves. Hybrid elimination by ecologic fac- 

 tors is suggested. 



We refer to such elimination as ecologi- 

 cal hybrid inviability in contrast to intrinsic 

 hybrid inviability, in which the mortaUty 

 is caused by internal lethal eflFects during 

 development. 



Another interfertile pair of species that 

 indicates ecological hybrid elimination is 

 found in the blue- winged warbler (Ver- 

 mivora pinus) and the golden- winged 

 warbler (V. chrysoptera) . In general, the 

 blue-winged warbler breeds in northern 

 and the golden-winged warbler in southern 

 regions. The blue-winged warbler is found 

 more commonly in low-lying bushy pas- 

 tures and second growth woods, while the 

 golden-winged warbler is characteristic of 

 uplands and oak woods. The breeding areas 

 of the two species overlap in the river val- 

 leys of portions of Connecticut, New York, 

 New Jersey, and Wisconsin, and in these 

 restricted habitats hybrids are occasionally 



found. The most conspicuous color charac- 

 ters of the two species seem to be Men- 

 delian characters (the whitish underparts 

 of the golden-winged warbler being domi- 

 nant over the yellowish underparts of 

 the blue-winged warbler, and the plain 

 throat of the blue-winged warbler domi- 

 nant over the black throat of the golden- 

 winged warbler in the first generation hy- 

 brid). The hybrid with the combination of 

 these two dominant characters is known as 

 Brewster's warbler. Back-crosses have been 

 observed. A double recessive hybrid is occa- 

 sionally produced, known as Lawrence's 

 warbler, with yellow underparts and a black 

 throat. Several other Mendelian characters 

 also segregate, and there is some evidence 

 that the throat patch may result from two 

 segregating genes. However, the blue- 

 winged and golden-winged warblers remain 

 pure in their respective ranges, and there 

 seems to be no effective transfer of genes 

 between the two species. This is circum- 

 stantial evidence of the elimination of the 

 hybrids over a few generations, either 

 through the long-run psychological isolation 

 of the species or through ecological hybrid 

 inviabiUty (see also Jewett, 1944; Alex- 

 ander, 1945). 



Thus selective hybrid elimination may 

 reproductively isolate two species otherwise 

 not completely separated. This mechanism 

 may reinforce the effectiveness of habitat 

 isolation in the origin of species (p. 616). 

 Hall ( 1946 ) favors the theory advanced by 

 Huxley (1939) to account for the main- 

 tenance of contiguous subspecies characters 

 (p. 602). Subspecies of Nevada mammals 

 seem to illustrate selective hybrid elimina- 

 tion. 



HYBRID ORIGIN OF SPECIES 



If an existing extrinsic isolating mecha- 

 nism separating two populations should 

 gradually or suddenly break down, the two 

 populations will be brought into contact 

 and will share their genetic characteristics, 

 provided no intrinsic bar to crossing has 

 developed during the period of isolation 

 (Table 52, p. 606). If genetic differences 

 that do not prevent interbreeding have 

 developed between two groups, either 

 through random fixation of genes in small 

 populations, through mutations in one 

 group only, or through survival of favorable 

 combinations and the elimination of the 



