Hybridization in Grosbeaks (Pheucticus) of the Great Plains

IN bisexual, cross-fertilizing organisms a species may be defined as a group of populations that shows intrinsic reproductive isolation (actual or potential) from other such groups of populations (Mayr, 1942).Sympattic populations are easily accommodated by this definition, but in al]opattic groups the existence of potential reproductive isolation is more difficult to determine.It is widely believed that spatial isolation is necessary for species formation (Mayr, 1942; Dobzhansky, 1951: 205).Once a population is divided and gene flow between the isolates ceases, differential selection and the occurrence of mutation and recombination lead to divergence.This is classical theory of speciation and needs no further elaboration.A later rejunction of these populations wi]] provide a natural test of the degree of intrinsic reproductive isolation that they have acquired.For this reason such rejunctions, or secondary contacts, are of interest to the student of evolution.There are two basic outcomes of a secondary contact: the populations are reproductively isolated, or they hybridize.Actually a continuous series of possibilities exists.Since the evolution of species is a gradual process, its interruption at any given time wi]] result in something between hybridization and lack of it.In addition, the ecological eompatabi]ity of the two populations will modify this simplified dichotomy.Hybridizatibn is of common occurrence in secondary contacts.Among birds the studies of Meise (1936), Sib]ey (1950, 1954, and 1958) , Dixon (1955), and Mayr and Gi]]iard (1952) may be mentioned. A distinction should be made between the occasional hybridization between sympatricspecies (e.g., ducks of the genus Anas), in which reduced success of the hybrids preserves the integrity of the species, and hybridization between a]]opatric populations in recent secondary contact.In this paper the latter case is considered.An example of a secondary contact with hybridization is found in two North American grosbeaks, the Rose-breasted Grosbeak, Pheucticus ludovicianus, and the Black-headed Grosbeak, P. melanocephalus.These birds, hereinafter referred to as ludovicianus and melanocephalus, are the North American representatives of a small genus that also includes P. chrysopeplus and P. aureoventris of Central and South America.They are more closely related than either is to the Latin American species.They occupy complementary ranges, ludovicianus east of the Great Plains, melanocephalus to the west.They are primarily woodland birds and do not breed widely in the Plains.They do occur in the riparian woodlands of, the Plains 399The Auk, 79: 399-424.