GENIC CHANGES ASSOCIATED WITH THE ESTABLISHMENT OF SYMPATRY IN ORIOLES OF THE GENUS ICTERUS.

Abstract

A study of the variation in plumage pattern and coloration in hybridizing orioles of the genus Icterus indicated that the two subspecies of the northern oriole, Icterus galbula galbula and I. g. bullockii, are sympatric in at least one locality in the Great Plains of North America (Corbin and Sibley, 1977). The genetic variation at 19 loci of the birds collected in that study is presented here. The observed allelic frequency differences are used to document further those changes that appear to be taking place in, and adjacent to, the of overlap and between the galbula and bullockii subspecies of the northern oriole. The observed sympatry between these two currently recognized subspecies might be a transient local condition resulting from a chance influx of the galbula phenotype into the range of distribution of the bullockii phenotype. However, we believe the evidence available supports the hypothesis that sympatry between the two subspecies has an underlying genetic basis, that sympatry developed in situ as a result of assortative mating and not because of rare dispersal events. As such,the condition may provide a better understanding of the genetic changes associated with the intermediate stages of the speciation process. We approach our hypothesis by considering: 1) how the allelic frequencies within and outside of the of contact might be altered following secondary contact between hybridizing, differentiated subspecies, and 2) how allelic frequencies might change within the of subsequent to selection against hybrids. Hybridization involving the orioles of the genus Icterus was first recognized by Sutton (1938) and subsequently studied in detail by Sibley and Short (1964), Sutton (1968), Rising (1968, 1969, 1970, 1973), Anderson (1971), Misra and Short (1974) and Corbin and Sibley (1977). In the mid1950s, the Baltimore oriole, Icterus galbula, and the Bullock's oriole, I. bullockii, were recognized as species. Morphologically, they could be distinguished on the basis of plumage pattern and coloration as well as on the basis of mensural characteristics such as wing length, tail length and body length. However, between them was extensive throughout the Great Plains of North America (Sibley and Short, 1964). Within the the average value of each of the various traits was intermediate between those of the parental phenotypes. (We use the term hybrid as defined by Short [1969:89], an area of where only hybrids occur, as distinguished from a zone of overlap and hybridization which is an area of secondary intergradation occupied by numerous hybrids and both parental forms.) There was evidence that introgression was bidirectional and that progeny were equally fit in comparison to the offspring of pure parental matings, although the latter was an inference based on the frequency of hybrids rather than on experimental breeding data. It appeared that premating isolating mechanisms were ab-