Rapid Evolution in Orioles of the Genus Icterus

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Abstract
Hybrid indices and character indices are presented for a series of Baltimore [Icterus galbula galbula] and Bullock''s [I. g. bullockii] orioles and their hybrids collected during the breeding seasons of 1970, 1971 and 1974 along an east-west transect extending from Madison, Connecticut to Glenwood Sprints, Colorado [USA]. The mean values and the frequency distributions of hybrid indices for some of the localities differ significantly from those obtained from specimens collected during 1955-1956. Within that portion of the transect extending along the Platte River of Nebraska and Colorado, a hybrid zone that separated allopatric populations of the parental types existed previously. A hybrid zone still appears in central Nebraska, but now a zone of sympatry of the 2 parental types exists within the zone of overlap and hybridization. Within this zone of sympatry in western Nebraska and eastern Colorado, the frequency distribution of the morphological types is now bimodal, with the hybrids comprising only about 25% of the population. Those populations lying to the west of this zone of sympatry are now characterized as being more like the Bullock''s type than they were in the 1950''s. Conversely, those populations lying to the east of the zone of sympatry are now significantly more like the Baltimore type than they were in 1955-1956. It therefore appears that in some areas at least the hybrid genotype is less fit than are the parental genotypes. It is impossible to determine the selective pressures that have resulted in the observed changes. The net results, however, appear to be the reinforcement of isolating mechanisms and an increase in the frequency of the Baltimore phenotype in certain areas of western Nebraska and eastern Colorado that were previously dominated by the Bullock''s Oriole.

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